Elimination of benzylic halides

A number of benzylic halides and onium salts under the influence of base undergo an apparent a-elimination to yield dimeric olefins, viz. 

Two types of mechanism have been suggested, (i) Carbene mechanism 

With sodamide in liquid ammonia, dimeric halides (20b) were isolated in good yield. Experiments with optically active a-phenylethyl chloride indicated that the displacement reaction (20b) proceeds with Walden inversion. Clearly, the bimolecular displacement mechanism operates in the benzyl and benzhydryl series at low temperatures. 

At higher temperatures, and with aqueous or alcoholic base, isolation of dimeric halides is not possible. Provided that proton transfers occur rapidly, the slow steps for the two mechanisms would be (19b) and (20b), steps which are respectively uni- and bimolecular. Determination of the kinetic order thus provides a method of distinguishing the operative mechanism. 

The reaction of 4-nitrobenzyl chloride in the presence of alkali to give 4,4 - 

---