The solvent effect on the dipole moments of acylpyrroles

Abstract

Acylpyrroles as the solutes in dioxan and benzene solutions mostly exist in the form of n-hydrogen-bonded or π-hydrogen-bonded complexes with the solvent; the dipole moments of these complexes can be regarded as equal to the resultant of the dipole moment of the uncomplexed species (μ 0) and an additional vector Δμ due to complexation. The fact that the moment of a closed-ring 2-acylpyrrole is markedly higher when measured in benzene or dioxan solution than the corresponding value measured in cyclohexane (taken as μ 0) clearly shows that the Δμ vector is not directed along the acidic HN bond, but lies approximately along the direction of the mesomeric acyl-conjugation moment. Analysis of the effects of benzene and dioxan on the dipole moments of both planar conformers allows the preferred conformations of 2-formyl- and 2-acetylpyrrole, and of 3-formyl- and 3-acetylpyrrole as benzene and dioxan solutes to be established; these conformations are cis (as in cyclohexane) and trans, respectively. 2-Acetyl-trialkylpyrroles and 3-acetyl-2,5-dimethyl-4- n-pyrrole appear to exist in a twisted- cis conformation.