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-hexaflu oro-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 stab ilities 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 exp erimental gas-phase cation-stabilities Delta G(o) (relative to l-adamantyl cation) was compared with the previously established similar relation for b ridgehead systems. It was revealed that the solvolysis of tert-butyl chlori de in 80% ethanol is nucleophilically assisted by 4-8 kcal mol(-1). The del ta Delta G double dagger vs Delta G(o) relation for heavily crowded 4-chlor o-2,2,3,6,6-pentamethylheptane was found to be comparable to that of bridge head compounds. The reversal of the ranking of stabilities of the tert-buty l cation and 1-adamantyl cation on going from the gas phase to aqueous solu tion was computationally assessed. The results agree with the fact that lar ger substituents around a cationic center increase the stability of the ion in the gas phase thanks to their larger polarizability, and that this effe ct is either offset or even dramatically reversed in solution.