Sulphonation of polymethylbenzensulphonic acids: Evidence for a buttressing effect☆☆☆

Add time:09/01/2019    Source:sciencedirect.com

The rate of sulphonation of a series of polymethylbenzene sulphonic acids in 99·6 % H2SO4 has been determined at 25·0°. The observed rate order for the various substrates (2,3,4-Me3 > 2,4-Me2 > 2,3-Me2 > 2,4,5-Me3 = 2,4,6-Me3 > 2-Me > 4-Me) is explained in terms of steric and electronic effects of the Me substituents on the rate of formation and the rates of decomposition of the σ-complex intermediate The relatively low reactivity of 2,4,5- and 2,4,6-trimethylbenzenesulphonic acid is ascribed to a very low rate of conversion of the σ-complex into the sulphonic acid product due to buttressing of the Me group ortho to the reaction centre at position 2 by the sulphonic acid substituent at position 1.For the sulphonation of benzenesulphonic acid and p-xylene-sulphonic acid in 104·4% H2SO4 at 25° the ratio of para- to meta- and of 6- to 5-substitution are 0·030 and 0·078 respectively; the corresponding activation parameter differences are ΔH≠p − ΔH≠m = 2·2 ± 0·2 kcal/mole, ΔH≠6 − ΔS≠5 = 2·4 ± 0·1 kcal/ mole, ΔS≠p − ΔS≠m = 1·3 ± 0·7 eu and ΔS≠6 − ΔS≠5 = 3·0 ± 0·4 eu. The entropy difference data illustrate that the steric requirements are smaller for sulphonation ortho to an isolated Me group than to a Me group which is flanked by an ortho-sulphonic acid group.

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