Question

Ortho, meta, and para-xylene (1,2-dimethylbenzene, 1,3-dimethylbenzene, and 1,4-dimethylbenzene) provide similar mass spectra which are very different from ethylbenzene. Explain why you would expect this based on the common fragmentation behavior of arenes. What would you expect to be the main fragments observed with ethylbenzene?

Answer 1

Solution -

Mass spectra of Ortho, meta, and para-xylene have similar spectra but ethyl benzene has diffents than the xylene.

Fragmentation pattern is same in the both the cases.

Fragmentation pattern in Ortho, meta, and para-xylene is as follows-

In this case initially methyl group of aromatic ring was break and form base peak 92 and M+ 91 and then second methyl group was broken it forms base peak 78, and M= 77

M ,M⁺ = 106, 105, 92, 91, 78, 77 (m/z)

In case In case of Ethyl benzene first methyl group of aliphatic carbon was break and form stable fragment 92 and M= 91 after that aromatic ring methyl was broken and formed base peak 78 and M= 77