Mechanism for nucleophilic substitution and elimination reactions at tertiary carbon in largely aqueous solutions: Lifetime of a simple tertiary carbocation

The rate constants and the yields of the products of the nucleophilic substitution and elimination reactions of 1-(4-methoxyphenyl)-3-methyl-3-butyl derivatives (1-X) have been determined in mostly aqueous solvents, and the absolute rate constant for reaction of the simple tertiary carbocation 1⁺ in 50:50 (v/v) trifluoroethanol/water has been estimated as k(s) = 3.5 x 10¹² s^-1. Product studies show that the acid-catalyzed reactions of 1-OH and 4-(4-methoxyphenyl)-2-methyl-1-butene (2) do not proceed exclusively through a common carbocation intermediate 1⁺. The observed formation of small amounts of the azide ion adduct 1-N₃ likely occurs by a preassociation reaction mechanism. The reactions of 1-Cl and 1-O₂CC₆F₅ in 50:50 (v/v) trifluoroethanol/water give 39% and 56%, respectively, of the alkene products of elimination, and the yields of the alkenes are unaffected by the addition of 0.50 M of the good nucleophile N₃^-. This result is consistent with a concerted unimolecular mechanism for the elimination reactions of 1-X.