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 × 10 12 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-N 3 likely occurs by a preassociation reaction mechanism. The reactions of 1-Cl and 1-O 2 CC 6 F 5 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 N 3 - . This result is consistent with a concerted unimolecular mechanism for the elimination reactions of 1-X .