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I have seen examples of dehydrohalogenation reaction that occur through E2 mechanism but never came across dehalogenation reaction that occur through E2 mechanism (for example, dehalogenation of 1,2-chloropropane).

Why is that all dehalogenation reaction occur through E1 reaction, if not can someone give a counterexample for it?

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  • $\begingroup$ Isn't dehydrohalogenation same as dehalogenation? $\endgroup$
    – Nisarg Bhavsar
    Jun 16, 2021 at 8:51
  • $\begingroup$ @NisargBhavsar I thought dehalogenation was removal of 2 halogen atoms $\endgroup$
    – Asher2211
    Jun 16, 2021 at 8:51
  • $\begingroup$ How can 2 halogens be eliminated simultaneously? Is an halogen acidic enough to get this done? $\endgroup$
    – Nisarg Bhavsar
    Jun 16, 2021 at 8:54
  • $\begingroup$ @NisargBhavsar Reaction of 1,2-chloropropane with Zn dust $\endgroup$
    – Asher2211
    Jun 16, 2021 at 8:56
  • $\begingroup$ That's via a radical mechanism and not E1 or E2. $\endgroup$
    – Nisarg Bhavsar
    Jun 16, 2021 at 9:04

1 Answer 1

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Dehalogenation of vicinal dihalides can occur through the E2 mechanism. After some digging, I found this article from 1955. The full text is behind a paywall for me, so I cannot cite the specific rate constants, but the general findings are that trans-1,2-dihalocylohexanes undergo iodide-mediated dehalogenation more rapidly than the cis-stereoisomer.

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These findings suggest the anti conformation is necessary, which supports the E2 mechanism.

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This answer suggests that the final step of the reductive dehalogenation of vicinal dihalides by zinc dust is E2-like.

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