Quantitative evaluation of the gem-dimethyl effect on the succinic acid ⇌ anhydride equilibrium. Conformations of the acids and anhydrides by empirical force field calculations

Abstract

In order to evaluate quantitatively the gem-dimethyl effect on the succinic acid ⇌ anhydride equilibrium, the conformations of succinic acid and its 2-methyl-, racemic 2,3-dimethyl-, tetramethyl-, and racemic 2,3-di-t-butyl-derivatives have been calculated by means of Allinger's 1977 empirical force field. An extension of the field was developed to calculate the conformations of the respective anhydrides. The calculated preferred conformations compare well with existing experimental data. No low-energy hydrogen-bonded minima for the acids were obtained. Increased substitution in the acids caused conformational changes facilitating ring closure: smaller torsion angles of conformations with gauche carboxy groups, favourable bond length and angle deformations, and a reduced number of preferred conformations. In the anhydrides, substitution leads to a twist around the C(2)–C(3)bond of the ring. The ΔΔH values estimated for the diacid ⇌ anhydride equilibria agree well with experimental data in water indicating that the main cause of the observed gem-dimethyl effect in the anhydrides is relief of intramolecular strain arising on substitution in the acids.