Kinetics of reactions in strongly acidic media

The hydrolysis and formation of esters in solutions of strong acids show very varied behaviour, and depend on a rather large number of different factors. Our understanding of the quantitative behaviour of these reactions has become much clearer in recent years, and since an attempt to follow the historical development of the subject might obscure parts of the pattern that has emerged, it seems best to base a discussion on recent results. 

A large amount of data is available for the hydrolysis of simple esters in sulphuric acid. In Table 4 are listed observed first-order rate coefficients for the hydrolysis of a series of acetate esters in sulphuric acid solutions of 

RATE COEFFICIENTS FOR ACID-CATALYZED ESTER HYDROLYSIS AT 25°Ca 

The rate of hydrolysis of a carboxylic ester in strong sulphuric acid generally shows one of the three types of dependence on acid concentration illustrated in Fig. 1. The simplest behaviour, a continuous increase in hydrolysis rate with increasing acid concentration, is shown by esters of tertiary alcohols, which are hydrolyzed very rapidly even in moderately concentrated acid, and by phenol esters, which are somewhat less reactive, but are hydrolyzed much faster than esters of simple primary and secondary alcohols with above about 60% H2S04. Substituted phenyl acetates behave very much like the parent compound, the p-chlorophenyl ester being hydrolyzed at almost the same rate as the unsubstituted compound, while p-nitrophenyl acetate is somewhat less reactive at low acid concentrations, but more reactive in above 70% sulphuric acid. 

Esters of primary aliphatic alcohols show an initial increase in rate with increasing acid concentration, but the curve, illustrated in Fig. 1 for ethyl acetate, passes through a maximum at an intermediate acid concentration (50-60% H2S04), and the rate coefficient subsequently falls almost to zero in the region of 80% acid. Finally, the rate increases slightly between 80% and 100% H2S04. Methyl acetate is rather more reactive than the ethyl ester in above about 50% H2S04, but /i-propyl acetate shows closely similar reactivity, so that the curve for ethyl acetate can be considered to represent the typical behaviour of a primary alkyl ester. 

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