Naphthalene dichloride and its derivatives. Part III. trans-1,2-Dichloro-1,2-dihydronaphthalene and a new naphthalene tetrachloride, r-1,c-2,c-3,t-4-tetrachlorotetralin

Abstract

The photochemical chlorination of naphthalene with equimolar amounts of chlorine in carbon disulphide at low temperatures gives trans-1,2-dichloro-1,2-dihydronaphthalene, which on heterolytic chlorination in methylene chloride gives a new (fifth) geometric isomer of naphthalene tetrachloride, r-1,c-2,c-3,t-4-tetrachlorotetralin. On alkaline dehydrochlorination in a mixture of methanol and acetone this tetrachloride gives, as the main intermediate,trans-1,2,4-trichloro-1,2-dihydronaphthalene. Some reaction through other intermediates, one of which can decompose unimolecularly, is indicated also. Further dehydrochlorination of trans-1,2,4-trichloro-1,2-dihydronaphthalene gives approximately equal amounts of 1,3- and 1,4-dichloronaphthalene by a bimolecular route, which is accompanied by a unimolecular pathway giving nearly exclusively 1,4-dichloronaphthalene. The results throw new light on the courses taken in the dehydrochlorination of the naphthalene tetrachlorides; comparisons are made of the reactivities of some naphthalene trans-dichlorides in bimolecular and unimolecular eliminations. Treatment of the new naphthalene tetrachloride with aluminium trichloride in nitrobenzene effects nearly complete conversion into naphthalene ε-tetrachloride, consistent with the view that this isomerisation involves heterolysis of 1- and 4-, but not of 2- and 3-chlorine substituents.