Does the Csp2—Csp3 barrier in ethylbenzene have a hyperconjugative component? An indirect experimental and theoretical approach

Abstract

The proton chemical shifts and the proton spin–spin coupling constants are reported for 1-phenyl-1-butyne and 1-phenyl-1-pentyne dissolved in CS2/C6D12 and acetone-d6. The long-range coupling constants between the methylene and ring protons are used to derive the twofold barriers to internal rotation in these molecules. They are 0.5 ± 0.1 kJ/mol; the perpendicular conformer is the most stable, the one in which the C(3)—C(4) bond of the side chain lies in a plane perpendicular to the phenyl group. This energetic preference is assigned to the difference between C—C and C—H hyperconjugative interactions with the aromatic π electron system, the C—C interaction being larger. Comparison of the twofold components of the internal rotational barriers in biphenyl and diphenylacetylene, that in the latter amounting to 40% of that in the former, implies that the hyperconjugative component of the internal barrier in ethylbenzene is 1.2 ± 0.3 kJ/mol, a minor component of the total magnitude. Molecular orbital computations of the conformational energies of 1 -phenyl-1-butyne all agree that the perpendicular conformer has the lowest energy but only to the extent of 0.1 kJ/mol at most.