Photochemical Isomerisation of Aromatic Compounds

Photochemistry of aromatic compounds provides a number of new photochemical pathways for these reactions which are not thermally feasible. The photoisomerisation of aromatic compounds differ from olefinic nonaromatic compounds. For example, an unsaturation provides two geometrical isomers [cis and trans), while disubstituted benzene gives three isomers (o-, m-and p-). 

Monocyclic aromatic compounds undergo remarkable photochemical rearrangements on irradiation, and provides photochemical isomerisation. For example an aromatic compound o-xylene an irradiation gives a mixture of o-, m-and p-xylenes. 

Mechanisms of the following photochemical isomerisations can be described as conversion of o-xylene into m-xylene and m-xylene into p-xylene, is accompained by 1, 2-alkyl shift. Similarly conversion of o-xylene into p-xylene and vice-versa is accompained by the 1, 3-alkyl group shift. 

2-alkyl shift takes place through benzvalene as well as prismane intermediates and obey following pathway. 

2-shift ofCHg group by benzvalene intermediate is as given above. 

---