Among the given halides, which one can be used as halide component for Friedel–Crafts reaction?

(a) Isopropyl chloride
(b) Bromobenzene
(c) Chlorobenzene
(d) Chloroethene

I don't understand what a "halide component" is. Is it the reactant that we use with existing molecule (a simple benzene ring, for example) on which we want to perform FC reaction on, or the reagent itself (i.e. anhyd. $\ce{AlCl3}$)?

  • 4
    $\begingroup$ Obviously, none of these is AlCl3. $\endgroup$ Dec 1 '21 at 4:58
  • $\begingroup$ Can you please elaborate? $\endgroup$
    – HarshDarji
    Dec 1 '21 at 5:24

Friedel–Crafts reactions proceed through the alkylation or acylation of an aromatic ring using an alkyl halide. Considering the alkyl halide compound of the form


where $\ce{R}$ is the alkyl group and $\ce{X}$ is the halide (whatever it may be), I assume the question is asking what compound, of the previously noted form, could be added to an aromatic ring using electrophilic aromatic substitution.

Given this, I would suggest imagining which of these four compounds are actually alkyl halides at all — only one of them will work for Friedel–Crafts alkylation. It's also important to understand one of the intermediate steps in the alkylation mechanism. Formation of the carbocation (if this is preferred) or formation of the complex with the Lewis acid catalyst is preferable only for alkyl halides.

This is due to the fact that carbocations are increasingly unstable as hybridization shifts (when there's double bond character).


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