Experimental and theoretical evaluation of energetics for nucleophilic solvent participation in the solvolysis of tertiary alkyl chlorides on the basis of gas phase bridgehead carbocation stabilities

The specific rates of solvolysis in 80% ethanol and 97% 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) for various tertiary alkyl chlorides having different steric requirement and experimental (FT ICR) gas-phase stabilities of the corresponding carbocations were determined. The experimental gas-phase stabilities were in good agreement with theoretical values computed at the MP2/6-311C(d,p) or the MP2/6-311G(d,p)//MP2/6-31G(d)(d) level. The relation of differential activation Gibbs energy changes for solvolysis [delta Delta G double dagger = -RT In(k/k(o))] (relative to 1-chloroadamantane) vs the experimental gas-phase cation-stabilities Delta G(o) (relative to l-adamantyl cation) was compared with the previously established similar relation for bridgehead systems. It was revealed that the solvolysis of tert-butyl chloride in 80% ethanol is nucleophilically assisted by 4-8 kcal mol(-1). The delta Delta G double dagger vs Delta G(o) relation for heavily crowded 4-chloro-2,2,3,6,6-pentamethylheptane was found to be comparable to that of bridgehead compounds. The reversal of the ranking of stabilities of the tert-butyl cation and 1-adamantyl cation on going from the gas phase to aqueous solution was computationally assessed. The results agree with the fact that larger substituents around a cationic center increase the stability of the ion in the gas phase thanks to their larger polarizability, and that this effect is either offset or even dramatically reversed in solution.