# Reaction of conjugated alkene with KNH2

I could not think of this reaction fitting into any of the reactions I know except the aryne mechanism. And hence I came up with the following mechanism after seeing the answer (which I've written in the picture next to question). Please tell if my proposed mechanism is correct.

Also, I am highly unsure of the fact that the last species formed will be able to extract a hydrogen atom from somewhere in the solution.

Question:

This is my proposed mechanism:

Source: Joint Entrance Exam (JEE) 1997 India

• If the product is indeed correct (diphenylacetylene), I would suggest alpha-elimination to form a carbene which rearranges with phenyl shift to give the alkyne product Apr 3 '18 at 20:48
• I have never seen any alkene form a carbene before. Could you please give an example of a reaction where you have seen such a thing. Or tell, why would it happen.. Please.. Apr 3 '18 at 20:55
• @orthocresol I believe you are correct. Deprotonation of the bromo-alkene to give the C(K)Br species will certainly give rise to a carbene (or metallocarbene) see en.wikipedia.org/wiki/Carbene and the phenyl shift seems the obvious route to the product Apr 3 '18 at 21:03

• I have a doubt in the migration step. I would have thought that the bottom phenyl migrates, not the top one - similar to how in Beckmann rearrangement, the group trans to the leaving $\ce{-OH2^+}$ migrates - isn't this reaction similar with $\ce{-OH2^+}$ replaced by $\ce{-Br}$, and a carbon in place of the nitrogen?