30. Hydrocarbons

Benzene and methylbenzene are arenes: aromatic rings with delocalised π electrons. Their characteristic reactions replace a ring hydrogen rather than add across the ring, preserving aromatic stabilisation.

Typical syllabus reactions include nitration, halogenation and Friedel–Crafts-type substitution. A methyl group activates the ring and directs substitution mainly to the 2- and 4-positions relative to itself.

Benzene gives nitrobenzene on nitration; methylbenzene gives a mixture of 2- and 4-nitromethylbenzene under comparable conditions.

Arene substitution is not ordinary alkene addition: the ring regains aromaticity after the substitution step.

In electrophilic substitution, an electron-rich arene attacks an electrophile to form a positively charged intermediate. A base then removes H⁺, restoring the delocalised π system.

The mechanism explains both the need for an electrophile and the final substitution product: addition is temporary, while loss of the ring proton regenerates aromatic stabilisation.

During nitration, NO₂⁺ attacks benzene, the sigma complex forms, and removal of H⁺ gives nitrobenzene plus restored aromaticity.

Do not draw a permanent addition product or leave the ring with one fewer π bond; the final product is aromatic again.

A methyl side chain and an aromatic ring react by different pathways. Radical conditions such as UV light favour substitution in the side chain, while a Lewis-acid catalyst with a halogen favours electrophilic substitution on the ring.

Look first for the reagent, catalyst and light/heat conditions; the same methylbenzene can therefore give different products.

Methylbenzene with Cl₂ under UV gives side-chain chlorination, whereas Cl₂/AlCl₃ gives ring chlorination, mainly the 2- and 4-isomers.

Do not choose a product from the substrate name alone. Conditions select the mechanism and therefore the position of substitution.

A substituent changes both the rate and the position of electrophilic substitution on benzene. The syllabus directing set includes –NH₂, –OH, –R, –NO₂, –COOH and –COR.

Electron-donating groups generally direct incoming electrophiles to the 2- and 4-positions; electron-withdrawing groups generally direct to the 3-position. The directing rule predicts mixtures, not one guaranteed product.

Methylbenzene gives mainly 2- and 4-nitro products, while nitrobenzene directs further substitution away from those positions, predominantly to the 3-position.

“Activating” and “directing” are different ideas: a group can slow the reaction yet still direct where substitution occurs.