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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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  • $\begingroup$ As many reasons already mentioned in previous answers. I also have one reason in mind. I think we can also say that as CO is released, with the evolution of gas , entropy of the reaction increases which makes it more favourable to occur. $\endgroup$
    – ASHUTOSH
    Aug 1, 2022 at 15:23

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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, 2015 at 11:41
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  • $\begingroup$ Does the rearrangement in the question asked also have a name? $\endgroup$
    – Charles
    Oct 7, 2015 at 14:35
  • $\begingroup$ It's not a rearrangement but F-C alkylation, only using unusual reagent. $\endgroup$
    – Mithoron
    Oct 7, 2015 at 14:38
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    $\begingroup$ @schrodinger_16 I'd expect some 1,4-bis(tert-butyl)benzene, but tert-butyl benzene should be main product if C6H6 is used in excess. $\endgroup$
    – Mithoron
    Jun 30, 2018 at 16:23

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