Electrophilic aromatic substitution. Part 35. Deviations from additivity of methyl substituent effects in detritiation of dimethylnaphthalenes: the effect of electron supply on bond fixation

Abstract

Rate coefficients have been measured for protiodetritiation in anhydrous trifluoroacetic acid at 70 °C of all positions of 1,3-, 1,4-, 1,5-, 1,8-, 2,3-, 2,6-, and 2,7-dimethylnaphthalenes. The derived partial rate factors show up to 10-fold departure from those calculated on the basis of additivity of the individual methyl substituent effects. Of the seventeen positions examined, all but three show reactivity which is greater than that calculated. The results point to a decrease in bond fixation in dimethylnaphthalenes compared to monomethylnaphthalenes, probably arising either from the increased electron supply in the former or from a sterically facilitated change in the shape of the naphthalene nucleus. Detritiation of 1,8-dimethylnaphthalene is accompanied by isomerisation to 1,7-dimethylnaphthalene due almost certainly to a sterically accelerated 1,2-methyl shift; this shift (which is the first observed during hydrogen exchange studies) accounts for discrepancies between previous measurements of the exchange rate of the (relatively unreactive) 3-position of 1,8-dimethylnaphthalene. The kinetics show that surface-catalysed protiodemethylation accompanies detritiation of 1,4-dimethylnaphthalene and this is aided by the high stability of the intermediate for this ipso-substitution.