Thermodynamic parameters for ionisation and dissociation of alkyl halides in water and nonaqueous solvents. Comments on the ion-pair mechanism of nucleophilic substitution

Abstract

Values of ΔG0, ΔH0, and ΔS0 have been calculated by the appropriate thermodynamic cycle for the ionisation of alkyl halides to the ion-pair R+X– and for the dissociation of alkyl halides to the pair of ions R++ X–, in water and in nonaqueous solvents. It is shown that values of ΔG0 and ΔH0 for ionisation of methyl and ethyl halides are so much higher than the observed values of ΔG‡ and ΔH‡ for solvolysis that it is energetically quite improbable that these halides undergo solvolysis or other nucleophilic substitutions by an ion-pair mechanism. For the isopropyl halides, ΔG0 and ΔH0 for ionisation are only slightly higher (by 1–5 kcal mol–1) than ΔG‡ and ΔH‡ for solvolysis, so in view of the experimental error in the calculated values it is just possible that simple secondary halides might undergo solvolysis by an ion-pair mechanism. Values of ΔG0 and ΔH0 for ionisation of t-butyl halides are less than ΔG‡ and ΔH‡ for solvolysis, thus indicating an SN1 or ion-pair-like transition state. This is confirmed by the marked correlation between the ionisation and activation parameters (G, H, and S) for reaction in six polar solvents.