18.11 : 弗里德尔-克拉夫茨反应的局限性

Friedel–Crafts reactions suffer from several limitations.

Firstly, Friedel–Crafts alkylations work only with alkyl halides where the halide is attached to an sp³ hybridized carbon. Vinylic and aryl halides cannot be used because they form unstable carbocations under the reaction conditions.

Secondly, Friedel–Crafts alkylations are prone to carbocation rearrangement to form a more stable carbocation. Therefore, the major products have a rearranged carbon skeleton.

No carbocation rearrangements occur for Friedel–Crafts acylations because the acylium ion is resonance-stabilized.

Thirdly, Friedel–Crafts alkylations often undergo polyalkylations, where the electron-donating alkyl group activates the ring towards further alkylation.

Conversely, Friedel–Crafts acylations favor monoacylation. The acyl group, an electron-withdrawing group, deactivates the ring for further substitutions.

Lastly, both the reactions fail on aromatic rings bearing either a strongly electron-withdrawing group or a basic amino group that can be protonated.

Such substituents make the ring electron-deficient, thereby decreasing the ring's reactivity towards further electrophilic substitution.