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In class we were told of an anomalous Friedel–Crafts reaction with a tertiary acid chloride — $\ce{(CH3)3CCOCl}$, or pivaloyl chloride. When this reacts with benzene in the presence of $\ce{AlCl3}$, instead of effecting the normal acylation, it instead breaks down into the tert-butyl carbocation, which then alkylates benzene to form tert-butylbenzene.

I could find no satisfactory explanation for this, however, except for the fact that a tertiary carbocation is a very stable carbocation — and nor did I find any mention of this reaction on searching. Could anyone please detail this further?

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You were on right path; tert-butyl carbocation is quite stable$\ldots$ so stable that the acylium cation, which normally reacts as an electrophile itself, instead decomposes via decarbonylation (loss of stable carbon monoxide molecule).

Generation of t-butyl cation from pivaloyl chloride with Lewis acid

The t-butyl carbocation isn't as stable as the acylium cation (which is stabilised by resonance), but the difference is small enough that with the help of the non-reversibility of CO loss (which is removed from the mixture as a gas) and reduction of the steric strain (which is lower in t-butyl than in acylium), it is enough to make it the main reaction route instead of a side route.

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  • $\begingroup$ Is there any other similar reaction which you know of? $\endgroup$ – Charles Oct 6 '15 at 11:41
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  • $\begingroup$ Does the rearrangement in the question asked also have a name? $\endgroup$ – Charles Oct 7 '15 at 14:35
  • $\begingroup$ It's not a rearrangement but F-C alkylation, only using unusual reagent. $\endgroup$ – Mithoron Oct 7 '15 at 14:38
  • $\begingroup$ What is the major product? Once tert-butyl benzene is formed, the ring is more activated towards substitution $\endgroup$ – arya_stark Jun 30 '18 at 13:20

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