CHEMICAL IONIZATION MASS SPECTROMETRY .8. ALKENES AND ALKYNES

The chemical ionization mass spectra of several alkenes and alkynes have been investigated using methane as reactant. The compounds studied comprised six straight-chain 1-monoolefins, eleven branched and internal monoolefins, and fourteen compounds of diverse types (di- and triolefins, monocyclic and bicyclic monoolefins, a cyclic diolefin, and acetylenes). The straight-chain 1-olefins undergo extensive fragmentation to produce the spectra consisting of series of alkyl and alkenyl ions. From the total intensities of these ion types it is concluded that the attack of the reactant ions on the olefin molecules is not random and occurs predominantly at the double bond. The (MW - 1)+ and (MW -15)+ intensities are largely in accordance with expectations; that is, they parallel the numbers of allylic hydrogens or methyl groups. The (MW + 1)+ intensities are generally appreciably larger for di-, tri-, and cyclic olefins than for monoolefins, but no enhancement is observed in the acetylenes. Ions with m/e values smaller than (MW - 15)+ are observed, but the reactions involved are not known. Evidence is presented for the occurrence of a 1,2-methide shift. © 1968, American Chemical Society. All rights reserved.