Influence of alkyl sulbstitution on the gas-phase stability of 1-adamantylcation and on the solvent effects in the solvolysis of 1-bromoadamantane

1-Adamantyl cations having three methyl groups or one, two, or three isopro pyl groups on the 3-, 5-, and 7-positions were found by FT ICR to be more s table than the l-adamantyl cation and that the stability increases with the number of isopropyl group. The relative stabilities calculated by PM3 were in good agreement with the experimental results. In contrast, the sequence of the rates for the solvolysis in nonaqueous solvents are 3,5,7-(Me)(3)-1 -AdBr < 1-bromoadamantane (1-AdBr) < 3,5,7-(n-Pr)(3)-1-AdBr < 3,5,1-(i-Pr)( 3)-1-AdBr. The rates of solvolysis of 3,5,7-(i-Pr)3-1-AdBr and 3,5,7-(n-Pr) (3)-1-AdBr relative to 1-AdBr at 25 <degrees>C are 15 and 3.8 in EtOH, resp ectively, but markedly decreases with the increase in the amount of added w ater, reaching 0.84 and 0.15, respectively, in 60% EtOH. Reflecting these e ffects of water, the Grunwald-Winstein (GW) relationship for 3,5,7(i-Pr)(3) -1-AdBr and 3,5,7-(n-Pr)3-1-Ad.Br against Y-Br is linear for nonaqueous alc ohols (EtOH, MeOH, TFE-EtOH, TFE, 97% HEIP), but marked downward deviations are observed for aqueous organic solvents, in particular, aqueous ethanol and aqueous acetone. The effect of the alkyl substituents to diminish relat ive solvolytic reactivity in EtOH-H2O mixtures may be ascribed to a blend o f steric hindrance to Bronsted base-type hydration to the P-hydrogens and h ydrophobic interaction of the alkyl groups with ethanol to make the primary solvation shell less ionizing. The introduction of one nonyl group to the 3-position showed much smaller deviations in the GW relationship than the c ase of 3,5,7-(n-Pr)3-1-AdBr. The markedly decelerated solvolysis of alkylat ed 1-bromoadamantanes in aqueous organic solvents is a kinetic version of a nomalously diminished dissociation of alkylbenzoic acids in aqueous ethanol and aqueous tert-butyl alcohol that was demonstrated by Wepster and co-wor kers a decade ago and ascribed to hydrophobic effects.