Friedel-crafts Reactions

Friedel-Crafts acylation introduces an acyl group onto an aromatic ring using an acid chloride or anhydride together with a Lewis acid catalyst such as anhydrous $AlC{l}_3$. The catalyst polarizes acetyl chloride to generate the acylium ion $C{H}_{3}C{O}^{+}$, which acts as the electrophile and attacks benzene. After loss of a proton to restore aromaticity, the product is acetophenone, $C_6{H}_{5}COC{H}_3$, an aryl ketone. Toluene is wrong because it would form from Friedel-Crafts alkylation with a methyl halide, not from an acyl chloride. Benzaldehyde is incorrect because formylation requires special reagents such as carbon monoxide and HCl (the Gattermann-Koch reaction), not acetyl chloride. Ethylbenzene would require alkylation with an ethyl group and does not arise from acylation, so it does not apply. A useful advantage of acylation is that, unlike alkylation, it does not suffer from carbocation rearrangement or polysubstitution because the acyl product deactivates the ring. The acylium ion is also resonance-stabilised, with the positive charge shared between carbon and oxygen, which makes it a well-defined electrophile that does not rearrange to a different skeleton. This is the NCERT account of Friedel-Crafts acylation and explains why ketones rather than rearranged alkyl products are obtained cleanly. A consistency check: the carbonyl-containing ketone is the logical product of adding an acyl, not an alkyl, group.