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 S_N1 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.