Q.What is Friedel-Crafts alkylation ?

The Core Intuition

Imagine an aromatic ring — benzene, say — as a very stable, electron-rich cloud. It's happy as it is. To make it react, you need to bring something that wants electrons badly enough to disturb that cloud. That something is an electrophile (electron-lover). The Friedel-Crafts reaction is simply a way to create that electrophile and then let it attack the ring.

The trick is that the electrophile you need — a carbocation (for alkylation) or an acylium ion (for acylation) — is too unstable to exist on its own. So you generate it in the reaction mixture using a Lewis acid catalyst like anhydrous $\ce{AlCl3}$ or $\ce{FeCl3}$.

The Precise Statement

Friedel-Crafts Alkylation: An alkyl group ($\ce{R}$) is introduced onto an aromatic ring using an alkyl halide ($\ce{R-X}$) and a Lewis acid catalyst.

Friedel-Crafts Acylation: An acyl group ($\ce{RCO}$) is introduced onto an aromatic ring using an acyl halide ($\ce{RCO-X}$) and a Lewis acid catalyst.

Both are electrophilic aromatic substitution reactions. The catalyst's job is to polarise the halogen-halide bond so strongly that the alkyl or acyl fragment breaks off as a powerful electrophile.

How It Works — Step by Step

Step 1: Generating the Electrophile

The Lewis acid ($\ce{AlCl3}$) has an empty orbital. It grabs the lone pair on the halogen of the alkyl halide ($\ce{R-X}$). This weakens the $\ce{C-X}$ bond enormously.

For alkylation, the complex $\ce{R-X\bond{->}AlCl3}$ can break to give a carbocation $\ce{R+}$ (if the alkyl group can stabilise a positive charge) or a highly polarised complex that behaves like one.

For acylation, the complex $\ce{RCO-X\bond{->}AlCl3}$ breaks to give an acylium ion $\ce{R-C+=O}$, which is resonance-stabilised and very stable.

Step 2: Attack on the Ring

The electrophile (carbocation or acylium ion) attacks the electron-rich aromatic ring. This forms a sigma complex (arenium ion) — a positively charged, non-aromatic intermediate. The ring loses its aromaticity temporarily, which costs energy, but the positive charge is delocalised over three carbon atoms.

Step 3: Regaining Aromaticity

A base (often the $\ce{AlCl4-}$ counterion) removes a proton from the sigma complex. The pair of electrons that was holding that proton goes back into the ring, restoring aromaticity. The product is an alkylbenzene or acylbenzene.

The Critical Difference Between Alkylation and Acylation