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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?

Mithoron
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Charles
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  • 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. – ASHUTOSH Aug 01 '22 at 15:23

1 Answers1

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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.

orthocresol
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Mithoron
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  • Is there any other similar reaction which you know of? – Charles Oct 06 '15 at 11:41
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    @Charles https://en.wikipedia.org/wiki/Hayashi_rearrangement, https://en.wikipedia.org/wiki/Wagner%E2%80%93Meerwein_rearrangement – Mithoron Oct 06 '15 at 22:23
  • Does the rearrangement in the question asked also have a name? – Charles Oct 07 '15 at 14:35
  • It's not a rearrangement but F-C alkylation, only using unusual reagent. – Mithoron Oct 07 '15 at 14:38
  • What is the major product? Once tert-butyl benzene is formed, the ring is more activated towards substitution – stoic-santiago Jun 30 '18 at 13:20
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    @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. – Mithoron Jun 30 '18 at 16:23
  • @Mithoron Why is acylium cation more stable than tert-butyl cation, exactly? – Yusuf Hasan Oct 25 '18 at 03:02
  • @YUSUFHASAN https://chemistry.stackexchange.com/questions/41162/why-is-the-acylium-ion-stable – Mithoron Oct 25 '18 at 16:44
  • Only one example or do all such tertiary compounds giving stable carbonation give similar reaction? – user137644 Jun 12 '20 at 08:26
  • @user137644 You mean if there's another tertiary group instead of t-butyl, there's gonna be similar effect? Yeah, it could be more or less pronounced, depending on bulky it is and how well is cation stabilised. – Mithoron Jun 12 '20 at 18:45