Knowledge Bank

In the spectroscopic proof of structure of polar olefins, the extent, kind, and distribution of substituents about the point of unsaturation is often of decisive importance. If at least one methylene group ``insulates'' the double bond from polar substitution or conjugation effects, it is generally safe to apply the hydrogen-wagging correlations well established for mono-olefinic hydrocarbons of the vinyl, vinylidene, cis and trans structure types. Direct polar substitution or conjugation effects can generally be recognized by shift of intensified double bond stretching absorption to longer wavelengths and often markedly alters absorption in the hydrogen-wagging region. For some special classes of polar olefins of practical interest (acrylates, methacrylates, vinyl ethers, thio ethers, etc.) these shifts are sufficiently consistent to establish helpful subcorrelations.