Topic 16 of 34

The ring and side-chain respond to different conditions

State the catalyst, light condition and temperature before predicting where substitution occurs.

A-Level 9476 (2026-2027)

The ring and side-chain respond to different conditions

State the catalyst, light condition and temperature before predicting where substitution occurs.

Reactions of benzene and methylbenzene
Reaction	Reagents and conditions	Products and catalytic role
Ring chlorination	Cl₂ with anhydrous AlCl₃; electrophilic substitution conditions.	Benzene → chlorobenzene + HCl. AlCl₃ is a Lewis acid catalyst.
Ring bromination	Br₂ with anhydrous AlBr₃.	Benzene → bromobenzene + HBr. AlBr₃ is a Lewis acid catalyst.
Benzene nitration	Concentrated HNO₃ + concentrated H₂SO₄, maintained at 50 °C.	Nitrobenzene + water. H₂SO₄ acts as a Bronsted-Lowry acid catalyst in generating the electrophile.
Methylbenzene nitration	The same concentrated-acid mixture, maintained at 30 °C.	Mainly 2- and 4-nitromethylbenzene under mononitration conditions.
Friedel-Crafts alkylation	Halogenoalkane with anhydrous AlCl₃ or AlBr₃, as appropriate.	Benzene + CH₃Cl → methylbenzene + HCl; the Lewis acid activates the halogenoalkane.
Methyl side-chain halogenation	Cl₂ or Br₂, UV light at room temperature.	C₆H₅CH₃ → C₆H₅CH₂X initially; further substitution can occur.
Complete side-chain oxidation	Hot alkaline KMnO₄ followed by dilute acid, or hot acidified KMnO₄.	Methylbenzene → benzoic acid. The aromatic ring is retained.

Concentrated sulfuric acid protonates nitric acid, enabling formation of NO₂⁺: HNO₃ + H₂SO₄ ⇌ NO₂⁺ + HSO₄^- + H₂O. Nitration then follows electrophilic substitution and the acid catalyst is regenerated. The syllabus explicitly specifies 30 °C for methylbenzene and 50 °C for benzene; these temperatures are part of these named conditions.

Positions relative to an existing substituent at carbon 1
Existing group	Favoured incoming positions	Electronic point
Alkyl, OH, NH₂	2 and 4 (with position 6 equivalent to 2 in the simple monosubstituted case).	Electron donation generally activates the ring and favours ortho/para substitution.
NO₂, COOH, CHO, COR, CN	3 (with position 5 equivalent).	Electron withdrawal generally deactivates the ring and favours meta substitution.
Cl or Br directly on the ring	2 and 4.	Halogens are the useful distinction: overall deactivating by withdrawal, yet ortho/para directing through lone-pair donation.

Activation describes how fast the ring reacts relative to benzene; direction describes where substitution occurs. They are not the same property. Steric hindrance can reduce substitution next to a bulky group, so do not assume that every allowed position is formed in equal amounts.

Worked example

Same starting compound, different reaction site

Compare methylbenzene with Br₂/AlBr₃ and with Br₂/UV at room temperature.

Lewis-acid conditions generate a strong electrophile for the aromatic pi system.
The methyl group directs ring substitution mainly to positions 2 and 4.
UV conditions initiate a radical chain and favour substitution in the alkyl side-chain.

Answer

Br₂/AlBr₃ gives mainly 2- and 4-bromomethylbenzene; Br₂/UV gives C₆H₅CH₂Br initially. Use structures to avoid confusing a ring bromine with a bromomethyl side-chain.

Ordinary vigorous side-chain oxidation requires a hydrogen on the carbon directly attached to the ring. Methylbenzene satisfies this condition: C₆H₅CH₃ + 3[O] → C₆H₅COOH + H₂O. In alkaline oxidation the carboxylate forms first, so acidification is needed to isolate the carboxylic acid.