Determination of double-bond position in some unsaturated terpenes and other branched compounds by alkylthiolation.

The electron-impact mass spectra of alpha, beta-bis(methylthio) derivatives of certain terpenes and other compounds with branched alkenyl groups contain diagnostic peaks that can be used for locating the position of the double bond in the parent compound. In contrast to the spectra of dimethyl disulfide (DMDS) derivatives obtained from compounds containing CH=CH type double bonds, which show two predominant fragment ions, most of the compounds examined in the current investigation showed only one predominant fragment ion, arising from that part of the molecule which possesses the more substituted carbon of the original double bond. For example, all the derivatives from compounds with an isopropylidene moiety showed a base peak at m/z89 which can be attributed to the formation of a [(CH3)2C=SCH3]+ fragment. The application of the DMDS procedure to naturally occurring terpenes is discussed.