The effect of ortho substitution on the mass spectral fragmentation of polychlorinated biphenyls

Isomeric chlorinated biphenyls are observed to yield very similar mass spectra, rarely exhibiting a [M  Cl]+ fragment of any siVgnificant magnitude. However, those isomers containing three ortho chloro substituents display a relatively intense [M  Cl]+ fragment. A rationalization for this observation is put forth on the basis of the relative stabilities of both product and reactant.