Evidence against appreciable internal ion pair return in the solvolyses of tertiary aliphatic halides. Measurement of α-methyl/hydrogen rate ratios in hexafluoropropan-2-ol–water

Abstract

A good linear correlation between the logarithms of rate constants for solvolyses of 1-adamantyl(I) and 2-methyl-2-adamantyl (III) chlorides is interpreted as evidence that these solvolyses proceed by the same mechanism, rate-determining formation of contact ion pairs. This interpretation conflicts with two previous interpretations based on β-deuterium kinetic isotope effects for solvolyses of tertiary substrates, (i) suggesting the occurrence of rate-determining elimination from contact ion pairs; (ii) claiming a linear correlation between log (CH₃/H) and log (CH₃/CD₃) rate ratios, where log (CH₃/H) is the logarithm of rate ratios for solvolyses of tertiary substrates (R¹R²-CH₃CX) and the corresponding secondary substrate (R¹R²HCX). Kinetic techniques for relatively fast solvolyses are used to study reactions in 97% w/w hexafluoropropan-2-ol–water, where nucleophilic solvent assistance is small, and so carbocation stabilities can be evaluated kinetically. Reaction of 2-chloro-2-methyladamantane is 10^7.4 times more rapid than that of 2-chloroadamantane, in agreement with studies using more nucleophilic solvents. The corresponding rate ratio for 2-bromo-2-methylpropane and 2-bromopropane is only 10^6.2, a higher ratio than is obtained in more nucleophilic solvents. It is proposed that nucleophilic solvent assistance is significant even for solvolysis of propan-2-yl substrates in hexafluoropropan-2-ol. Kinetic data measured directly at 25° for 2-exo- and 2-endo-norbornyl methanesulphonates give an exo/endo rate ratio of 1.59 x 10³, in agreement with results obtained by temperature extrapolations. Some of the factors influencing (or helping to prevent) internal ion pair return are discussed.