Reactivity of sterically hindered diarylbenzhydryl carbocations – competing trapping by nucleophiles and elimination

Abstract

The bis-(pentamethylphenyl)carbocation 11c, can be formed from the corresponding alcohol 10c in concentrated sulfuric acid or from the sec-alkyl chloride 12c in ionizing solvents. The pK R value for the equilibrium between the cation and the alcohol was measured as -4.80, while the pK a of the carbocation 11c was estimated as -6.04. Elimination (to give the xylylene 3) competes with trapping of this carbocation in a ratio of 22:1 in aqueous dioxanee. Trapping of the carbocation by N 3 -, H 2 O and alcohols show that this carbocation is selective, but particularly so for reactions with secondary alcohols. In each case comparison is made with the trapping of bis-(mesityl)carbocation 11b and the bis-(o-tolyl)carbocation 11a, neither of which undergoes competing elimination in the reaction with water.

Reaction mechanisms : Part (iii) Polar reactions

“…The pK R and pK a values have been determined for diarylbenzhydryl carbocations in aqueous solution using an 'azide clock' method. 31 Elimination to xylylenes was observed to compete with reaction with solvent and azide ion in aqueous dioxane.…”