The Effect of α-Substituents on Benzylic Spin–Spin Coupling Constants. Derivatives of 2,6-Dichlorotoluene

Abstract

Spin–spin coupling constants between methylene protons and ring protons over six and five bonds, [Formula: see text], are reported for α-substituted derivatives of 2,6-dichlorotoluene. Potential energy calculations suggest that the C—X bond of the CH₂X group lies perpendicular to the plane of the ring. The six-bond coupling is dominated by a π electron mechanism and depends on the conformation of CH₂X with respect to the aromatic plane and on the electronegativity of the substituent, X. The five-bond coupling is independent of X and it is suggested that the decrease in the π electron contribution to this coupling caused by the increasing electronegativity of X, is compensated by a corresponding increase in the sigma electron contribution.