Calculations of steric effects. Part I. Uncatalysed SE2 substitution of alkylmercury(II) salts by mercury(II) salts in ethanol

Abstract

Steric effects of alkyl groups in reaction (1) have been calculated by setting up nonbonded potential functions, RHgX + HgX₂ [graphic ommitted] RHgX + HgX₂(1) and obtaining the various nonbonded interaction energies in both initial states and transition states. These interactions lead to restricted rotation of methyl groups both in the initial state (β-rotation) and transition state (α- and β-rotation), and a treatment is given that allows the effect of restricted rotation on the relative rate constants to be determined. Calculations carried out on an open transition state model lead to close agreement between observed and calculated relative rate constants for R = Me, Et, and Bu^tCH₂(X = Br at 100°) and for R = Me and Bu^s(X = OAc at 60°), viz. observed (Me : Et : Bu^tCH₂ : Bu^s), 1 : 0·42 : 0·33 : 0·06 and calculated, 1 : 0·50 : 0·38 : 0·05, and it is shown that the above sequence is mainly due to the effect of restricted rotation of groups in the transition state. Similar calculations carried out using a cyclic transition state model do not yield relative rate constants that are at all compatible with the observed relative rates.