Nuclear magnetic resonance study of α-halogenobutyraldehydes and a re-appraisal of rotational isomerism in dihalogenoaldehydes and related sp2–sp3 hybridised systems

Abstract

From the temperature and solvent dependence of the vicinal coupling between the aldehydic and α-protons the rotational isomerism about the C(1)–C(2) bond in α-chloro-, α-bromo-, and α-iodo-butyraldehydes has been interpreted in terms of a three-fold barrier to rotation. The rotamer with the halogen atom eclipsing the carbonyl group contributes to a negligible extent in non-polar solvents. Published data for dichloro- and dibromo-acetaldehyde have been shown to be more consistent, for non-polar solvents, with an equilibrium involving skew rotamers. For these compounds the positions of the minima in the potential energy curve appear to be sensitive to solvent. Trends in the effect of halogen substitution on the rotational isomerism about the sp²–sp³ hybridised bond in carbonyl-containing compounds are discussed, and the anomalous effect of fluorine is rationalised.