Why Benzene Would Rather Swap an Atom Than Add One

Benzene, C${}_6$H${}_6$, is usually drawn with three double bonds. Yet it does not decolourise bromine water the way alkenes do, and it prefers to swap a hydrogen for another group rather than add atoms across its bonds.

The reason is aromaticity — a special stability that comes from electrons spread evenly around the ring.

This part covers benzene's structure and Huckel's rule, naming and preparation, electrophilic substitution, and the directing effects of substituents.

**Benzene is a resonance hybrid of two structures with alternating double bonds; its six sp${}^2$ carbons overlap their p orbitals into a ring of six delocalised pi electrons above and below the plane; and a ring is aromatic if it is planar, cyclic, fully conjugated and has $(4n+2)$ pi electrons.

Evidence. All six C–C bonds in benzene are $139$ pm**, between a single bond ($154$ pm) and a double bond ($134$ pm).

Worked example 1 — extra stability. Adding hydrogen to one C=C of cyclohexene releases about $120$ kJ/mol, so three separate double bonds should release about $3\times 120=360$ kJ/mol. Benzene releases only about $208$ kJ/mol — roughly **$150$ kJ/mol more stable than expected.

Worked example 2 — Huckel's rule.

- Benzene**: $6$ pi electrons, $n=1$ — aromatic
- Naphthalene: $10$ pi electrons, $n=2$ — aromatic
- Cyclopentadienyl anion: $6$ pi electrons — aromatic
- Cyclopentadienyl cation: $4$ pi electrons — not aromatic
- Cyclooctatetraene: $8$ pi electrons and non-planar — not aromatic

An everyday example. Common medicines such as paracetamol and aspirin are built around a benzene ring, whose stability survives in the body.

The substance. Huckel's rule applies only to planar, fully conjugated rings — counting pi electrons is not enough on its own.

Monosubstituted benzenes are named by adding the substituent's name to "benzene" or by accepted names such as toluene, phenol and aniline; disubstituted benzenes use locants 1,2 (ortho), 1,3 (meta) or 1,4 (para); and benzene is prepared by polymerising ethyne, decarboxylating sodium benzoate or reducing phenol.

Monosubstituted: methylbenzene (toluene), hydroxybenzene (phenol), aminobenzene (aniline), nitrobenzene, chlorobenzene, benzoic acid.

Worked example 1 — disubstituted names.

- Two methyl groups side by side — 1,2-dimethylbenzene (ortho-xylene)
- Two chlorines opposite each other — 1,4-dichlorobenzene (para)
- Cl and NO${}_2$ opposite each other — 1-chloro-4-nitrobenzene, with substituents in alphabetical order
- OH and NO${}_2$ side by side — 2-nitrophenol, since OH gives the parent name and takes position 1

Preparation:

- From ethyne: three molecules passed through a red-hot iron tube form benzene
- Decarboxylation: sodium benzoate heated with soda-lime gives benzene and Na${}_2$CO${}_3$
- Reduction of phenol: heating phenol with zinc dust gives benzene

Worked example 2 — yield. $14.4$ g of sodium benzoate ($144$ g/mol) is $0.1$ mol, giving $0.1\times 78=7.8$ g of benzene.

An everyday example. Toluene is the solvent in many paint thinners, a benzene ring carrying one methyl group.

The substance. Ortho, meta and para describe only disubstituted rings; three or more groups need numbered locants.

**Each is electrophilic substitution in three steps: a strong electrophile is generated, it attacks the pi cloud to form a positively charged arenium ion that has lost aromaticity, and the ring loses H${}^{+}$ to regain its aromatic stability.

The four reactions:

- Nitration** — concentrated HNO${}_3$ and H${}_2$SO${}_4$ generate the nitronium ion, NO${}_2^{+}$; the product is nitrobenzene
- Sulphonation — fuming sulphuric acid supplies SO${}_3$; the product is benzenesulphonic acid
- Halogenation — Cl${}_2$ with anhydrous FeCl${}_3$ in the dark forms an electrophilic chlorine; the product is chlorobenzene
- Friedel-Crafts alkylation — CH${}_3$Cl with anhydrous AlCl${}_3$ gives CH${}_3^{+}$; the product is toluene
- Friedel-Crafts acylation — CH${}_3$COCl with AlCl${}_3$ gives the acylium ion CH${}_3$CO${}^{+}$; the product is acetophenone

Worked example — nitration yield. $7.8$ g of benzene ($0.1$ mol) gives up to $0.1$ mol of nitrobenzene ($123$ g/mol):

$$0.1\times 123=12.3\text{ g}$$

Addition is possible, but only under harsh conditions. In ultraviolet light with excess chlorine, benzene adds six chlorine atoms, losing its aromaticity.

An everyday example. Aniline dyes used for colouring fabrics are made from nitrobenzene, which comes from the nitration of benzene.

The substance. **The arenium ion loses H${}^{+}$ instead of adding a nucleophile**, because substitution restores the stable aromatic ring.

**Groups that release electrons into the ring, such as –OH, –NH${}_2$, –OCH${}_3$ and –CH${}_3$, activate it and direct new groups to ortho and para positions; groups that withdraw electrons, such as –NO${}_2$, –CN, –CHO, –COOH and –SO${}_3$H, deactivate it and direct to meta; halogens deactivate but still direct ortho and para.

Why.** –OH donates a lone pair into the ring by resonance, building up extra electron density at the ortho and para carbons. –NO${}_2$ withdraws electrons, leaving the ortho and para carbons most electron-poor, so attack happens at meta.

Worked examples.

- Nitrating toluene — mainly 2-nitrotoluene and 4-nitrotoluene
- Nitrating nitrobenzene — mainly 1,3-dinitrobenzene, and more slowly than benzene
- Chlorinating chlorobenzene — 1,2- and 1,4-dichlorobenzene

Halogens. Their strong –I effect makes the ring less reactive, but their lone pairs still favour ortho and para attack.

Polynuclear aromatic hydrocarbons. Compounds with several fused benzene rings, such as benzopyrene, form when tobacco, coal or petroleum burns incompletely. They are toxic and carcinogenic because, after changes in the body, they can damage DNA.

An everyday example. Smoke from burning rubbish and from tobacco contains polynuclear aromatic hydrocarbons, one reason such smoke is harmful to breathe.

The substance. Halogens are the key exception — deactivating yet ortho-para directing.