Protium–deuterium exchange of alkylated benzenes in dilute acid at elevated temperatures

Abstract

Benzene (1), toluene (2), o-xylene (3), m-xylene (4), p-xylene (5), 1,2,3-trimethylbenzene (6), 1,2,4-trimethylbenzene (7), 1,3,5-trimethylbenzene (8), 1,2,3,4-tetramethylbenzene (9), 1,2,3,5-tetramethylbenzene (10), 1,2,4,5-tetramethylbenzene (11), pentamethylbenzene (12), hexamethylbenzene (13), hexaethylbenzene (14), ethylbenzene (15), and tetralin (16) have been labelled with deuterium by the high temperature – dilute acid (HTDA) method. In dilute DCl/D2O (0.14–0.27 M) at temperatures of 250–285 °C, the ring hydrogens of these compounds equilibrate with the deuterium pool. At temperatures above 250 °C the methyl groups of 3, 4, 5, 6 and 11 undergo slow H/D exchange; the methyl groups of 8, 9, 10, and 13 undergo exchange much more readily and hexamethylbenzene (13) can be readily perlabelled. Hexamethylbenzene exhibits a lower reactivity than 13 at the benzylic sites. Keywords: deuterium, exchange, alkyl groups, benzenes, acid-catalyzed.