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Esterification

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Esterification will be considered in the broadest sense as including all processes by which esters are produced. The importance of esters has increased enormously in recent years. Esters made from petroleum base stock are made using a fundamental process involving esterification, filtration and distillation.

Esterification involves a reaction between a monohydric alcohol and the oil in the presence of a catalyst. The triacylglycerols in the oil are transformed into fatty acid esters and glycerol. Normally methanol is the alcohol used in this type of reaction. The catalyst promoting the reaction may be acid or alkali.

What is Ester?

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Most natural oils whether they are petroleum or vegetable based, contain impurities and waxes and thus their properties are not uniform. Synthetic products on the other hand are made when distinct and purer components of the oils are synthesized so that their properties are more uniform than in their natural forms. Esters are primarily defined as natural or synthetic.
Natural esters are derived from vegetable oils or animal fats. An ester is formed when an alcohol is combined with an organic acid, with water as a by-product.

Ester

Synthetic esters are made using acids and alcohols and due to their purity offer thermal stability and cold temperature =performance far superior to their base materials. The chemistry of vegetable oils is generally similar in that they are triglycerides or esters derived from glycerin and a range of fatty acids from carbon shows a triglyceride and an adipate.

Definition

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The reaction of an acid with an alcohol to give an ester is known as esterification. The reaction is reversible, in the reverse state it is known as hydrolysis (when catalyzed by acid) or saponification (when catalyzed by base).

R-COOH + R'-OH $\rightleftharpoons$ RCOOR' + H2O

The exact point of equilibrium of the reaction depends upon the structure of the alcohol and acid. However, the reaction may be caused to go in either of the two directions by the use of a large excess of one of the reactants, in the presence of large excess of alcohol the reaction mainly proceeds in the right direction whereas in the presence of a large excess of water, hydrolysis is the main reaction.

Esterification Reaction

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A very important reaction occurs when carboxylic acids are heated with alcohols or phenols in the presence of an acid catalyst. A molecule of water splits out and a carboxylic ester is formed.

The general reaction is shown below.

Esterification Reaction

As before, R' indicates that the two R groups can be the same or different. The process of ester formation is called esterification, and the carbonyl carbon-oxygen single bond of the ester group is called the ester linkage.

Ester Linkage

The ester functional group is a key structural feature in fats, oils and other lipids. Also widely found in fruits and flowers, many esters are very fragrant and represent some of natures most pleasant odors. Because of this characteristic esters are commonly used as flavoring agents in foods and as scents in personal products.

Specific example is given below.

Esterification Example

Mechanism

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Acid catalyzed esterification mechanism is shown below. Esterification involves a reaction between a monohydric alcohol and the oil in the presence of a catalyst. The triacylglycerols in the oil are transformed into fatty acid esters and glycerol. Normally methanol is the alcohol used in this type of reaction. The catalyst promoting the reaction may be acid or alkali,

Mechanism of Esterification

Esterification of optically active 2-octanol with the acetic acid results in complete preservation of optical activity showing that the acyl fission has taken place. Protonation of the carbonyl oxygen activates the carbonyl group toward the nucleophilic addition.

Esterification Mechanism

All the steps in esterification reaction are reversible. To force equilibrium towards the ester usually requires an excess of the alcohol together with a dehydration agent, which removes the water from the reaction solution as soon as it is formed. Alternatively the water can be removed from the reaction solution by distillation.
The reactivity of ester which is greater than that of the free acid, makes it suitable for many synthesis. For example, sarcosine, chloroacetamide, thioglycolic acid ester for pharmaceuticals (vitamin A) and crop protection agents (dimethoate). Condensation with aldehydes and ketones gives glycide esters. Other uses are the synthesis of heterocyclic compounds, for example, 2-phenylimino-4-oxooxazolidine from the ester and phenyl urea, and the well known condensation of chloroacetic acid and its esters with thioureas to form pseudothiohydantoins.

Esters are used in perfumes, solvents and flavorings. The alcohol part of the ester is commonly used as the solvent for alcohols with low boiling points. Formation of an ethyl ester would use ethanol as a solvent and the formation of a methyl ester would use methanol. If an ester is formed from an alcohol with a high boiling point a solvent such as benzene or toluene is often used and an excess of the alcohol is then added to that solvent.

Carboxylic Acid Plus Alcohol

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Alcohols react with carboxylic acids to form esters and water, the reactions are called esterification. Esterification reactions are reversible. They are therefore carried out in the presence of a dehydrating agent, such as concentrated sulphuric acid, which removes water formed in the reaction allowing it to proceed in the forward direction.

RCOOH + HO-R' $\xrightarrow[conc H_{2}SO_{4}]{\Delta}$ RCOOR' + H2O

Specific Example

CH3COOH + HOC2H5 $\xrightarrow[conc H_{2}SO_{4}]{\Delta}$ CH3COOC2H5 + H2O

The order of reactivity of alcohols with a given carboxylic acid varies as