Photochemical addition Reactions Shown by Aromatic Compounds

PHOTOCHEMICAL ADDITION REACTIONS SHOWN BY AROMATIC COMPOUNDS  

Addition reactions given by aromatic compounds are studied in two categories. First consists, addition reactions by benzene and its derivatives, while second category contains other aromatic compounds. 

The photochemistry of benzene is in contrast to ground state chemistry and provides a full exploitation in synthetic applications, which is thermally not possible. The addition of an alkenes, dienes, alkynes, amines, alcohols or carboxylic acids to benzene, can occur across the ortho-, meta-, and para-positions to give three distinct products. The addition of alkene gives ortho-, meta- and paro-cycloadditions. All the addition are streospecific with respect to alkene component. 

All of these three additions [o-, m- and p-] can occurs from the singlet excited state of aromatic compound. The excitation can be caused by excitation of aromatic chromosphere, i.e.,initial excitation of alkene addend or charge transfer complex formed between addends. 

The reaction of benzene and alkenes like an isopropyldiene (Me2C==C) moiety produces an non-cyclic ene-type of reaction product with effective p-substituent. The reaction is a non-concerted process and is considered to proceed either via a biradical or from an equivalent polarised complex. The reaction involves charge transfer by donation of an electron from alkene to excited benzene. 

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