Proton Magnetic Resonance Studies of Rotational Isomerism in Halotoluenes VIII. Stereospecific 1H–9F Coupling Constants and Conformations of some Fluoro and Chloro Derivatives of Benzalchloride

Abstract

In solution the 2-fluoro-5-chloro-, 2-fluoro-6-chloro-3-nitro-, 2-fluoro-6-chloro-5-nitro-, and 2,4,5-trichloro-benzalchlorides prefer conformations in which the C— bond of the side-chain lies in the plane of the aromatic ring. This C—H bond eclipses that ortho C—X bond (X = H, F, Cl) in which X is smaller than Y = H, F, Cl in the C—Y bond, also ortho to the dichloromethyl group. The long-range spin–spin coupling constants between the proton in the side-chain and the ring protons or fluorine nucleus are stereospecific. In particular, the coupling over four bonds between the side-chain proton and the ring fluorine is −0.3 Hz when the C—H and C—F bonds are arranged cis to each other but is −2.5 Hz when these bonds have a transoid planar arrangement.