STRUCTURAL STUDIES BY NUCLEAR MAGNETIC RESONANCE .21. CONFORMATIONAL ANALYSIS OF DICHLORACETALDEHYDE AND DIBROMACETALDEHYDE

The following conclusions were reached from the solvent and temperature dependence of the vicinal spin-spin coupling constants of dichloroacetaldehyde and dibromoacetaldehyde. (1) The data are consistent with a threefold barrier to rotation about the carbon-carbon bond. (2) In nonaromatic solvents whose dielectric constant is less than six, ∆G° for XIII→ XIVa is positive, i.e., the rotamer with the C-H bond eclipsing the carbonyl group is of lower energy than the others. (3) In nonaromatic solvents whose dielectric constant is higher than 7, ∆G° is negative. (4) Both free energies and enthalpies for XIII→ XIVa vary over a range of about 2.5 kcal/mol, as the dielectric constant of the solvent varies from about 2 to 45. © 1969, American Chemical Society. All rights reserved.