Organic Carbonates. Part XIII. Separation of Polar and Steric Effects in the Hydrolyses of Substituted Ethylene and Trimethylene Carbonates

The rate constants, the free energies (ΔF^#), the energies (ΔH^#) and entropies (ΔS^#) of activation, and the steric parameter of substituent (Es), for both the acid and the alkaline hydrolyses of four highly branched ethylene carbonates (Ia - d), and 11 trimethylene carbonates (IIa - k) were analysed according to Taft's procedure for a quantitative separation of polar and steric effects of alkyl substituents in the total effect of structure upon reactivity in hydrolysis. Application of Eq. (II) with Σσ* to the most hindered substrates (“α-axial methyl effect”) (IId - f) and to the moderately hindered substrates (IIa - c) gave well-separated parallel straight lines for the two groups. Deviations from Eq. (II) were manifested by the rate-enhanced group of compounds, geminally 2,2-disubstituted trimethylene carbonates (IIg - k) (“2,2-gem-dialkyl effect”), shown to exert a polar “field effect”; the assessment of its substituent constant σ_F* (= +0.17) is provided by Eq. (III). Analysis of 0¹⁸ exchange experiments and ΔΔF_A^# vs ΔΔF_B^# correlation (Eq. (IV)) show that the acid- and base-catalysed reactions occur in parallel via similar transition states, the attainment of each designating the rate-determining step in both systems. A value of 0.5 kcal/mole was found for the net steric interaction (E_S^B - E_S^A) exerted by an α-axial methyl substituent on the attacking hydroxide ion in attaining the transition state. It was concluded that in alkaline hydrolysis, the transition states of compounds exhibiting an “α-axial methyl effect” should closely resemble the tetrahedral intermediate (IX), while the transition states of the 2,2-gem-dialkyl series should more closely resemble the ground-state.