Unusual Friedel–Crafts alkylation with pivaloyl chloride

Question

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?

Answer 1

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).

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.