Solvent effects and steric effects in electrophilic and nucleophilic substitution reactions

• Main Author: Grellier, Priscilla Louise
• Published: University of Surrey 1973
• Subjects: 547.13
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.457412

The SE2 reaction (1) between tetra-alkyltins and mercury(II) salts has been studied kinetically (R = Me, Et, Prn, Pri, Bun, Bui, Peneo; X = OAc in solvents methanol and tertiary butanol at 30 °C and R = Me, Et; X = Cl, I, OAc in solvents methanol, ethanol, n-propanol, n-butanol and tertiary butanol at 25 °c). R4Sn + HgX2 → R3SnX + RHgX (l) The following order of reaction rates was observed for (1), X = OAc, in solvent methanol: R = Me > Et > Prn > Bun > Bui > Peneo > Pri (2) Although this sequence is probably steric in origin, it is not the same sequence as is observed in SN2 reactions, and in SE2 reactions which proceed with inversion of configuration at the substituted carbon atom (SE2(open)Inv reactions). It is suggested that reaction (1) in methanol proceeds with retention of configuration at the substituted carbon atom, by mechanism SE2(open)Ret. It is further suggested that the stereochemical course of SE2(open) reactions of substrates RMXn may be deduced from a consideration of the constitutional effects of alkyl groups (R) on the rate of reaction; if these effects parallel steric effects in SN2 reactions, then mechanism SE2(open)Inv is indicated, but if the constitutional effects parallel those found for reaction (1) in methanol (sequence (2)) then mechanism SE2(open)Ret is indicated. In this way, stereochemical assignments for a number of other SE2(open) reactions have been deduced. Calculations have shown qualitatively and semi-quantitatively that the reactivity sequence (2) does indeed arise from steric effects, mainly in the transition state, in reaction (1). Similar calculations were also carried out on the SN2 reaction between bromide ion and alkyl bromides, and it was demonstrated that the different steric effects in inversion and retention reactions are a result of the different geometrical shapes of the transition states. Standard free energies of the reactants in (1) (R = Me, Et; X = Cl, I, OAc) were determined for transfer between the five alcoholic solvents, and combination of these initial state solvent effects with the kinetic data enabled transition state solvent effects to be calculated for the reactions. It was concluded that the transition states in these SE2(open)Ret reactions behave as polarisable species, with quite a high degree of charge separation. The SN2 reaction (3) was also studied kinetically, in solvents water, methanol, ethanol, n-propanol, isopropanol, n-butanol, tertiary butanol, ethyl acetate, ethyl benzoate and acetonitrile, at 25 °C. Et3N + EtI → Et4N+I- (3) Gas-liquid chromatography was used to determine terminal values of Raoult's law activity coefficients for both reactants, in a large number of solvents, thus permitting a dissection of solvent effects on the reaction into initial and transition state effects, for more than 30 solvents. It was concluded that the transition state in this Menschutkin reaction behaves as though it were a polarisable non-electrolyte, rather than an ion pair, and that in many solvents, the effects are due to specific solvent-solute interactions.