Electrophilic aromatic substitution. Part XVII. Protiodetritiation of some cycloalkyl- and secondary alkyl-benzenes. A linear free energy relationship for ortho-aromatic substitution

Abstract

From the measured rates of protiodetritiation of some cycloalkyl- and secondary alkyl-benzenes (RPh) by tri-fluoroacetic acid at 70°, partial rate factors for ortho- and para-substitution have been determined as follows: (R =) cyclobutyl, 455, 1 070; cyclopentyl, 473,1 195: cyclohexyl, 406, 886: 1-methylethyl, 223, 544; 1-methylpropyl, 244, 690: 1-ethylpropyl, 196, 682: 1-ethylbutyl, 217, 685. With the exception of the last two compounds, each alkylbenzene gives a constant log fo : log fp ratio of 0.865 ± 0.025 as do a large number of other aromatic compounds in hydrogen exchange; this ratio is precisely predicted by the charge distribution in the Wheland intermediate. This correlation permits the first assignment of meaningful σ+ortho substituent constants for application to electrophilic substitutions of these compounds (but not their analogous side-chain reactions). Exchange at the ortho-positions of 1-ethylbutyl- and 1-ethylpropyl-benzene may be slightly hindered; exchange in these compounds was also accompanied by an intermolecular surface-catalysed migration of the alkyl group which may be sterically accelerated. Reasons for the greater electron-supplying abilities of the cyclic substituents relative to their open chain analogues and for the greater electron-supplying ability of these and other bulky substituents in hydrogen exchange compared with side-chain solvolyses, are discussed; the greater activation by the cyclopentyl substituent relative to the other cycloalkyl groups (excluding cyclopropyl) may be due to steric enhancement of hyperconjugation, and this phenomenon is proposed to account for the very high reactivity of acenaphthene in electrophilic substitution.