Why does Friedel–Crafts alkylation fail on aniline and nitrobenzene?

Nitrobenzene carries the strongly deactivating –NO2 group, which drains electron density from the ring so the arene can no longer attack the electrophile. Aniline fails for a different reason: its basic –NH2 nitrogen has a lone pair that forms a Lewis acid–base complex with anhydrous AlCl3. This ties up the catalyst and converts the ring into a deactivated anilinium-type system, so neither alkylation nor acylation proceeds normally on aniline.

Why does Friedel–Crafts acylation avoid carbocation rearrangement?

The acylation electrophile is the acylium ion RCO+, which is resonance-stabilised by the adjacent oxygen lone pair (R–C≡O+ form). Because the positive charge is delocalised onto oxygen, the acylium ion is stable and does not rearrange its carbon skeleton. By contrast, alkylation generates a free alkyl carbocation R+, which readily rearranges to a more stable carbocation by hydride or alkyl shifts.

Why does monoacylation stop cleanly while alkylation gives polysubstitution?

In acylation the product is an aryl ketone whose –COR group is electron-withdrawing and deactivates the ring towards a second electrophilic attack, so the reaction stops at one substituent. In alkylation the product carries an alkyl group, which is electron-releasing (+I effect) and activates the ring further; the more reactive product competes for the electrophile and polyalkylation results.

How do you make a straight-chain alkylbenzene such as n-propylbenzene?

Direct Friedel–Crafts alkylation with 1-chloropropane gives mainly isopropylbenzene because the n-propyl carbocation rearranges to the more stable secondary cation. The clean route is to first acylate benzene with propanoyl chloride and AlCl3 to give an aryl ketone, then reduce the carbonyl to a –CH2– group using the Clemmensen reduction (Zn-Hg/HCl) or the Wolff–Kishner reduction (NH2NH2 then KOH/heat). This delivers the unrearranged straight chain.

What is the electrophile in Friedel–Crafts alkylation versus acylation?

In alkylation the electrophile is the alkyl carbocation R+, generated when anhydrous AlCl3 abstracts the halide from the alkyl halide. In acylation the electrophile is the acylium (acyl) cation RCO+, generated when AlCl3 abstracts the chloride from an acyl chloride or coordinates to an anhydride.

Why must the AlCl3 catalyst be anhydrous?

Aluminium chloride is the Lewis acid that pulls the halide off the alkyl or acyl halide to generate the electrophile. Water hydrolyses AlCl3 to aluminium hydroxide and hydrogen chloride, destroying its Lewis-acid character, so the electrophile is never formed and the reaction fails. The catalyst must therefore be strictly anhydrous.

Friedel–Crafts Alkylation & Acylation — NEET Chemistry

What the Friedel–Crafts reactions do

Benzene is electron-rich and reacts with electron-deficient species rather than adding across its ring like an alkene. The NIOS chapter on hydrocarbons lists four characteristic electrophilic substitutions of benzene — halogenation, nitration, sulphonation and the Friedel–Crafts reaction. The Friedel–Crafts family is the one that builds carbon–carbon bonds onto the ring, and it comes in two flavours that share a single requirement: anhydrous aluminium chloride as the catalyst.

In alkylation, benzene is heated with an alkyl halide; in acylation, it is heated with an acyl halide or acid anhydride. The skeleton of each reaction is the same — an electrophile is generated, the ring attacks it, and a proton is lost to restore aromaticity.

Feature	Alkylation	Acylation
Reagent	R–X (alkyl halide)	RCOCl or (RCO)₂O
Catalyst	anhydrous AlCl₃	anhydrous AlCl₃
Electrophile	carbocation R⁺	acylium ion RCO⁺
Group added	alkyl, –R	acyl, –COR (a ketone)
Product class	alkylbenzene	aryl ketone

Friedel–Crafts alkylation

Here benzene reacts with an alkyl halide in the presence of anhydrous aluminium chloride to give an alkylbenzene. The simplest NIOS example uses chloromethane to make toluene:

$$\ce{C6H6 + CH3Cl ->[\text{anhyd. } AlCl3][\Delta] C6H5CH3 + HCl}$$

The hydrogen on the ring is replaced by the methyl group, and HCl leaves. Written generally, with any alkyl halide R–X:

$$\ce{C6H6 + R-X ->[\text{anhyd. } AlCl3] C6H5-R + HX}$$

The reactive species that the ring actually attacks is not the neutral halide but the carbocation $\ce{R+}$ that aluminium chloride pulls out of it. The general principles of organic reactions describe exactly this kind of electron-deficient, electron-seeking carbon as the alkyl carbocation, and list such cations among the electrophiles that attack positions of high electron density.

Friedel–Crafts acylation

In acylation, benzene is heated with an acyl halide (or an acid anhydride) and anhydrous aluminium chloride; the product is an aromatic ketone. The standard NIOS example uses acetyl chloride to give acetophenone:

$$\ce{C6H6 + CH3COCl ->[\text{anhyd. } AlCl3][\Delta] C6H5COCH3 + HCl}$$

Here the group introduced is the acyl group $\ce{-COCH3}$, so the ring now carries a carbonyl. Using an anhydride works the same way:

$$\ce{C6H6 + (CH3CO)2O ->[\text{anhyd. } AlCl3] C6H5COCH3 + CH3COOH}$$

The electrophile is the acylium ion $\ce{RCO+}$ (for acetyl, $\ce{CH3CO+}$). The general-principles chapter lists $\ce{CH3CO+}$ explicitly among the recognised electrophiles. The acylium ion is stabilised by resonance with the oxygen lone pair, a fact that turns out to control the whole personality of acylation.

Build the toolkit

Friedel–Crafts is one node in a web of named conversions. See how rings and chains are stitched together in Wurtz & Wurtz–Fittig reactions.

Mechanism as electrophilic aromatic substitution

Both reactions follow the same three-step electrophilic aromatic substitution (EAS) pattern. NIOS frames every aromatic substitution this way: an electrophile attacks the electron-rich ring and one ring hydrogen is the leaving group.

Step 1 — Generate the electrophile

Aluminium chloride, a Lewis acid, abstracts the leaving group from the reagent. For alkylation it pulls off the halide; for acylation it pulls the chloride off an acyl chloride. This heterolytic fission is the carbocation-forming step described in the general-principles chapter.

$$\ce{R-Cl + AlCl3 -> R+ + AlCl4-}$$

$$\ce{CH3CO-Cl + AlCl3 -> CH3CO+ + AlCl4-}$$

Figure 1 · Electrophile generation

The acylium ion is drawn with a C≡O triple-bond resonance form because the oxygen lone pair delocalises the positive charge — the structural reason it stays intact.

Step 2 — Form the arenium ion

The π electrons of benzene attack the electrophile, breaking aromaticity and producing a resonance-stabilised carbocation called the arenium ion (or sigma complex). The positive charge is spread over three ring carbons.

Figure 2 · The arenium-ion intermediate

E⁺ is the electrophile — R⁺ for alkylation, RCO⁺ for acylation. The dashed arc in the middle ring marks the delocalised positive charge of the non-aromatic arenium intermediate.

Step 3 — Lose a proton

The arenium ion is not aromatic and is therefore unstable. It sheds the proton from the carbon now bearing the electrophile; $\ce{[AlCl4]-}$ takes up that proton, regenerating both the aromatic ring and the aluminium chloride catalyst as $\ce{HCl}$ is released. Because aromaticity is recovered, the ring substitutes rather than adds — the central reason benzene behaves so differently from an alkene.

Limitations of alkylation

Friedel–Crafts alkylation is plagued by three problems, all traceable to the free carbocation electrophile.

1. Carbocation rearrangement

A free carbocation seeks the most stable arrangement available. The general-principles chapter notes that, in the presence of a Lewis acid such as $\ce{AlCl3}$, alkyl skeletons rearrange — for example 1-chlorobutane converts to 2-chlorobutane. The same drive operates here. The order of carbocation stability follows the +I (electron-releasing) order of the chapter, tertiary > secondary > primary:

$$\ce{(CH3)3C+ > (CH3)2CH+ > CH3CH2+}$$

So when 1-chloropropane is used to alkylate benzene, the primary $\ce{CH3CH2CH2+}$ rearranges by a hydride shift to the more stable secondary $\ce{CH3\overset{+}{C}HCH3}$. The product is mostly isopropylbenzene (cumene), not the intended n-propylbenzene.

NEET Trap

"n-propyl chloride must give n-propylbenzene"

A common error is to assume the alkyl group transfers unchanged. The primary propyl cation rearranges before it reaches the ring, so the major product is the branched isopropylbenzene. Whenever a straight-chain primary or longer alkyl halide is offered for direct alkylation, suspect rearrangement.

Rule: direct Friedel–Crafts alkylation cannot reliably install an unrearranged straight chain longer than ethyl.

2. Polyalkylation

The alkyl group introduced is electron-releasing (+I effect), so the product alkylbenzene is more reactive towards electrophiles than the starting benzene. The freshly made product therefore competes for the electrophile and gets alkylated again, giving di- and poly-substituted mixtures that are hard to control.

3. Failure on deactivated rings and on aniline

Strongly deactivating, electron-withdrawing groups starve the ring of the electron density needed to attack the electrophile. NIOS classes $\ce{-NO2}$ as a meta-directing deactivator; nitrobenzene therefore does not undergo Friedel–Crafts reactions. Aniline fails for a separate, subtler reason: its basic $\ce{-NH2}$ nitrogen has a lone pair that forms a Lewis acid–base complex with $\ce{AlCl3}$. This consumes the catalyst and leaves the nitrogen positively polarised, deactivating the ring.

Ring substituent	Effect on ring	Friedel–Crafts?
–OH, –OCH₃, –CH₃ (activating)	electron-releasing	Proceeds (alkylation may over-react)
–NO₂, –SO₃H, –COR (deactivating)	electron-withdrawing	Fails — ring too poor
–NH₂ (aniline)	complexes AlCl₃ at N	Fails — catalyst tied up

Why acylation is clean

Acylation escapes two of these three problems, and the reason is the same in both cases: the acylium ion.

Figure 3 · Alkylation vs acylation contrast

The single structural fact — a stable acylium versus a free carbocation — explains both the absence of rearrangement and the absence of polysubstitution in acylation.

No rearrangement

The acylium ion $\ce{RCO+}$ is resonance-stabilised: the oxygen lone pair delocalises the positive charge to give a $\ce{R-C#O+}$ contributor. A stabilised ion has no thermodynamic incentive to rearrange its carbon skeleton, so the acyl group transfers to the ring exactly as drawn. The straight chain is preserved.

No polysubstitution

The product of acylation is an aryl ketone, and the $\ce{-COR}$ group is electron-withdrawing — it deactivates the ring towards further electrophilic attack. The product is therefore less reactive than the starting benzene, so the reaction stops cleanly after one acyl group has been added. Monoacylation is the rule.

NEET Trap

Acylation still fails on deactivated rings

Acylation cures rearrangement and polysubstitution, but it does not overcome strong deactivation. Nitrobenzene resists acylation just as it resists alkylation, and aniline still ties up the $\ce{AlCl3}$ at nitrogen. The acylium advantage is about the electrophile, not about the ring's electron density.

Rule: a strongly deactivated ring blocks both Friedel–Crafts variants.

Ketone to straight-chain alkyl

The two facts above combine into the classic NEET strategy for making a straight-chain alkylbenzene without rearrangement. Rather than alkylate directly, you acylate first, then reduce the carbonyl to a methylene. The acylium delivers the unrearranged chain, and a carbonyl-to-CH₂ reduction removes the oxygen.

Two reductions reach the same destination:

Method	Reagent	Conditions suited to
Clemmensen reduction	Zn–Hg (zinc amalgam) / conc. HCl	acid-stable substrates
Wolff–Kishner reduction	NH₂NH₂, then KOH/ethylene glycol, heat	base-stable substrates

Both convert the carbonyl group $\ce{>C=O}$ of an aldehyde or ketone into $\ce{>CH2}$. The NIOS hydrocarbons chapter independently records that aldehydes and ketones can be reduced to alkanes with HI in the presence of red phosphorus, the same net carbonyl-to-CH₂ conversion in aliphatic form:

$$\ce{RCOR' + 4HI ->[\text{red P}][423\,K] RCH2R' + 2I2 + H2O}$$

For the aromatic case the two-step Friedel–Crafts route gives the clean product:

$$\ce{C6H6 + CH3CH2COCl ->[\text{anhyd. } AlCl3] C6H5COCH2CH3}$$

$$\ce{C6H5COCH2CH3 ->[\text{Zn-Hg, HCl}] C6H5CH2CH2CH3}$$

The product is n-propylbenzene — the very compound that direct alkylation could not deliver, because the acylium chain never rearranged and the carbonyl was simply stripped to a methylene.

Worked examples

Worked Example 1

Predict the major product when benzene is treated with 1-chloropropane and anhydrous AlCl₃.

AlCl₃ abstracts chloride to give the primary cation $\ce{CH3CH2CH2+}$, which rearranges by a 1,2-hydride shift to the more stable secondary cation $\ce{(CH3)2CH+}$. The ring attacks the rearranged cation, so the major product is isopropylbenzene (cumene), not n-propylbenzene.

$$\ce{C6H6 + CH3CH2CH2Cl ->[AlCl3] C6H5CH(CH3)2 + HCl}$$

Worked Example 2

How would you prepare n-propylbenzene cleanly from benzene?

Avoid direct alkylation. Acylate benzene with propanoyl chloride and AlCl₃ to give 1-phenylpropan-1-one (ethyl phenyl ketone); the acylium does not rearrange. Then reduce the carbonyl to a methylene with Clemmensen (Zn-Hg/HCl) or Wolff–Kishner (NH₂NH₂, KOH, heat).

$$\ce{C6H6 ->[CH3CH2COCl,\ AlCl3] C6H5COCH2CH3 ->[Zn\text{-}Hg / HCl] C6H5CH2CH2CH3}$$

Worked Example 3

Why does acetyl chloride with AlCl₃ give only acetophenone and not a polysubstituted product, while methyl chloride tends to over-react?

Acetophenone carries the electron-withdrawing $\ce{-COCH3}$ group, which deactivates the ring, so a second electrophile is not accommodated and the reaction stops at monosubstitution. Toluene, the product from methyl chloride, carries the electron-releasing $\ce{-CH3}$ group, which activates the ring and invites further alkylation — hence polysubstituted mixtures.

Quick Recap

Friedel–Crafts in one screen

Alkylation: $\ce{ArH + R-X ->[AlCl3] Ar-R}$; electrophile = carbocation $\ce{R+}$.
Acylation: $\ce{ArH + RCOCl ->[AlCl3] Ar-COR}$; electrophile = acylium $\ce{RCO+}$.
Both follow EAS: generate electrophile → arenium ion → lose H⁺ to restore aromaticity.
Alkylation suffers carbocation rearrangement and polyalkylation; both arise from the free carbocation and the activating product.
Acylation avoids both — the acylium is resonance-stabilised (no rearrangement) and the $\ce{-COR}$ product deactivates the ring (no polysubstitution).
Neither variant works on strongly deactivated rings (nitrobenzene) or on aniline, whose $\ce{-NH2}$ complexes the $\ce{AlCl3}$.
For an unrearranged straight chain: acylate, then reduce the ketone with Clemmensen (Zn-Hg/HCl) or Wolff–Kishner (NH₂NH₂/KOH, Δ).

Friedel–Crafts Alkylation & Acylation