Esterification
the production of esters by reacting alcohols with acids. For example, carboxylic acids are esterified through the reaction
Esterification is a reversible reaction; water reacts with esters to form the parent compounds (seeHYDROLYSIS).
The position of the equilibrium in esterification depends on the nature and amount of the acid and alcohol used. For example, when ethyl alcohol and acetic acid are combined in equimolar proportions, the equilibrium is established after two-thirds of the alcohol and acid have been converted to the ester (ethyl acetate). The temperature has a great effect on the rate at which maximum esterification is attained. Thus, in the previous example, only 55 percent of the maximum is formed at room temperature in 368 days, but 55.7 percent is formed at 100°C in 32 hours, and 66 percent is formed at 150°C in 24 hours. The reaction is also accelerated by the addition of strong acids, for it increases the concentration of hydronium ions, which are catalysts of esterification. According to the law of mass action, in order to increase the yield of the ester, either the concentration of one of the reactants must be increased or one of the products (ester or water) must be removed from the reaction vessel as it is formed. In the latter case, compounds that form azeotropic mixtures with water are sometimes added to the reaction mixture in order to facilitate the removal by distillation.
Between 1877 and 1879, N. A. Menshutkin established that primary alcohols undergo esterification the most readily and that tertiary alcohols undergo esterification with the greatest difficulty. In 1938 the American chemists D. Roberts and H. Urey used isotopically labeled compounds (tracers) to show that the hydroxyl group is split from the acid molecule in esterification, as shown in the reaction
Of the various methods for producing esters, esterification is the simplest and most convenient method, and it has the greatest practical importance. Esterification is one of the best studied and most commonly used organic reactions; it was first studied by P. Berthelot in 1862 and 1863.
Esterification is common in living cells and is important because of the formation of ester bonds between individual nucleotide molecules in nucleic acids. These bonds link the phosphoric radical attached to the fifth carbon of the carbohydrate pentose in one mononucleotide with the alcohol group attached to the third carbon of the pentose of another mononucleotide. The formation of the bond is catalyzed by aminoacyl-RNA synthetases, which are specific enzymes of the ligase class. The reaction of cyclization with the loss of pyrophosphate in the formation of cyclic nucleotides is also essentially an esterification that occurs between the acid and alcohol groups within a single molecule. This reaction is accomplished by means of the specific enzymes adenylate cyclase and guanylidate cyclase.
E. A. POLENOV and E. P. FEDENKO
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