In Section 23.7.4, we learned that Friedel–Crafts alkylation suffers from overalkylation.
(a) Draw the product of a Friedel–Crafts reaction that resulted in three methyl groups adding to the ring.
(b) Why is hard to stop at the addition of one alkyl group?

Friedel–Crafts alkylation is an electrophilic aromatic substitution reaction where an alkyl group is introduced to an aromatic ring using an alkyl halide and a Lewis acid catalyst. This reaction allows for the formation of new carbon-carbon bonds, enhancing the complexity of aromatic compounds. However, it often leads to multiple alkylation events, resulting in overalkylation.

Overalkylation refers to the phenomenon where multiple alkyl groups are added to an aromatic ring during a Friedel–Crafts reaction, rather than just one. This occurs because the alkylated aromatic compound can still act as a nucleophile, making it susceptible to further electrophilic attack. The stability of the resulting carbocation intermediates also contributes to this tendency.

Carbocation stability is a key factor in determining the reactivity of intermediates in organic reactions. In Friedel–Crafts alkylation, the formation of a stable carbocation from the alkyl halide is crucial for the reaction to proceed. More substituted carbocations are generally more stable, which can lead to increased reactivity and the likelihood of multiple alkylation events occurring.