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Organic Carbonates. Part XIII. Separation of Polar and Steric Effects in the Hydrolyses of Substituted Ethylene and Trimethylene Carbonates
Author(s) -
Katzhendler Joshua,
Poles L. A.,
Sard Shalom
Publication year - 1972
Publication title -
israel journal of chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.908
H-Index - 54
eISSN - 1869-5868
pISSN - 0021-2148
DOI - 10.1002/ijch.197200019
Subject(s) - chemistry , steric effects , substituent , reactivity (psychology) , hydrolysis , reaction rate constant , alkyl , medicinal chemistry , polar , ethylene , crystallography , stereochemistry , organic chemistry , kinetics , catalysis , medicine , physics , alternative medicine , pathology , quantum mechanics , astronomy
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 0 18 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.