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

Abstract

AbstractThe 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 σF* (= +0.17) is provided by Eq. (III). Analysis of 018 exchange experiments and ΔΔFA# vs ΔΔFB# 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 (ESB ‐ ESA) 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.