Alkyl nitrite isomerism. PMR investigation of substituted ethyl nitrites XC [H( β)] 2C[H( α)] 2ONO

Abstract

New microwave results confirm Brown and Hollis' trans-cis assignment for alkyl nitrites. In the present paper, the experimental material is widened by investigating compounds of the type XCH 2CH 2ONO where X = F, Cl, Br, I, CN. The barrier to trans-cis conversion is fixed to ca. 10 kcalmole −1 in all cases. Likewise, E cis - E trans is close to 1 kcalmole −1, and S cis - S trans ∼ 2 gcalmole −1 deg −1. Rotation about the C,C bond generates 3 isomers, 2 in which X is gauche, and 1 in which X is trans to the ONO group. By measuring proton chemical shifts and proton spin-spin couplings at room temperature where effective averaging over rotations about the C, O single bond and the ON single bond takes place, it is shown that the gauche forms are more abundant than statistics would indicate ( 2 3 ). Since the same gauche:trans ratio is observed in the case X = F for the pure trans (O,N) form at −70°C, we conclude that gauche preference also applies to the cis isomer. This rules out the idea of some intramolecular bond between non-neighbouring atoms, stabilizing the cis isomer. The new compounds X = F, Br, I, CN are characterized by their infrared spectra.