Rotational isomerism. Part X. Steric effects on rotamer populations in cis- and trans-1,3-dichloroprop-1-enes

Abstract

The n.m.r. spectra of cis-(I) and trans-(II)-1,3-dichloropropene and cis- and trans-1-chloropropene have been analysed in a number of solvents. The solvent-dependence of the vicinal CH·CH couplings in (II) can be interpreted quantitatively on the basis of two interconverting rotamers, with the gauche-form more stable by 0·5 kcal/mol in the vapour, increasing to 1·2 kcal/mol in polar solvents. The long-range CH:C·CH couplings in (II) are obtained as J_g–3·8 and J_t+ 1·7 Hz.

The solvent-dependence of the couplings in (I) cannot be interpreted on this simple picture. Calculations of the steric energies in propenes based on the Westheimer–Hendricksen method are given which correctly reproduce the observed barrier heights in some chloropropenes and reasonably fit the rotamer energy differences in allyl chloride and (II).

These calculations show that (I) does not consist of two discrete rotamers, but of only one flexible rotamer. This rotamer changes its geometry with solvent, and this is in accord with the n.m.r. results and also of calculations of the solvation energies of the molecule.