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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:

enter image description here

This is my proposed mechanism:

enter image description here


Source: Joint Entrance Exam (JEE) 1997 India

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    $\begingroup$ 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 $\endgroup$ – orthocresol Apr 3 '18 at 20:48
  • $\begingroup$ 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.. $\endgroup$ – Kavita Juneja Apr 3 '18 at 20:55
  • $\begingroup$ @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 $\endgroup$ – Waylander Apr 3 '18 at 21:03
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Here is a flowchart of what the commenters have stated. An α-elimination occurs stepwise 1 --> 2 --> 3. There is a phenyl migration (bridged?) to afford tolane 4. Look here for this chemistry.

enter image description here

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