Rotational isomerism—XIII : Rotational isomerism in furfuraldehyde

The equilibrium between the cis and trans isomers of furfuraldehyde has been studied by measuring the stereospecific coupling constants between the aldehyde and ring protons in various solvents. These values when combined with the observed values of the couplings in the individual conformers at low temperatures give the energy difference between the cis and trans forms in these solvents. These values together with other previous measurements demonstrate the large solvent effect on this equilibrium. E(cis)-E(trans) is +1·5 kcals/mole in the vapour, much smaller in non-polar solvents (+0·3 kcals/mole in CF₂Cl₂), ca 0 in CHCl₃ and becomes negative in polar media (−1·0 kcals/mole in the pure liquid). This solvent dependence is given a quantitative explanation in terms of a known theory of solvation.