Carbon skeletal rearrangements in 2-phenylthiophene and 2,5-diphenylthiophene under conditions of electron-impact

The mass spectra of ¹³C-labelled 2-phenylthiophenes and 2,5-diphenylthiophenes were studied. The label distributions for the HCS]⁺, C₂H₂S]^+·, C₈H₆]^+·, C₉H₇]⁺ and C₇H₅]S⁺ ions from 2-phenylthiophene and the HCS]⁺, C₉H₇]⁺, C₇H₅S]^+·, and C₁₅H₁₁]⁺ ions from 2,5-diphenylthiophene were interpreted in terms of both carbon skeletal rearrangements in the thiophene ring and migration of the phenyl substituent. The degree of carbon scrambling in the thiophene ring appeared to be almost independent of the electron beam energy. The formation of some of the fragment ions studied seems to be so fast that no carbon scrambling could be detected at all; in neither case was complete scrambling of the carbon atoms of the thiophene ring observed.