Esters, Nucleophilic Substitution on the Unsaturated Carbon Atom

2 Esters, Nucleophilic Substitution on the Unsaturated Carbon Atom 

An alcohol is a good nucleophile because it has two electron lone pairs on the oxygen atom. In the reaction with a carboxylic acid, the oxygen atom of alcohol attacks the carbon atom of the carboxylic acid group. The mechanism is analogous to the previously discussed nucleophilic additions of aldehydes and ketones. The main difference between the two mechanisms is that the reaction involving carboxylic acid also involves removing a molecule of water. The final product is an ester. 

The mechanism of this reaction which is called esterification is a nucleophilic substitution on the unsaturated carbon (the carbon atom is unsaturated because it is bound with the double bond). In summary, esters are formed by the reaction of a carboxylic acid and an alcohol with the elimination of a water molecule. However, removing the water molecule is only possible in an acidic medium, so that in the preparation of esters a few drops of strong acid must be added (see the scheme 

The nomenclature of esters is relatively simple, because it is similar to the nomenclature of salts. For instance, the ester obtained from methanol and ethanoic acid (acetic acid) is called methyl ethanoate (methyl acetate). Esters are compounds that comprise a large number of structures depending on what the alcohol and acid components are. Esters with a small molecular mass are volatile substances, in most cases with a pleasant odor. For instance, butyl acetate is responsible for the odor of apples. In nature, some esters serve as pheromones for insects, for example isoamyl acetate which attracts butterflies. Other kinds of esters can be large molecules as for instanee waxes and fats where both the alcohol and the acid components can be long-ehained or eomplicated structures. These natural esters will be diseussed in the ehapter on lipids. In industry, esters are used in the fabrication of polymeric fibers for textile materials and plastic materials for various uses. The most common material is polyethylene terephthalate (PET), the ester prepared from terephthalic acid and ethylene glycol. 

One of the most typical reactions of esters is the saponification or hydrolysis of esters by strong bases such as NaOH or KOH. Boiling ester with a water solution of KOH or NaOH yields the salt of a carboxylic acid and the corresponding alcohol. 

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