[SVXX]

Greetings folks. Been a while since I've created a thread...and now I have a bothersome mechanism question which has prompted me to do so!
I discussed this with my friend to no avail : The compound Ph₂C=CHBr when reacted with KNH₂ yields? The answer is Ph-C≡C-Ph.
We figured it would go through either of two mechanisms -

1. The potassium variant of sodamide given here is a strong base. So it abstracts the proton present on the carbon having Br. This forms a carbanion. Now a migration occurs, and the Ph anti in conformation to the anion migrates, somehow causing the formation of the triple bond.

2. An alpha elimination occurs, and both -H and -Br are lost, leading to the formation of a carbene. The -Ph group previously cis in conformation to the bromine atom migrates to fulfill the carbene's need for electrons. Now Ph-C⁺=C^--Ph becomes the product given above.

Which mechanism is correct? And why?

[AWK]

Check reaction of Va and Vb at:

http://article.pubs.nrc-cnrc.gc.ca/ppv/RPViewDoc?issn=1480-3291&volume=33&issue=11&startPage=1633

[SVXX]

Well, this document proves that the answer is indeed correct, but it doesn't seem to indicate what mechanism could have been followed for the reaction(it rather gives experimental procedures and data)...thanks for your help nevertheless! Still need the outline of the mechanism.

[AWK]

Name of reaction is given. Any good textbook, and even wikipedia give you answer

[SVXX]

Name of reaction is given. Any good textbook, and even wikipedia give you answer

It's given?? I didn't find it in the pdf...could you please tell me what the name of the reaction is?

[AWK]

Wolff Kishner

[sjb]

Wolff Kishner

Mis-thread? Whilst I see the similarity, is this not a http://en.wikipedia.org/w/index.php?title=Fritsch%E2%80%93Buttenberg%E2%80%93Wiechell_rearrangement&oldid=319058101 , instead?

[SVXX]

Ah thanks a bunch sjb!! That indeed is the reaction I was looking for. So it confirms the carbene mechanism...just one little doubt, are there any conformational considerations, i.e., does the migrating aryl group have to be syn or anti in conformation wrt leaving group?

[movies]

I found some examples where H is the migrating group (forming a terminal alkyne) and either E or Z alkenes worked, so I would say there is not a geometric requirement for the migration.  If there is indeed a vinylidene intermediate then I don't think you would expect any, but if it were step-wise then you probably would expect it.

[AWK]

Sorry, I just discussed two subjects and put  answer for the second thread. My first link to canadian journal concerns this subject and exclude the second proposal of the mechanism given by SVXX. Canadian authors cited  A.A. Bothner-By, JACS 77 3293-6 (1955) which gives the answer for name of this rearrangement.

[SVXX]

Thanks everyone for your valuable contributions.