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Abstract

The homolytic aromatic substitution reactions of chlorinated benzenes was investigated by studying their phenylation reactions. Dibenzoyl peroxide was used as the source of phenyl radicals and reactions were effected in sealed, thick walled tubes at different temperatures with various additives.
All the chlorosubstituted benzenes (chlorine = 1 - 6) were investigated namely chlorobenzene, three isomers each of di-, tri and tetra-chlorobenzenes, penta- and hexachlorobenzenes. The methylation of polychlorobenzenes was examined qualitatively, using tertiary butyl peroxide as the source of methyl radicals.
The phenylation of chlorosubstituted benzenes was found to involve the normal processes of displacement, disproportionation and dimerization. Products which could be formed by displacement of hydrogen (phenyldehydrogenation) and of chlorine (phenyldechlorination) occurred in all cases where possible. In addition, as the number of chlorine atoms increased, other novel reaction products became important and suggestions are made as to the mechanisms of their formation.
It is suggested that σ-complexes formed could take up a hydrogen atom and then form products involving the elimination of hydrogen chloride or less commonly of molecular chlorine. Ipso σ-complexes formed by the attack of a phenyl radical at a carbon bearing chlorine could give products arising from migration of either phenyl or chlorine to the ortho or meta position. Occasionally it might be possible for these pathways to give a novel route to a conventional product.
Thus in the phenylation of the isomeric di-, tri- and tetra-, and penta- and hexachlorobenzenes, the products are those of phenyl-dehydrogenation and phenyldechlorination reactions. However, additional novel by-products were also found in these reactions which possibly arise by the addition - elimination reaction sequence described above and the ipso rearrangement reaction.
Therefore the products of the phenylation reactions of the chloro-substituted benzenes differ substantially from those of the corresponding fluorosubstituted benzenes which were only reported to undergo replacement of hydrogen and fluorine.
The relative rates of the isomeric chlorosubstituted benzenes were determined and also the partial rate factors at the various positions of the aromatic ring in the absence and presence of several additives.

Publication Type:	Thesis (Doctoral)
Subjects:	Q Science > Q Science (General) Q Science > QD Chemistry
Departments:	School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses

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