Studies of the halogenation of some substituted naphthalenes

Abstract

A general survey of the reaction of electrophilic halogens with polynuclear aromatic substrates has led to the conclusion that in many cases addition may take place concurrently with substitution. This is important in understanding the mechanism of this kind of reaction, especially as to the sequences leading to the isolated products, which seem to represent an alternative fate of a common carbonium ionic intermediate. The addition of molecular chlorine to a number of simple naphthalene derivatives has, therefore, been investigated under a wide range of experimental conditions. The products of these reactions were isolated and char= acterised as far as possible, particularly by using column chromatography, proton magnetic resonance spectroscopy, and kinetic techniques. The chlorination of 1-, 2-, 1,2-, and 1,4-substitu= ted naphthalenes, with activating, deactivating, or mixed groups on the nucleus, gave products of substitution, accompanied mainly by tetrachlorides and acetoxytrichlo rides, whose structures and predominant conformations in solution have been elucidated. On the whole it is found that addition of chlorine always occurs on the ring to which substitution takes place in accordance with the rules governing electrophilic substitution. Of the six possible orientations of substituents in theresulting tetralin ring system, two forms clearly pre= dominate, having the atoms (or groups) added to the alicyclic ring according to a '-e-e-a' or a'-e-e-e' spatial orientation. The relative rates and products of alkaline dehy= drochlorination have also been studied and are those expected from the configurations of the individual pro= ducts examined, and throw some light on their possible modes of hydrogen chloride elimination.Finally, in discussing the reaction paths leading to the addition products, the results confirm the possibility that polar addition of chlorine can proceed both in the cis- and trans-sense; but, when heterolytic conditions are chosen, the former is a most frequent process that has usually been recognized.