The following reaction was given, which uses potassium t-butoxide.

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The solution to this problem was given as follows:

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Step 1 is the attack of strong base, t-butoxide, on the vinyl proton.

Step 2 seems to have the chlorophenyl group shift to the neighbouring carbon, forcing the cleavage of the $\ce{C–Br}$ bond.

Here, the bromine is conjugated to the alkene, since the lone pair on bromine is in resonance with the alkene, leading to partial double bond character between $\ce{C}$ and $\ce{Br}$. In such a case, the $\ce{C–Br}$ bond usually cannot be cleaved.

My question is

  • Is this carbanion intermediate responsible for the product?
  • What is the mechanism to form 1-chloro-4-(phenylethynyl)benzene?

1 Answer 1


As your heading correctly states, this reaction take the path of Fritsch–Buttenberg–Wiechell rearrangement (Wikipedia). The reaction mechanism of Fritsch–Buttenberg–Wiechell rearrangement (FBW) is described as:

The strong base deprotonates the vinylic hydrogen, which after alpha-elimination forms a vinyl carbene. A 1,2-aryl migration forms the 1,2-diaryl-alkyne product. The mechanism of the FBW rearrangement was a subject of on-surface studies where the vinyl radical was visualized with sub-atomic resolution (Ref.1).

However, if you carefully read this reference, you'd realize they use special method to generate diradical from gem-dibromo-alkene stepwise. Thus, the proposed mechanism for every FBW-type reactions. Earlier, A. A. Bothner-By (Ref.2) from Harvard reported that in a diaryl-substituted system of a bromoalkene (e.g., see structures 1 and 4 in the scheme 1), the major migrating group is the aryl moiety located anti to the halogen (the fact, which has been proven by $\ce{^{14}C}$-labeling experiments). Particularly, this migration of an aryl group between multiply-bonded carbons is one of the several unusual features of the rearrangement (Ref.2):


Yet, there are plethora of data to support the 1,2-phenyl group migration through single bonds (e.g., the benzilic acid rearrangement), and at least one example for migration of an aryl group between multiply-bonded carbons (e.g., the Beckmann rearrangement). Thus, based on these facts, I suggests following arrow-pushing mechanism, which would be very reasonable:

Alkene-Alkyne Mechanism


  1. N. Pavliček, P. Gawel, D. R. Kohn, Z. Majzik, Y. Xiong, G. Meyer, H. L. Anderson, L. Gross, “Polyyne formation via skeletal rearrangement induced by atomic manipulation,” Nature Chemistry 2018, 10, 853–858 (https://www.nature.com/articles/s41557-018-0067-y).
  2. Aksel A. Bothner-By, “Stereochemistry of the rearrangement involving migration between multiply-bonded carbons,” J. Am. Chem. Soc. 1955, 77(12), 3293–3296 (https://doi.org/10.1021/ja01617a042).

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