Aromatic detritiation. Part III. Deviations from additivity in detritiation of xylenes

Abstract

Partial rate factors for the detritiation of some xylenes by anhydrous trifluoroacetic acid at 70° have been determined as follows: [2-³H]-p-xylene 1460, [2-³H]-m-xylene 39,800, and [4-³H]-m-xylene 78,900. For the m-xylenes, which are 200 times as reactive as toluene, the observed reactivities are 20% less than predicted by the additivity principle, but in free-energy terms the ratio of 2- : 4-, i.e., ortho : para substitution is the same as in toluene. The anomalous ratio of 3 : 4, i.e., ortho: para substitution previously found for o-xylene is therefore thought to arise from solvation rather than electronic effects previously proposed. Possible explanations for the 10% greater reactivity of [2-³H]-p-xylene over that of [3-³H]-o-xylene (both of which have activation of the reaction site by an ortho- and a meta-methyl group) are proposed.

The kinetic data indicated that rearrangement occurs in formation of the Grignard reagent (in ether) from 1-bromo-2,6-dimethylbenzene; this does not occur on forming the organolithium reagent.