The kinetics of hydrogen isotope exchange reactions. Part XV. Acid-catalysed tritiation of mesitylene and correlation of exchange rates with Hammett's acidity function

Abstract

The rate of tritium uptake by mesitylene from aqueous hydrochloric acid has been studied under homogeneous conditions at 25° over the acid concentration range 0·1–9·16M, which corresponds to a nearly million-fold variation in exchange velocity. The salting-out of mesitylene by hydrochloric acid has been measured up to 4·6M-acid. The correlation between exchange velocity constant (λ) and Hammett's acidity function H₀ is considered in relation to known information about the mechanism of aromatic exchange. The function (log λ+H₀+ logf_B/f_Mesitylene) is found to be more nearly independent of acid concentration (up to 2M) than the function (log λ+H₀).

This may mean that the failure of the simpler equation is for the most part due to effects in the activity coefficients of mesitylene and of the indicator base used to establish the acidity function over this concentration range.