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 pKR value for the equilibrium between the cation and the alcohol was measured as -4.80, while the pKa 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 N3, H2O 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.