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Condensation Reaction

Condensation reaction

A condensation reaction (also known as a dehydration reaction or dehydration synthesis when water is lost) is a chemical reaction in which two molecules or moieties react and become covalently bonded to one another by the concurrent loss of a small molecule, often water or methanol. It may be considered as the opposite of a hydrolysis reaction (the cleavage of a chemical entity into two parts by the action of water).

Mechanism

Many condensation reactions follow a nucleophilic acyl substitution or an aldol condensation reaction mechanism. Other condensations, such as the acyloin condensation are triggered by radical or single electron transfer conditions.

Condensation reactions in polymer chemistry

In polymer chemistry, a series of condensation reactions take place whereby monomers or monomer chains add to each other to form longer chains. This may also be termed as 'condensation polymerization', 'polycondensation', 'stepgrowth polymerization', or 'stepwise polymerization'. It occurs either as a homopolymerization of an A-B monomer or a polymerization of two co-monomers A-A and B-B. Small molecule condensates are usually liberated, unlike in polyaddition where there is no liberation of small molecules. A high conversion rate is required to achieve high molecular weights as per Carother's equation. In general, condensation polymers form more slowly than addition polymers, often requiring heat. They are generally lower in molecular weight. Monomers are consumed early in the reaction; the terminal functional groups remain active throughout and short chains combine to form longer chains. Bifunctional monomers lead to linear chains (and therefore thermoplastic polymers), but when the monomer functionality exceeds two, the product is a thermoset polymer.

Applications

This type of reaction is used as a basis for the making of many important polymers for example: nylon, polyester and other condensation polymers and various epoxies. It is also the basis for the laboratory formation of silicates and polyphosphates. The reactions that form acid anhydrides from their constituent acids are typically condensation reactions. Other organic condensation reactions are Aldol condensations, self-condensation, the acyloin condensation and the benzoin condensation. Nearly all biological transformations are condensation reactions. Polypeptide synthesis, polyketide synthesis, terpene syntheses, phosphorylation, glycosylations, are just a few examples. Category:Organic reactions

Chemical reaction

A chemical reaction is a process that results in the interconversion of chemical substances . The substance(s) initially involved in a chemical reaction are called reactants. Chemical reactions are characterized by a chemical change and it yields one or more product(s) which are different from the reactants. Classically, chemical reactions encompass changes that strictly involve the motion of electrons in the forming and breaking of chemical bonds, although the general concept of a chemical reaction, in particular the notion of a chemical equation, is applicable to transformations of elementary particles, as well as nuclear reactions. Many different chemical reactions are used in combinations in chemical synthesis in order to get a desired product. In biochemistry, series of chemical reactions form metabolic pathways, since straight synthesis of a product would be energetically impossible in conditions within a cell. Chemical reactions are also divided into organic reactions and inorganic reactions.

Reaction types

There are five major classifications of chemical reactions. Some common and widely used terms are:
- Isomerization in which a chemical compound undergoes a structural rearrangement without any change in its net atomic composition; see stereoisomerism
- Direct combination or synthesis, in which two or more chemical element or compounds unite to form a more complex product; f.e. formation of water from hydrogen and oxygen
- Chemical decomposition or analysis, in which a compound is decomposed into smaller compounds; f.e. combustion of hydrocarbons
- Single displacement or substitution, characterized by an element being displaced out of a compound by a more reactive element; f.e. acid-base reactions
- Double displacement or coupling substitution , in which two compounds in aqueous solution (usually ionic) exchange elements or ions to form different compounds. Some branches of chemistry include any minor changes in chemical conformation in the reaction types, while others consider these changes merely as physical properties of a compound. The collision of more than two particles into the ordered structure necessary to perform chemical transformations is extremely unlikely; which is why ternary reactions in practice are not observed. A chemical reaction may require three or more reagents, but the process can generally be decomposed into a stepwise series or a set of stepwise reactions of the above. The large diversity of chemical reactions makes it difficult to establish simple criteria for functional (as opposed to mechanistic) classification. However, some kinds of reactions have similarities which make it possible to define some larger groups. A few examples are:
- Organic reactions, which encompass several different kinds of reactions involving compounds which have carbon as the main element in their molecular structure. These reactions occur mostly according to, within, by, or via functional groups. Reactions in petrochemistry aren't always classified as organic.
- Redox reactions, which involve augmenting or decreasing the electrons associated with a particular atom. according to its oxidation number.
- Combustion, where a substance reacts with oxygen gas;

Thermochemistry

See main article: Thermochemistry. Thermochemistry deciphers whether a specific chemical reaction can or cannot occur. Thermodynamics (or what is now known as equilibrium thermodynamics) understands the reaction in terms of the initial and final states of the reaction mixture. Reactions very seldom occur directly. Usually, reactants must collide to form an activated complex. This complex has a higher internal energy than the original reactants combined, having gained some from the kinetic energy of the reactant substances' collision. This energy allows for the rearrangement of bonds which constitutes the reaction. In some reactions, the reactants may pass through several reactive intermediates before becoming products. Thermodynamics does not attempt to figure out the process by which a reaction occurs. This field of study is taken up by the field of chemical kinetics. Another question "How fast is the reaction?" is also left completely unanswered by it. Chemical kinetics attempts to put all these phenomena into perspective.

Chemical equilibrium

Every chemical reaction is, in theory, reversible. In a forward reaction the substances defined as reactants are converted to products. In a reverse reaction products are converted into reactants. Chemical equilibrium is the state in which the forward and reverse reaction rates are equal, thus preserving the amount of reactants and products. However, a reaction in equilibrium can be driven in the forward or reverse direction. This is done by changing the reaction conditions such as temperature or pressure. Le Chatelier's principle can be used to predict whether products or reactants will be formed. Although all reactions are reversible to some extent, some reactions can be classified as irreversible. An irreversible reaction is one that "goes to completion." This phrase means that nearly all of the reactants are used to form products. These reactions are very difficult to reverse even under extreme conditions.

Exothermic reactions

Le Chatelier's principle According to energy balance criteria, that is, chemical reaction equilibria criteria, any closed system will tend to minimize its free energy. Without any outside influence, any reaction mixture, too, will try to do the same. For many cases, an analysis of the enthalpy of the system will give a decent account of the energetics of the reaction mixture. The enthalpy of a reaction is calculated using standard reaction enthalpies and the Hess' law of constant heat summation. Many of these enthalpies may be found in beginners' books on thermodynamics. For example, consider the combustion of methane in oxygen: :CH₄ + 2 O₂ → CO₂ + 2 H₂O By calculating the amounts of energy required to break all the bonds on the left ("before") and right ("after") sides of the equation using collected data, it is possible to calculate the energy difference between the reactants and the products. This is referred to as ΔH, where Δ (Delta) means difference, and H stands for enthalpy, a measure of energy which is equal to the heat transferred at constant pressure. ΔH is usually given in units of kilojoules (kJ) or in kilocalories (kcal). If ΔH is negative for the reaction, then energy has been released often in the form of heat. This type of reaction is referred to as an exothermic reaction (literally, outside heat, or throwing off heat). An exothermic reaction is more favourable and thus more likely to occur. An example reaction is combustion, known from everyday experience, since burning gas in air produces heat.

Endothermic reactions

combustion A reaction may have a positive ΔH. If a reaction has a positive ΔH, it consumes energy as the reaction moves towards completion. This type of reaction is called an endothermic reaction (literally, inside heat, or absorbing heat). The above rule, "Exothermic reactions are favourable", is usually true. However, there may be situations where exothermic reactions may not be favourable. This happens when the stability obtained due to loss of enthalpy is off set by a corresponding decrease in entropy (a measure of disorder). The exact rule is that a reaction is favourable when the Gibbs free energy of that reaction is negative where ΔG = ΔH − TΔS; ΔG being the change in Gibbs free energy, ΔH being the change in enthalpy, and ΔS is the change in entropy A reaction is called spontaneous if its thermodynamically favoured, by that meaning that it causes a net increase on entropy. Spontaneous reactions (in opposition to non-spontaneous reactions) do not need external perturbations (such as energy supplement) to happen. In a system at chemical equilibrium, it is expected to have larger concentrations of the substances formed by the spontaneous direction of the process. Thus, in a global isolated system (which it strictly isn't, see entropy), spontaneous reactions may be understood to occur without human interference. Most spontaneus reactions in this system are exothermic (such as rusting) or metamorphosis, thus increasing the global entropy, though photosynthesis is an important exeption (in a global system).

Chemical kinetics

See main article: Chemical kinetics. The rate of a chemical reaction is a measure of how the concentration of the involved substances changes with time. Analysis of reaction rates is important for several applications, such as in chemical engineering or in chemical equilibrium study. Rates of reaction depends basically on:
- Reactant concentrations, which usually make the reaction happen at a faster rate if raised,
- Surface Area, the amount of the substance being used,
- Pressure, By increasing the pressure, you squeeze the molecules together so you will increase the frequency of collisions between the molecules.
- Activation energy, which is defined as the amount of energy required to make the reaction start and carry on spontaneously. Higher activation energy implies that a reaction will be harder to start and, therefore, slower.
- Temperature, which hastens reactions if raised, because higher temperature means that the involved species will have more energy, thus making the reaction easier to happen,
- The presence or absence of a catalyst. Catalysts are substances which increases the speed of a reaction by lowering the activation energy needed for the reaction to take place. A catalyst is not destroyed or changed during a reaction, so it can be used again. Reaction rates are related to the concentrations of substances involved in reactions, as quantified by the law of mass action. Reactions whose rates are independent of reactant concentrations are called zero-order reactions.

External links

- [http://www.purchon.com/chemistry/rates.htm#surface Rate of reaction]

References

- IUPAC Gold Book [http://www.iupac.org/goldbook/C01033.pdf Definition]
- Category:Chemistry ko:화학 반응 ja:化学反応

Moiety

- In chemistry, a moiety is a specific segment of a molecule. For example, aniline has a phenyl and an amino moiety. Ethidium bromide also has both of these.
- In anthropology, moiety is a term used to describe each descent group in a culture which is divided exactly into two descent groups.
- In law, moiety means half of something, such as an inheritance.
- Julius Caesar uses the term moiety in William Shakespeare's Antony and Cleopatra after he learns of the death of Antony: : The breaking of so great a thing should make : A greater crack. The round world : Should have shook lions into civil streets : And citizens to their dens. The death of Antony : Is not a single doom; in the name lay : A moiety of the world. Caesar's message symbolizes the gravity of Antony's death, as it represented the triumph of Rome — and of Western civilization — over Cleopatra and Egypt.

Water

:This article focuses on water as it is experienced in everyday life. See water (molecule) for information on the chemical and physical properties of pure water (H₂O, hydrogen oxide). Water (from the Old English word wæter; c.f German "Wasser", from PIE
- wod-or, "water") is a tasteless, odorless, and nearly colorless (it has a slight hint of blue) substance in its pure form that is essential to all known forms of life and is known also as the most universal solvent. Water is an abundant substance on Earth. It exists in many places and forms. It appears mostly in the oceans and polar ice caps, but also as clouds, rain water, rivers, freshwater aquifers, and sea ice. On the planet, water is continuously moving through the cycle involving evaporation, precipitation, and runoff to the sea. Water fit for human consumption is called potable water. This natural resource is becoming more scarce in certain places as human population in those places increases, and its availability is a major social and economic concern.

Molecular properties

Forms of water

potable water] Water takes many different shapes on earth: water vapor and clouds in the sky, waves and icebergs in the sea, glaciers in the mountain, aquifers in the ground, to name but a few. Through evaporation, precipitation, and runoff, water is continuously flowing from one form to another, in what is called the water cycle. Because of the importance of precipitation to agriculture, and to mankind in general, different names are given to its various forms: while rain is common in most countries, other phenomena are quite surprising when seen for the first time. Hail, snow, fog or dew are examples. When appropriately lit, water drops in the air can refract sunlight to produce rainbows. Similarly, water runoffs have played major roles in human history as rivers and irrigation brought the water needed for agriculture. Rivers and seas offered opportunity for travel and commerce. Through erosion, runoffs played a major part in shaping the environment providing river valleys and deltas which provide rich soil and level ground for the establishment of population centers. Water also infiltrates the ground and goes into aquifers. This groundwater later flows back to the surface in springs, or more spectacularly in hot springs and geysers. Groundwater is also extracted artificially in wells. Because water can contain many different substances, it can taste or smell very differently. In fact, humans and other animals have developed their senses to be able to evaluate the drinkability of water: animals generally dislike the taste of salty sea water and the putrid swamps and favor the purer water of a mountain spring or aquifer.

Water in biology

From a biological standpoint, water has many distinct properties that are critical for the proliferation of life that set it apart from other substances. Water carries out this role by allowing organic compounds to react in ways that ultimately allows replication. It is a good solvent and has a high surface tension, and thus allows organic compounds and living things to be transported in it. Fresh water has its greatest density at 4°C, then becoming less dense as it freezes or heats up from this point. As a stable, polar molecule prevalent in the atmosphere, it plays an important atmospheric role as an absorber of infrared radiation, crucial in the atmospheric greenhouse effect without of which, the average surface temperature would be −18° Celsius. Water also has an unusually high specific heat, which plays many roles in regulating global and regional climate, such as the Gulf Stream climate, allowing life to survive. Water is a very good solvent, chemically not unlike ammonia, and dissolves many types of substances, such as various salts and sugar, and facilitates their chemical interaction, which aids complex metabolisms. Some substances, however, do not mix well with water, including oils and other hydrophobic substances. Cell membranes, composed of lipids and proteins, take advantage of this property to carefully control interactions between their contents and external chemicals. This is facilitated somewhat by the surface tension of water. Water drops are stable due to the high surface tension of water caused by the strong intermolecular forces called cohesive forces. This can be seen when small quantities of water are put onto a nonsoluble surface such as polythene: the water stays together as drops. On extremely clean glass the water may form a thin film because the molecular forces between glass and water molecules (adhesive forces) are stronger than the cohesive forces. This property plays a key role in plant transpiration. A simple but environmentally important and unique property of water is that its common solid form, ice, floats on the liquid. This solid phase is less dense than liquid water, due to the geometry of the strong hydrogen bonds which are formed only at lower temperatures. For almost all other substances and for all other 11 uncommon phases of water ice except ice-XI, the solid form is more dense than the liquid form. Fresh water is most dense at 4°C, and will sink by convection as it cools to that temperature, and if it becomes colder it will rise instead. This reversal will cause deep water to remain warmer than shallower freezing water, so that ice in a body of water will form first at the surface and progress downward, while the majority of the water underneath will hold a constant 4°C. This effectively insulates a lake floor from the cold. While this behavior may seem obvious, even intuitive, it should be noted that almost all other chemicals are denser as solids than they are as liquids, and freeze from the bottom up. Life on earth has evolved with and adapted itself to the important features of water. The existence of abundant liquid, vapor and solid forms of water on Earth has been an important factor in the abundant colonization of Earth's various environments by life-forms adapted to those varying and often extreme conditions. Civilizations have historically flourished around rivers and major waterways; Mesopotamia, the so-called cradle of civilization, is situated between two major rivers. Large metropolises like London, Paris, New York, and Tokyo owe their success in part to their easy accessibility via water and the resultant expansion of trade. Islands with safe water ports, like Singapore and Hong Kong, have flourished for precisely this reason. In places such as North Africa and the Middle East, where water is scarcer, access to clean drinking water was and is a major factor in human development.

Astronomical position of Earth and impact on its water

Mesopotamia The coexistence of the solid, liquid, and gaseous phases of water on Earth is vital to the origin, evolution, and continued existence of life on Earth. However, if the Earth's location in the solar system were even marginally closer or further from the Sun (ie, a million miles or so), the conditions which allow the three forms to be present simultaneously would be far less likely to exist. Earth's mass allows gravity to hold an atmosphere. Water vapor and carbon dioxide in the atmosphere provides a greenhouse effect which helps maintain a relatively steady surface temperature. If Earth were less massive, a thinner atmosphere would cause temperature extremes preventing the accumulation of water except in polar ice caps (as on Mars). According to the solar nebula model of the solar system's formation, Earth's mass may be largely due to its distance from the Sun. The distance between Earth and the Sun and the combination of solar radiation received and the greenhouse effect of the atmosphere ensures that its surface is neither too cold nor too hot for liquid water. If Earth were more distant, most water would be frozen. If Earth were nearer to the Sun, its higher surface temperature would limit the formation of ice caps, or cause water to exist only as vapor. In the former case, the low albedo of oceans would cause Earth to absorb more solar energy. In the second case, a runaway greenhouse effect and inhospitable conditions similar to Venus would result. It has been proposed that life itself may maintain the conditions that have allowed its continued existence. The surface temperature of Earth has been relatively constant through geologic time despite varying solar flux, indicating that a dynamic process governs Earth's temperature via a combination of greenhouse gases and surface or atmospheric albedo. This proposal is known as the Gaia hypothesis.

Human uses of water

Gaia hypothesis All known forms of life depend on water. Water is a vital part of many metabolic processes within the body. Significant quantities of water are used during the digestion of food. (Note however that some bacteria and plant seeds can enter a cryptobiotic state for an indefinite period when dehydrated, and come back to life when returned to a wet environment) About 72% of the fat free mass of the human body is made of water. To function properly the body requires between one and seven litres of water per day to avoid dehydration, the precise amount depending on the level of activity, temperature, humidity, and other factors. It is not clear how much water intake is needed by healthy people. However, for those who do not have kidney problems, it is rather difficult to drink too much water, but (especially in warm humid weather and while exercising) dangerous to drink too little. People do often drink far more water than necessary while exercising, however, putting them at risk of water intoxication, which is frequently fatal. The "fact" that a person should consume eight glasses of water per day cannot be traced back to a scientific source. However, leading dieticians and nutritionists will tell you that this is the RDI (Recommended Daily Intake) of water. [http://ajpregu.physiology.org/cgi/content/full/283/5/R993]. The latest dietary reference intake report by the National Research Council recommended 2.7 liters of water total (including food sources) for women and 3.7 liters for men[http://www.iom.edu/report.asp?id=18495]. Water is lost from the body in urine and feces, through sweating, and by exhalation of water vapor in the breath. Humans require water that does not contain too much salt or other impurities. Common impurities include chemicals and/or harmful bacteria, such as crypto sporidium. Some solutes are acceptable and even desirable for perceived taste enhancement and to provide needed electrolytes.

Water as a precious resource

:See water resources for information about fresh water supplies. fresh water Because of the growth of world population and other factors, the availability of drinking water per capita is shrinking. The issue of water shortage can be solved through more production, better distribution and less waste of it. For this reason, water is a strategic resource for many countries. Many battles and wars, such as the Six-Day War in the Middle East, have been fought to gain access to it. Experts predict more trouble ahead because of the world's growing population, increasing contamination through pollution, and global warming. UNESCO's World Water Development Report (WWDR, 2003) from its World Water Assessment Program indicates that, in the next 20 years, the quantity of water available to everyone is predicted to decrease by 30%. 40% of the world's inhabitants currently have insufficient fresh water for minimal hygiene. More than 2.2 million people died in 2000 from diseases related to the consumption of contaminated water or drought. In 2004, the UK charity WaterAid reported that a child dies every 15 seconds due to easily preventable water-related diseases. Some have predicted that clean water will become the "next oil", making Canada, with this resource in abundance, possibly the richest country in the world.

Regulating water distribution

Drinking water is often collected at springs or extracted from artificial borings in the ground, or wells. Building more wells in adequate places is thus a possible way to produce more water assuming the aquifers can supply an adequate flow. Other water sources are the rainwater and river or lake water. This surface water, however, must be purified for human consumption. This may involve removal of undissolved substances, dissolved substances and harmful microbes. Popular methods are filtering with sand which only removes undissolved material while chlorination and boiling kill harmful microbes. Distillation does all three functions. More advanced techniques exist, such as reverse osmosis. Desalination of abundant ocean or seawater is a more expensive solution used in coastal arid climates. The distribution of drinking water is done through municipal water systems or as bottled water. Governments in many countries have programs to distribute water to the needy at no charge. Others argue that the market mechanism and free enterprise are best to manage this rare resource, and to finance the boring of wells or the construction of dams and reservoirs. Reducing waste, that is using drinking water only for human consumption, is another option. In some cities, such as Hong Kong, sea water is extensively used for flushing toilets citywide in order to conserve fresh water resources. Polluting water may be the biggest single misuse of water; to the extent that a pollutant limits other uses of the water, it becomes a waste of the resource, regardless of benefits to the pollutor. Pharmaceuticals consumed by humans often end up in the waterways and can have detrimental effects on aquatic life if they bioaccumulate and if they are not biodegradable.

The impact of water on human culture

Water is considered a purifier in most religions, including Christianity, Islam, Judaism, and Shinto. For instance, baptism in Christian churches is done with water. In addition, a ritual bath in pure water is performed for the dead in many religions including Judaism and Islam. In Islam, the daily Salah can only be done after ablution (Wodoo), that is, washing parts of the body in clean water. In Shinto, water is used in almost all rituals to cleanse a person or an area. Water is often believed to have spiritual powers. In Celtic mythology, Sulis is the local goddess of thermal springs; in Hinduism, the Ganga is also personified as a goddess. Alternatively, gods can be patrons of particular springs, river or lakes: for example in Greek and Roman mythology, Peneus was a river god, one of the three thousand Oceanids. The Greek philosopher Empedocles held that water is one of the four classical elements along with fire, earth and air, and was regarded as the ylem, or basic stuff of the universe. Water was considered cold and moist. In the theory of the four bodily humours, water was associated with phlegm. Water was also one of the Five Elements in traditional Chinese philosophy, along with earth, fire, wood, and metal. A common misconception about water is that it is a powerful conductor of electricity. Any electrical properties observable in water are due to the ions of mineral salts and carbon dioxide dissolved in it. Water does self-ionize (two water molecules become one hydroxide anion and one hydronium cation), but only at a very slight, almost immeasurable level. Pure water can also be electrolized into oxygen and hydrogen gases but without any dissolved ions, this is a very slow process and thus very little current is conducted. Many bottled water companies exploit another common misconception, advertising both purity and taste, even though pure water is tasteless.

External links

- [http://www.lsbu.ac.uk/water/phase.html Phase diagrams of water]
- [http://www.publicforuminstitute.org/issues/oceans/index.htm Oceans and Water Issues Page]
- [http://www.greenfacts.org/water-disinfectants/index.htm Scientific Facts on Water disinfectants] A faithful summary by GreenFacts of a leading scientific consensus report on Drinking Water Disinfectants published by the International Programme on Chemical Safety of the WHO.
- [http://www.hkc22.com/residentialwater.html Residential water problems and markets] Study paper from Helmut Kaiser Consultancy
- [http://www.hkc22.com/watermarketsworldwide.html Water markets worldwide] Study paper from Helmut Kaiser Consultancy
- [http://www.worldwaterforum.org/ World Water Forum]
- [http://www.unesco.org/water/wwap/ World Water Assessment Program]
- [http://unesdoc.unesco.org/images/0012/001295/129556e.pdf United Nations' World Water Development Report]
- [http://www.gemswater.org/ United Nations GEMS/Water Programme]
- [http://www.lsbu.ac.uk/water/ Water Structure and Behaviour]
- [http://www.wateraid.org/ WaterAid]
- [http://www.sahra.arizona.edu/newswatch/ SAHRA—Global Water Newswatch]
- [http://www.siwi.org/ Stockholm International Water Institute] (SIWI)
- [http://www.c-win.org/ California Water Impact Network (C-WIN)]
- [http://news.bbc.co.uk/2/hi/science/nature/3752590.stm BBC: The water debate]
- [http://www.geocities.com/tapvsbottled/ Tap Water Vs Bottled Water] - Interesting site providing facts about tap and bottled water.
- [http://www.emagazine.com/september-october_2003/0903feat1.html E the Environmental Magazine piece on bottled water] (Oct 2003).
- [http://www.iapws.org/ International Association for the Properties of Water and Steam]
- [http://ga.water.usgs.gov/edu/watercycle.html US Geological Survey: Comprehensive discussion of the water cycle, in many languages]
- [http://www.dartmouth.edu/~etrnsfer/water.htm Why is water blue?]
- [http://www.water.org.uk/home/resources-and-links/water-for-health/ask-about/adults Water requirements in adults]
- [http://www.hkc22.com/environmentaltechnology.html/ Climate change raises markets for environmental technology, drinking water and clean energies]

References

- OA Jones, JN Lester and N Voulvoulis, Pharmaceuticals: a threat to drinking water? TRENDS in Biotechnology 23(4): 163, 2005
- Category:Beverages Category:Hydrology Category:Materials Category:Natural resources Category:Nutrition zh-min-nan:Chúi als:Wasser ko:물 ja:水 ms:Air simple:Water th:น้ำ

Nucleophilic acyl substitution

Nucleophilic acyl substitution is a type of substitution reaction between nucleophiles and acyl compounds. Acyl compounds are carboxylic acid derivatives such as esters, amides and acid halides. Nucleophiles are a diverse group of reactive intermediates such as alkoxide compounds and enolates.

reaction mechanism

enolate (L)]]The reaction of a nucleophile with a polar carbonyl group such as a ketone or an aldehyde results in nucleophilic addition with a tetrahedral alkoxide as primary reaction product. However, in acyl compounds the carbonyl group is bonded to a substituent that can act as a leaving group. Upon attack of the nucleophile to the carbonyl group, as before, a tetrahedral intermediate is formed with the nucleophile, the leaving group and the oxygen anion attached to the central carbon atom. The alkoxy group can now revert back to the carbonyl group and at the same time expell the leaving group. The leaving group has in effect taken over the position previously occupied by the nucleophile as a free anion. Acyl substitution is basically a two-step nucleophilic addition and elimination reaction. Both reaction steps are reversible reactions and the process can in principle revert. The relative strength of both nucleophilic species determines the reaction outcome but in practical reactions the leaving group is by far the poorest nucleophile.

Reactions

Many condensation reactions are nucleophilic acyl substitutions. Carboxylic acids react with chlorine donors such as thionyl chloride or phosphorous trichloride to acid chlorides, with alcohols to esters in esterfication and carboxylic acids selfcondense to acid anhydrides. With amines they form amides. Esters react with Grignard reagents in a nucleophilic acyl substitution followed by a nucleophilic addition to tertiary alcohols. Esters also react with Enolate nucleophiles. For example ethyl acetate reacts with acetone to acetylacetone. The Baker-Venkataraman rearrangement is a nucleophilic acyl substitution used in the synthesis of flavones. In the Weinreb ketone synthesis ketones are synthesized from carboxylic acid precursors.

External links

- Reaction of ethyl acetate with acetone in Organic Syntheses Coll. Vol. 3, p.16; Vol. 20, p.6 [http://www.orgsyn.org/orgsyn/prep.asp?prep=cv3p0016 Article]

References

# Organic Chemistry John McMurry 2nd Ed. ISBN 0534079687 category:organic reactions

Aldol condensation

An Aldol condensation is a organic reaction where a enolate ion reacts with a carbonyl compound to form a β-hydroxyaldehyde or β-hydroxyketone followed by dehydration to a conjugated enone. The first part of this reaction is a aldol reaction, the second part a elimination reaction. Dehydration may be accompanied by decarboxylation when a activated carboxyl group is present. The base used in this reaction is a strong base like t-butoxide,potassium hydroxide or sodium hydride.

Condensation types

It is important to distinguish the Aldol condensation from other addition reactions to carbonyl compounds.
- When the base is an amine and the active hydrogen compound is sufficiently activated the reaction is called a Knoevenagel condensation.
- In a Perkin reaction the aldehyde is aromatic and the enolate generated from an anhydride.
- A Claisen condensation involves two ester compounds.
- A Dieckmann condensation involves two ester groups in the same molecule and yields a cyclic molecule
- A Henry reaction involves an aldehyde and an aliphatic nitro compound.
- A Robinson annulation involves a α,β-unsaturated ketone and a carbonyl group who first engage in a Michael reaction prior to the Aldol condensation

Examples

Ethyl 2-methylacetoacetate and campholenic aldehyde react in an Aldol condensation. The synthetic procedure is typical for this type of reactions. Michael reaction Ethyl 2-methylacetoacetate (2) (727 mg, 4.53 mmol) is added to a stirred solution of sodium hydride (184 mg, 4.60 mmol) in dioxane (25 mL). Then campholenic aldehyde (1) (707 mg, 3.72 mmol) is added and the mixture refluxed for 15 h. Then 2N hydrochloric acid (15 mL) is added and the mixture extracted with diethyl ether (3x15 mL). The combined organic layers are washed with 2N hydrochloric acid (2x15 mL), saturated sodium bicarbonate (2x15 mL) and brine (3x15 mL). The organic phase is dried over anhydrous sodium sulfate and the solvent evaporated under reduced pressure to yield a residue (900 mg) which was purified by vacuum distillation to give 3 (505 mg, 2.63 mmol, 58%). ¹ Ethyl glyoxylate and diethyl-methylglutaconate react to isoprenetricarboxylic acid with sodium ethoxide. This reaction product is very unstable with initial loss of carbon dioxide and followed by many secondary reactions. This is believed to be due to steric strain resulting from the methyl group and the carboxylic group in the cis-dienoid structure ². 2

References

- ¹ (E)-6-(2,2,3-Trimethyl-cyclopent-3-enyl)-hex-4-en-3-one Concepcion Bada, Juan M. Castro, Pablo J. Linares-Palomino, Sofia Salido, Joaquan Altarejos Manuel Nogueras, Adolfo Sanchez, Molbank 2004, M388 [http://www.mdpi.net/molbank/molbank2004/m0388.htm Online Publication]
- ² 2-Methyl-(1Z,3E)-butadiene-1,3,4-tricarboxylic Acid, "Isoprenetricarboxylic Acid" Mayer B. Goren, Edward A. Sokoloski, and Henry M. Fales J. Org. Chem., 70 (18), 7429 -7431, 2005 [http://pubs.acs.org/cgi-bin/abstract.cgi/joceah/2005/70/i18/abs/jo0507892.html Abstract] category:Organic reactions

Reaction mechanism

In chemistry, a reaction mechanism is the step by step sequence of reactions by which overall chemical change occurs. Although only the net chemical change is directly observable for most chemical reactions, experiments can often be designed that suggest the possible sequence of steps in a reaction mechanism. An overall description of how a reaction occurs. A mechanism describes in detail exactly what takes place at each stage of a chemical transformation. It describes the transition state and which bonds are broken and in what order, which bonds are formed and in what order, and what the relative rates of the steps are. A complete mechanism must also account for all reactants used, the function of a catalyst, stereochemistry, all products formed and the amount each. A reaction mechanism must also account for the order in which molecules react. Often what appears to be a single step conversion is in fact a multistep reaction. Consider the following example: CO + NO₂ → CO₂ + NO In this reaction, it has been experimentally determined that this reaction takes place according to the rate law

R = k[NO_2]^2

. Therefore, a possible mechanism by which this reaction takes place is 2 NO₂ → NO₃ + NO (slow)
NO₃ + CO → NO₂ + CO₂ (fast)
Each step is called an elementary step, and each has it’s own rate law and molecularity. All of the elementary steps must add up to the original reaction. When determining the overall rate law for a reaction, the slow step is the step that determines the reaction rate. Because the first step is the slow step, it is the rate-determining step. Because it involves the collision of 2 NO₂ molecules, it is a bimolecular reaction with a rate law of

R = k[NO_2]^2

. If one were to cancel out all the molecules that appear on both sides of the reaction, you would be left with the original reaction. In organic chemistry one of the first reaction mechanisms proposed was that for the benzoin condensation in 1903 by A. J. Lapworth. Mechanism

Radical

Radical is derived from the Latin word radix, which means "root". In various fields of endeavor, it can mean: ;Sciences
- in chemistry, either an atom or molecule with at least one unpaired electron, or a group of atoms, charged or uncharged, that act as a single entity in reaction. These two definitions are not functionally identical. (see radical (chemistry)).
- in mathematics:
- the n-th radical or root of a number a, written as

\sqrt[n]

, which is a number whose n-th power is a (see radical (mathematics)).
- the radical of an algebraic group is a concept in algebraic group theory.
- the radical of an ideal is an important concept in abstract algebra.
- in linguistics, a radical consonant involves the root of the tongue.
- in grammatology, it is part of a Chinese character (see radical (Chinese character)). ;Social sciences
- in sociology:
- one who advocates thoroughgoing analysis or change "at the root"
- in politics:
- can refer to a supporter of a revolutionary social movement
- can refer to the Radicalism (historical) movement which developed in 19th century Britain with the primary aim of electoral reform
- can refer to Radicalism (historical) left wing liberal movements in continental Europe
- can refer to a counterpart to reactionary; see Radical Republican
- can refer to member of a Radical Party
- can refer to a progressive liberal, e.g. the Radicals (UK), a group of left-wing MPs in the 19th-century British Parliament

Carother's equation

In step-growth polymerization, Carother's equation gives the number-average degree of polymerization, X_n, for a given fractional monomer conversion, p. :

\bar_n=\frac

Notes: :
- X_n is also the average chain length (in monomer units) :
- p = (N₀-N)/N₀, where: ::N₀ is the number of molecules present initially ::N is the number of unreacted molecules at time t ::p is also a measure of the extent of reaction, or yield A high monomer conversion is required to achieve a high number-average degree of polymerization. For example, a monomer conversion, p, of 98% is required for X_n = 50, and p = 99% is required for X_n = 100.

Related equations

Related to the Carother's equation are the following equations: :

\begin
\bar_w & = & \frac \\
\bar_n & = & M_o\frac \\
\bar_w & = & M_o\frac\\
PDI & = & \frac=1+p\\
\end

where: :
- X_w is the weight average degree of polymerization, :
- M_n is the number average molecular weight, :
- M_w is the weight average molecular weight, :
- M_o is the molecular weight of the repeating monomer unit, :
- PDI is the polydispersity index The last equation shows that the minimum value of the PDI is 2, which occurs at a monomer conversion of 100%. In practise the average length of the polymer chain is limited by such things as the purity of the reactants, the absence of any side reactions (i.e. high yield), and the viscosity of the medium. category:polymer chemistry

Polymer

Polymer is a generic term used to describe a very long molecule consisting of structural units and repeating units connected by covalent chemical bonds. The key feature that distinguishes polymers from other molecules is the repetition of many identical, similar, or complementary molecular subunits in these chains. These subunits, the monomers, are small molecules of low to moderate molecular weight, and are linked to each other during a chemical reaction called polymerization. Instead of being identical, similar monomers can have varying chemical substituents. The differences between monomers can affect properties such as solubility, flexibility, and strength. In proteins, these differences give the polymer the ability to adopt a biologically-active conformation in preference to others. (See self-assembly.) Identical monomers with nonreactive side groups result in a polymer chain that will tend to adopt a random coil conformation, as described by an ideal chain mathematical model. Although most polymers are organic, with carbon-based monomers, there are also inorganic polymers; for example, the silicones, with a backbone of alternating silicon and oxygen atoms. Polymers are typically classified according to three main groups:
- thermoplastics (linear or branched chains)
- thermosets (crosslinked chains)
- elastomers The term polymer covers a large, diverse group of molecules, including substances from proteins to stiff, high-strength Kevlar fibres. For example, the formation of polyethene (also called polyethylene) involves thousands of ethene molecules bonded together to form a straight (or branched) chain of repeating -CH₂-CH₂- units (with a -CH₃ at each terminal): image:example_polymerization.png Polymers are often named in terms of the monomer from which they are made. Because it is synthesized from ethene in a process during which all the double bonds in the vinyl monomers are lost, polyethene has the unsaturated structure: image:polyethene_monomer.png If it were named according to its final structure, it would have the alkane designation "polyethane". Because synthetic polymer formation is governed by random assembly from the constituent monomers, polymer chains within a solution or substance are generally not of equal length. This is unlike basic, smaller molecules in which every atom is stoichiometrically accounted for, and each molecule has a set molecular mass. An ensemble of differing chain lengths, often obeying a normal (Gaussian) distribution, occurs because polymer chains terminate during polymerization after random amounts of chain lengthening (propagation). Proteins are polymers of amino acids. Typically, hundreds of the (nominally) twenty different amino acid monomers make up a protein chain, and the sequence of monomers determines its shape and biological function. (There are also shorter oligopeptides which function as hormones.) But there are active regions, surrounded by, as is believed now (Aug 2003), structural regions, whose sole role is to expose the active regions. (There may be more than one on a given protein.) So the exact sequence of amino acids in certain parts of the chains can vary from species to species, and even given mutations within a species, so long as the active sites are properly accessible. Also, whereas the formation of polyethylene occurs spontaneously under the right conditions, the synthesis of biopolymers such as proteins and nucleic acids requires the help of enzyme catalysts, substances that facilitate and accelerate reactions. Unlike synthetic polymers, these biopolymers have exact sequences and lengths. (This does not include the carbohydrates.) Since the 1950s, catalysts have also revolutionised the development of synthetic polymers. By allowing more careful control over polymerization reactions, polymers with new properties, such as the ability to emit coloured light, have been manufactured. The characterization of a polymer requires several parameters which need to be specified. This is because a polymer actually consists of a statistical distribution of chains of varying lengths, and each chain consists of monomer residues which affect its properties. Some of these parameters are described below.

Physical properties of polymers

Physical properties of polymers include the degree of polymerization, molar mass distribution, crystallinity, as well as the thermal phase transitions:
- T_g, glass transition temperature
- T_m, melting point (for thermoplastics).

Branching

During the propagation of polymer chains, branching can occur. In free-radical polymerization, this occurs when a chain curls back and bonds to an earlier part of the chain. When this curl breaks, it leaves small chains sprouting from the main carbon backbone. Branched carbon chains cannot line up as close to each other as unbranched chains can. This causes less contact between atoms of different chains, and fewer opportunities for induced or permanent dipoles to occur. A low density results from the chains being further apart. Lower melting points and tensile strengths are evident, because the intermolecular bonds are weaker and require less energy to break. Besides branching, polymers can have other topologies: linear, network (cross-linked 3D structure), IPN (integrated polymer network), comb, or star as well as dendrimer and hyperbranched structures.

Stereoregularity

Stereoregularity or tacticity describes the isomeric arrangement of functional groups on the backbone of carbon chains. Isotactic chains are defined as having substituent groups aligned in one direction. This enables them to line up close to each other, creating crystalline areas and resulting in highly rigid polymers. In contrast, atactic chains have randomly aligned substituent groups. The chains do not fit together well and the intermolecular forces are low. This leads to a low density and tensile strength, but a high degree of flexibility. Syndiotactic substituent groups alternate regularly in opposite directions. Because of this regularity, syndiotactic chains can position themselves close to each other, though not as close as isotactic polymers. Syndiotactic polymers have better impact strength than isotactic polymers because of the higher flexibility resulting from their weaker intermolecular forces.

Constitution of polymers

Copolymers

Copolymerization with two or more different monomers results in chains with varied properties. There are twenty amino acid monomers whose sequence results in different shapes and functions of protein chains. Copolymerising ethene with small amounts of 1-hexene (or 4-methyl-1-pentene) is one way to form linear low-density polyethene (LLDPE). (See polyethylene.) The C₄ branches resulting from the hexene lower the density and prevent large crystalline regions from forming within the polymer, as they do in HDPE. This means that LLDPE can withstand strong tearing forces whilst remaining flexible. A block copolymer is formed when the reaction is carried out in a stepwise manner, leading to a structure with long sequences or blocks of one monomer alternating with long sequences of the other. There are also graft copolymers, in which entire chains of one kind (e.g., polystyrene) are made to grow out of the sides of chains of another kind (e.g., polybutadiene), resulting in a product that is less brittle and more impact-resistant. Thus, block and graft copolymers can combine the useful properties of both constituents and often behave as quasi-two-phase systems. The following is an example of step-growth polymerization, or condensation polymerization, in which a molecule of water is given off and nylon is formed. The properties of the nylon are determined by the R and R' groups in the monomers used. nylon The first commercially successful, completely synthetic polymer was nylon 6,6, with alkane chains R = 4C (adipic acid) and R' = 6C (hexamethylene diamine). Including the two carboxyl carbons, each monomer donates 6 carbons; hence the name. In naming nylons, the number of carbons from the diamine is given first and the number from the diacid second. Kevlar is an aromatic nylon in which both R and R' are benzene rings. Copolymers illustrate the point that the repeating unit in a polymer, such as a nylon, polyester or polyurethane, is often made up of two (or more) monomers.

Chemical properties of polymers

Intermolecular forces

The attractive forces between polymer chains play a large part in determining a polymer's properties. Because polymer chains are so long, these interchain forces are amplified far beyond the attractions between conventional molecules. Also, longer chains are more amorphous (randomly oriented). Polymers can be visualised as tangled spaghetti chains - pulling any one spaghetti strand out is a lot harder the more tangled the chains are. These stronger forces typically result in high tensile strength and melting points. The intermolecular forces in polymers are determined by dipoles in the monomer units. Polymers containing amide groups can form hydrogen bonds between adjacent chains; the positive hydrogen atoms in N-H groups of one chain are strongly attracted to the oxygen atoms in C=O groups on another. These strong hydrogen bonds result in, for example, the high tensile strength and melting point of kevlar. Polyesters have dipole-dipole bonding between the oxygen atoms in C=O groups and the hydrogen atoms in H-C groups. Dipole bonding is not as strong as hydrogen bonding, so ethene's melting point and strength are lower than kevlar's, but polyesters have greater flexibility. Ethene, however, has no permanent dipole. The attractive forces between polyethene chains arise from weak van der Waals forces. Molecules can be thought of as being surrounded by a cloud of negative electrons. As two polymer chains approach, their electron clouds repel one another. This has the effect of lowering the electron density on one side of a polymer chain, creating a slight positive dipole on this side. This charge is enough to actually attract the second polymer chain. Van der Waals forces are quite weak, however, so polyethene melts at low temperatures.

Polymer characterization

A variety of lab techniques are used to determine the properties of polymers. Techniques such as wide angle X-ray scattering, small angle X-ray scattering, and small angle neutron scattering are used to determine the crystalline structure of polymers. Gel permeation chromatography is used to determine the number average molecular weight, weight average molecular weight, and polydispersity. FTIR is used to determine composition. Thermal properties such as the glass transition temperature and melting point can be determined by differential scanning calorimetry and dynamic mechanical analysis. Pyrolysis followed by analysis of the fragments is one more technique for determining the possible structure of the polymer. Polymer known as polymer substrate is used for everyday banknotes in Australia and New Zealand, and is also used in commemorative notes in other countries. See also: Polymerization -- Biopolymer -- Condensation polymer -- Addition polymer -- Synthetic polymer -- Glass transition temperature -- Polymer physics -- Important publications in polymer chemistry

External links

- [http://www.borealisgroup.com/public/dictionary/Dictionary.jsp Polymer dictionary]
- [http://www.vivamer.com/ Responsive Biopolymers for Drug Delivery and Imaging]
- [http://web.umr.edu/~wlf/ Polymer Chemistry Hypertext, Educational resource]
- [http://www.polychemistry.com/ Polymer Chemistry Innovations]
- [http://www.odcad.com/html/organicdevice_appearance1.HTM Materials for Organic devices]
- [http://www.pslc.ws/macrog/index.htm The Macrogalleria - a cyberwonderland of polymer fun!] Category:Polymers Category:Polymer chemistry ko:중합체 ms:Polimer ja:重合体 th:โพลีเมอร์

Polyester

For the film, see the article Polyester (film) Polyester (film) Polyester (film) with a seven-lobed cross section]] Polyester is a category of polymers, or, more specifically condensation polymers, which contain the ester functional group in their main chain. Although polyesters do exist in nature, polyester generally refers to the large family of synthetic polyesters (plastics) which includes polycarbonate and above all polyethylene terephthalate (PET). PET is one of the most important thermoplastic polyesters. The first synthetic polyester, glycerine phthalate, was used in the First World War for waterproofing. Natural polyesters have been known since around 1830.
Applications

- Fibers (and microfibers) for fabric
- Bottles
- Films such as Mylar, often aluminized
- Photographic film (after cellulose triacetate, polyester is the most important substrate)
- A common matrix for glass-reinforced plastic (commonly called "fiberglass") and other composite materials.
- Liquid crystal displays
- Holograms
- Filters Liquid crystalline polyesters are among the first industrially used liquid crystalline polymers. In general they have extremely good mechanical properties and are extremely heat resistant. For that reason, they can be used as an abradable seal in jet engines. Thermosetting polyester resins are commonly used as casting materials, fiberglass laminating resins, and non-metallic auto-body fillers. In such applications, polymerization and cross-linking are initiated through an exothermic reaction involving an organic peroxide, such as methyl ethyl ketone peroxide or benzoyl peroxide.
Synthesis
Synthesis of polyesters is generally achieved by a polycondensation reaction.
Azeotrope esterification
In this classical method an alcohol and a carboxylic acid react to form a carboxylic ester. To assemble a polymer, the water formed by the reaction must be continually removed by azeotrope distillation.
Alcoholic transesterification
See main article on transesterification.
Acylation (HCl method)
The acid begins as an acid chloride, and thus the polycondensation proceeds with emission of hydrochloric acid (HCl) instead of water. This method can be carried out in solution or as an enamel. :Silyl method :In this variant of the HCl method, the carboxylic acid chloride is converted with the trimethyl silyl ether of the alcohol component; trimethyl silyl chloride is produced.
Acetate method (esterification)
:Silyl acetate method
Ring-opening polymerization
Aliphatic polyesters can be assembled from lactones under very mild conditions, catalyzed anionically, cationically or metallorganically.
Common usage and culture
When the word polyester is used by the layman, it is usually in reference into the fiber; this is the most common general usage of the term. Polyester clothing is considered to have a "less natural" feeling to it in comparison to natural fibers. Quite frequently, polyester fibers are spun together with fibers of cotton, producing a cloth with some of the better properties of each. Category:Esters Category:Plastics Category:Fibers Category:Organic polymers Category:Synthetic resins ja:ポリエステル

Condensation polymer

Condensation polymers are any class of polymers formed through a condensation reaction, as opposed to addition polymers which involve the reaction of unsaturated monomers. Types of condensation polymers include polyamides, polyacetals and polyesters. Condensation polymerization is a process by which two molecules join together, with the loss of a small molecule which is often water. The type of end product resulting from a condensation polymerization is dependent on the number of functional end groups of the monomer which can react. Monomers with only one reactive group, give end products with a low molecular weight. Linear polymers are created using monomers with two reactive end groups and monomers with more than two end groups give three dimensional polymers which are cross linked, such as polysiloxanes. This often involves linking monomers with an -OH (hydroxyl) group and a freely ionized -H on either end (such as a hydrogen from the -NH₂ in nylon or proteins). Normally, two or more different monomers are used in the reaction. The bonds between the hydroxyl group, the hydrogen atom and their respective atoms break forming water from the hydroxyl and hydrogen, and the polymer. Polyester is created through ester linkages between monomers, which involve the functional groups carboxyl and hydroxyl (an organic acid and an alcohol monomer). Nylon is a common condensation polymer. It can be manufactured by reacting di-amines with carboxyl derivatives. In this example the derivative is a di-carboxylic acid, but di-acyl chlorides are also used. Another approach used is the reaction of di-functional monomers, with one amine and one carboxylic acid group on the same molecule: image:con_polymer.png The carboxylic acids and amines link to form peptide bonds, also known as amide groups. Proteins are condensation polymers made from amino acid monomers. Carbohydrates are also condensation polymers made from sugar monomers such as glucose and galactose. Condensation polymerization is occasionally used to form simple hydrocarbons. This method, however, is expensive and ineffective, so the addition polymer of ethene (polyethylene) is generally used. Condensation Polymers, unlike Addition polymers are bio-degradable. The peptide or ester bonds between monomers can be hydrolysed by acid catalysts or bacterial enzymes breaking the polymer chain into smaller pieces. The most commonly known condensation polymers are proteins, fabrics such as nylon, silk, or polyester. :See also: Biopolymer, Polyester, Polyamide Category:Polymers

Silicate

In chemistry, a silicate is a compound consisting of silicon and oxygen (Si_xO_y), one or more metals, and possibly hydrogen. It is also used to denote the salts of silica or of one of the silicic acids. In common conditions, the most stable form is silicon dioxide, SiO₂, often called quartz, and similar species. This always has, in equilibrium, a minute amount of silicic acid, H₄SiO₄. Chemists consider quartz as insoluble, but it moves around at longer timescales. Also, in basic conditions, we find H₂SiO₄^2-. Silicate minerals are noted for their tetrahedral form. Sometimes the tetrahedra are joined in chains, double chains, sheets, and three-dimensional frameworks. They are subclassified into groups based on the degree of polymerization of the tetrahedra, such as nesosilicates, cyclosilicates, and so forth. In geology and astronomy, the term silicate is used to denote a type of rock that consists of silicon and oxygen (usually as SiO₂ or SiO₄), one or more metals, and possibly hydrogen. Such rocks range from granite to gabbro. Most of the Earth's crust is made up of silicate rocks, as are the crusts of other terrestrial planets. Mineralogically, silicate minerals are divided according to their molecular structure into the following groups:
- Olivine (single tetrahedron) - Nesosilicates
- Epidote (double tetrahedra) - Sorosilicates
- Tourmaline (rings of tetrahedra) - Cyclosilicates
- Pyroxene (single chain) - Inosilicates
- Amphibole (double chain) - Inosilicates
- Micas and clays (sheet) - Phyllosilicates
- Feldspars (framework) - Tectosilicates
- Quartz (SiO₂ framework) Silicate was also the name given to the bone-sucking monsters in the British horror movie Island of Terror (aka Night of the Silicates). These were silicon-based organisms created by cancer research gone wrong, which consumed the calcium phosphate in the bones of carbon-based lifeforms.

Anhydride

In chemistry, an anhydride is typically an oxide of a nonmetallic element or an organic radical, capable of forming an acid by uniting with the elements of water. The anhydride is so called because it may be formed from an acid by the removal of water. Examples of inorganic anhydrides include dinitrogen pentoxide, which is the anhydride of nitric acid, and sulfur trioxide, which is the anhydride of sulfuric acid. Useful organic anhydrides include acetic anhydride (an acid anhydride), formed by the condensation of acetic acid: : 2 CH₃COOH → (CH₃CO)₂O + H₂O Anhydrides are typically more reactive than their corresponding acids, as they can react with water to form their corresponding acid. They often are good dehydrating agents. Acetic anhydride is useful in the acetylation of salicylic acid, as using acetic acid to do the reaction leaves water behind that can destroy the product, acetylsalicylic acid, or aspirin. In biology, most of the high energy phosphate compounds are formed from the condensation of the phosphate ion with a phosphorylated sugar. The resulting pyrophosphate bond is a classic anhydride bond.

Aldol condensation

Condensation types

Examples

References

Self-condensation

Self-condensation is a organic reaction where a chemical compound containing a carbonyl group acts both as the electrophile and the nucleophile in a aldol condensation. it is also called a symmetrical aldol condensation as opposed to a mixed aldol condensation where electrophile and nucleophile are different species. 2 CH₃COCH₃ → (CH₃)₂C=CH(CO)CH₃ + H₂O For instance 2 molecules of acetone condense to a single compound mesityl oxide with an ion exchange resin ^[1].

References

- ^[1] Notes - Ketone Condensations Using Sulfonic Acid Ion Exchange Resin N. Lorette; J. Org. Chem.; 1957; 22(3); 346-347. category:organic reactions

Acyloin condensation

Acyloin condensation is a reductive coupling of two carboxylic esters using metallic sodium to yield an α-hydroxyketone, also known as an acyloin. sodium The reaction is most successful when R is aliphatic and inert. To achieve the condensation, chemists may employ solvents with a high boiling point, such as benzene and toluene.

References

- Bouveault, L., and R. Loquin. Comptes. Rendus. 1905, 140, 1593.
- Finley, K. T. Chem. Rev. 1964, 64, 573.

Benzoin condensation

The Benzoin condensation is a condensation reaction between two aromatic aldehydes, especially benzaldehyde that is catalyzed by a cyanide. The reaction product is an aromatic α-hydroxyketone with benzoin as the parent compound. The reaction mechanism for this organic reaction was already proposed in 1903 by A. J. Lapworth. center In the first step in this reaction the cyanide ion (as sodium cyanide) reacts with the aldehyde in a nucleophilic addition. Umpolung reverses the polarity of the carbonyl group and the rearranged intermediate adds to the second carbonyl group in a second nucleophilic addition. Proton transfer and elimination of the cyanide ion affords the benzoin. This is a reversible reaction. The cyanide ion is a very specific catalyst and serves three different purposes in the course of the reaction. It acts as a nucleophile, it facilitates proton abstraction in the umpolung by its inductive effect and it is also the leaving group in the final step. The benzoin condensation is in effect a dimerization and not a condensation because a small molecule like water is not released in this reaction. For this reason the reaction is also called a benzoin addition. Both aldehydes have a different purpose. One aldehyde donates a proton and one aldehyde accepts a proton. 4-Dimethylaminobenzaldehyde is an efficient proton donor while benzaldehyde is both a proton and a donor. In this way it is possible to synthesise asymmetric benzoins. The reaction can be extended to aliphatic aldehydes with base catalysis in the presence of thiazolium salts. The reaction mechanism is essencially the same. The corresponding product is called a acyloine. These compounds are important in the synthesis of heterocyclic compounds. The addition is also possible with α-β-unsaturated ketones for instance methylvinylketon in the Stetter reaction. In biochemistry, the coenzyme Thiamine is responsible for biosynthesis of acyloine-like compounds. This coenzyme also contains a thiazolium moiety.

References

- Main text & image German Wiki original
- CXXII.—Reactions involving the addition of hydrogen cyanide to carbon compounds. Part II. Cyanohydrins regarded as complex acids Arthur Lapworth, Journal of the Chemical Society, Transactions, 1904, 85, 1206 - 1214 [http://www.rsc.org/publishing/journals/article.asp?doi=CT9048501206 Abstract]
- Benzoin Roger Adams and C. S. Marvel Organic Syntheses, Coll. Vol. 1, p.94; Vol. 1, p.33 [http://www.orgsyn.org/orgsyn/prep.asp?prep=cv1p0094 Article] Category:Organic reactions

Infantry Tank

The Infantry tank was a concept developed by the British in the years leading up to World War II. They followed from the principle of separating tank functions into two areas - the Infantry tanks that would support the infantry units in making a breakthrough in the enemy lines of defence and Cruiser tanks which would exploit the gaps moving through into the enemy rear cutting lines of supply. Since the Infantry tanks were to work at the pace of the infantry units which would be attacking on foot, high speed was not a requirement and they were able to carry heavier armour. The first two Infantry tanks, the Mark I "Matilda" and Mark II "Matilda" were armed with a machine gun and 2 pounder anti-tank gun respectively. They were followed in by the Valentine and Churchill designs. In practice, although able to resist hits from tanks and anti-tank guns, and designed for good, albeit slow, cross country performance, the separation of tank functions into specialised areas such as infantry and cruiser types was not effective, and although the Churchill was successful in its area the Infantry tank idea faded as tank design progressed during the war. The concept was also employed by the