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In the reaction of alkylation of aromatic systems with the help of Friedel Crafts alkylation - the reagent used is alkyl halide in presence of appropriate Lewis acid.

When is rearrangement observed?

If the alkyl halide is primary, the carbocation is usually not stable - as a result, a free carbocation shouldn't be formed and the attack should occur from the acid base complex itself (this is what my instructor said). However, in cases where the alkyl halide is tertiary, or where the carbocation is stable enough to be formed - rearrangement (if possible) is observed and the reaction proceeds through a free carbocation intermediate which acts as electrophile.

So in the former case, where a full fledged carbocation isn't formed, is rearrangement possible?

For example, is rearrangement possible in alkylation of benzene with n-propyl chloride in presence of aluminium chloride? Clearly, the primary carbocation is not stable - and may arrange to secondary carbocation (only if it is formed in first place)

So what really happens, and when in general and in particular can we expect rearrangements in Friedel Crafts alkylation? Also, when not to expect rearrangements? (Of course, when the carbocation isn't formed, but when is that?)

It'd be great if you could quote some credible sources along with your answer, if you wish to add certain specific reactions etc.

P.S.

  • Which Lewis acids allow for rearrangement to occur? As far as I know, only in presence of stronger Lewis acids does rearrangement occur - but how strong?
  • Does temperature also play a role for rearrangement to occur?
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When you conduct the alkylation of benzene with n-propyl chloride, the product you obtain is isopropylbenzene, so a rearrangement occurred by virtue of the fact that the primary carbocation that forms will undergo a 1,2-hydride shift to form a secondary one. Most alkylation ( aromatic ring and alkyl halides ) are conducted in the presence of aluminium chloride which is one of the strongest Lewis acids. The principle is that the alkyl halide reacts with the aluminium chloride to form a salt: a carbocation and a complex ion of tetrachloroaluminate. You can have methyl chloride react in this way because even if it forms an unstable carbocation, it will readily react with the aromatic ring ( which acts as a Lewis base ). You can have methyl carbocation in solution such as in alkylation reactions, but you can’t isolate a salt containing methyl carbocation in solid state. If you had methyl chloride and aluminium chloride in solid state ( in contact with each other, they would not react to form carbocations.

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  • $\begingroup$ Actual methyl cations in solution react to make t-butyl cations and hydrogen and are made with superacids not AlCl3, which makes adduct with methyl chloride. $\endgroup$ – Mithoron Dec 24 '18 at 19:34

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