Cycloheptatrienocarbonium bromide

Tropylium bromide (cycloheptatrienocarbonium bromide), CrHTBr, required for reaction (2a) above may be prepared by bromination of cycloheptatriene in carbon tetrachloride, followed by removal of the carbon tetrachloride and heating the residue in vacuo (60° at 20 mm.) for several days. The product so obtained is washed with tetrahydrofuran. Recrystallization is unnecessary. Tropylium bromide is deliquescent and must be stored in a dry atmosphere. Allyl chloride or bromide may be substituted for tropylium bromide in reaction (2a). 

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