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

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.