Hydrocarbons · ⁨Hydrocarbon⁩

Arenes 芳烃 are aromatic hydrocarbons, built on the benzene 苯 ring. The delocalised ring of electrons is stable and electron-rich, so benzene mostly reacts by electrophilic substitution 亲电取代 — keeping the ring — rather than by addition.

Reactions of benzene and methylbenzene

In the side-chain oxidation, the whole side-chain 侧链 (such as the $\text{–CH}_3$ on methylbenzene) is turned into a $\text{–COOH}$ group, giving benzoic acid. In the hydrogenation, three molecules of $\text{H}_2$ add to the benzene ring 苯环 to make a saturated cyclohexane ring.

The mechanism: electrophilic substitution

Take nitration as the example. The acid mix makes the electrophile $\text{NO}_2^+$. Then:

1. the delocalised electrons of the ring form a bond to the electrophile, giving an unstable intermediate.
2. an $\text{H}^+$ is lost from that carbon, which restores the stable ring.

Substitution wins over addition because the delocalisation 离域 (aromatic stabilisation) of the ring is kept. Addition would destroy this stable system, so it is not favoured.

Side-chain or ring?

Where a halogen reacts depends on the conditions:

• with a halogen-carrier catalyst (such as $\text{AlCl}_3$) and no UV light → substitution in the ring.
• with UV light and no catalyst → free-radical substitution in the side-chain.

Directing effects

A group already on the ring decides where the next group goes — its directing effect 定位效应:

Worked example. Methylbenzene and nitrobenzene are each nitrated. Predict where the new $\text{NO}_2$ group goes, and which compound reacts faster. Look at the group already on the ring. In methylbenzene the $\text{–CH}_3$ is an alkyl group: it directs the new group to positions 2 and 4, and it releases electrons into the ring, making the ring more attractive to an electrophile - so methylbenzene nitrates faster than benzene. In nitrobenzene the $\text{–NO}_2$ directs to position 3, and it withdraws electrons from the ring - so nitrobenzene nitrates more slowly than benzene. The group already present controls both the position and the rate, and the two always travel together: 2,4-directors activate the ring, 3-directors deactivate it.

• Benzene undergoes electrophilic substitution, not addition, because addition would destroy the stable delocalised ring.
• Learn nitration (concentrated $\text{HNO}_3$/$\text{H}_2\text{SO}_4$, $50-60\ ^\circ\text{C}$) and halogenation (halogen + $\text{AlCl}_3$) with the electrophile-generating step.
• Compare reactivity: benzene resists addition far more than an alkene because of delocalisation.