Dehydrohalogenation carbene formation

The acid was added to copper wool and rinsed down with DMSO. Within 20 s, the contents of the flask were ejected and the neck was distorted by intense heat [1], Adding the copper wool to a solution of the acid in the solvent gave an exothermic but controlled reaction [2], (Formation of a reactive carbene species by dehydrohalogenation seems a remote possibility.)... 

Although the observed products are consistent with initial formation of 4a, Untch proposed that they might arise via the intermediacy of cyclic allene 3a.23 Subsequent experiments by W.M. Jones and co-workers supported the initial formation of 3a in the dehydrohalogenation reactions, but left open the possibility of a rapid equilibrium between cycloheptatetraene (3a) and cycloheptatrienylidene (4a, Scheme 9).22,24 It was also found that fusion of benzene or naphthalene rings to different positions on the seven-membered ring strongly influenced whether a carbenic or allenic structure predominates.25... 

In the case of difluoromethylene, Hine 4- suggested that formation of the carbene is so favored that dehydrohalogenation occurs in a concerted fashion, with no carbanion intermediate. 

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