Esterification

Esterification is the chemical process for making esters, which are compounds of the chemical structure R-COOR', where R and R' are either alkyl or aryl groups. The most common method for preparing esters is to heat a carboxylic acid, R-CO-OH, with an alcohol, R'-OH, while removing the water that is formed. A mineral acid catalyst is usually needed to make the reaction occur at a useful rate.

Esters can also be formed by various other reactions. These include the reaction of an alcohol with an acid chloride (R-CO-Cl) or an anhydride (R-CO-O-COR'). Early studies into the chemical mechanism of esterification, concluded that the ester product (R-CO-OR') is the union of the acyl group (R-C=O-) from the acid, RCO-OH, with the alkoxide group (R'O-) from the alcohol, R'-OH rather than other possible combinations.

The chemical structure of the alcohol, the acid, and the acid catalyst used in the esterification reaction all effect its rate. Simple alcohols such as methanol (CH₃OH) and ethanol (CH₃CH₂OH) react very fast because they are relatively small and contain no carbon atom sidechains that would hinder their reaction. These differing rates of reaction were first reported by Nikolay Menschutkin (1842-1907) in 1879-83. He also noted that simple acids such as acetic acid or vinegar (CH₃CO₂H) form esters very easily. The most common acid catalysts are hydrochloric acid, HCl, and sulfuric acid, H₂SO₄, because they are very strong acids. At the end of the esterification reaction, the acid catalyst has to be neutralized in order to isolate the product. German chemists, during World War II, developed solid acid catalysts or ion exchange resins for use in the manufacture of esters. These solid catalysts work well with acid sensitive esters because they can be separated from the product by filtration and therefore, the catalyst does not spend very much time in contact with the acid unstable product.

The esterification process has a broad spectrum of uses from the preparation of highly specialized esters in the chemical laboratory to the production of millions of tons of commercial ester products. These commercial compounds are manufactured by either a batch or a continuous synthetic process. The batch procedure involves a single pot reactor that is filled with the acid and alcohol reactants. The acid catalyst is added and the water removed as the reaction proceeds. This method is most often used by chemists in the laboratory, but in a few cases, it is used by industry to make large quantities of esters. This batch process usually requires reactors that hold extremely large volumes of reactants. Butyl acetate is commonly prepared from butanol and acetic acid by this method. The continuous process for making esters was first patented in 1921 and has been used extensively in the manufacture of large quantities of esters. This procedure involves the mixing of streams of the reactants into a reaction chamber while the product is removed at the same time. Continuous esterification has the advantage that larger quantities of products can be prepared in shorter periods of time. This procedure can be run for days or weeks without interruption, but requires special equipment and special chemical engineering considerations. The continuous esterification process is used industrially to make methyl acetate from acetic acid and methanol and ethyl acetate from acetic acid and ethanol.

The alternative process of making esters from the reaction of an alcohol with an anhydride is important in the manufacture of drugs. This reaction gives an acid as a by-product.

Acetic anhydride, CH₃-CO-O-CO-CH₃, a derivative of acetic acid, the acid in vinegar, is the most commonly used anhydride reactant. Phenyl acetate, one ester prepared industrially by this method, is an important intermediate in the synthesis of acetaminophen. Aspirin, or acetylsalicylic acid, is also prepared in large scale by the esterification reaction of an alcohol with acetic anhydride. This anhydride is important in the production of cellulose acetate by the esterification of cellulose or cotton. Cellulose acetate was first prepared in 1865 and was used extensively during World War I to coat airplane wings. Today, cellulose acetate finds its largest application as the fibrous material used in cigarette filters. It is also used in various yarns and textiles and to make the tips of felt-tip pens. Phthalic anhydride, an anhydride derivative of a benzene or phenyl ring, yields dimethyl phthalate when reacted with methanol in a reaction to that described for acetic anhydride. Dimethyl phthalate is used as a mosquito repellent and in the manufacture of certain polyesters. It is also a component in hair sprays and is added to plastics to soften them.