Question

Using a Mass Spectrum, a strong peak at 77 amu (atomic mass units) indicates the presence of what organic group? Why is the peak so strong?

Answer 1

A strong peak at 77amu indicates the C6H5+. , the phenyl cation in the molecule. The presence of this peak is shown by most of the aromatic compounds.

Though in many reactions of aromatic comounds phenyl cation is not formed as it is highly unstable, ( the positive charge is not stabliised by resonance), in mass spectrometry we often encounter phenyl cations in almost all aromatic compounds.

The reason is though the positive charge is not involved in resonance, still the benzene ring is aromatic and thus makes phenyl cation much more stable relatively than many other positive ions formed in the mass spectrometer.

Thus we find phenyl cation ingood abundance as a strong peak.