Why Chlorobenzene Refuses to React Like Chloroethane

Warm aqueous alkali swaps the chlorine in chloroethane for a hydroxide group, but the chlorine in chlorobenzene survives the same treatment untouched. Haloalkanes can also lose HX to form alkenes, and a few simple halogen compounds — chloroform, iodoform and DDT — have had large practical uses and consequences.

This lesson covers the low reactivity of haloarenes in nucleophilic substitution, elimination reactions of haloalkanes, and the properties and uses of chloroform, iodoform and DDT.

Haloarenes resist nucleophilic substitution because resonance gives the carbon-halogen bond partial double-bond character, the sp2 carbon holds that bond tightly, the phenyl cation is too unstable for SN1, and the ring's electron cloud repels incoming nucleophiles.

Four reasons:

- Resonance — a lone pair on chlorine spreads into the ring, giving the C-Cl bond partial double-bond character
- Hybridisation — the carbon is $\mathrm{s}{\mathrm{p}}^2$, with more s-character than an $\mathrm{s}{\mathrm{p}}^3$ carbon, so the C-Cl bond is shorter and stronger: about 169 pm in chlorobenzene against 177 pm in chloromethane
- Unstable phenyl cation — it cannot be stabilised by resonance, so SN1 is ruled out
- Repulsion — the pi electron cloud repels an approaching nucleophile, and the ring blocks backside attack

Forcing the reaction:

- Chlorobenzene gives phenol only with aqueous sodium hydroxide at about 623 K and high pressure
- Electron-withdrawing groups at ortho and para positions make substitution easier: 4-nitrochlorobenzene reacts at 443 K, and 2,4,6-trinitrochlorobenzene reacts with warm water

Why nitro groups help. They stabilise the negatively charged intermediate formed when the nucleophile attacks — but only from ortho and para positions, where resonance can place the negative charge on the nitro group.

Electrophilic substitution still occurs. The halogen deactivates the ring but directs new groups to the ortho and para positions.

An everyday example. One industrial route to paracetamol, a common fever tablet in India, begins with 4-nitrochlorobenzene, whose nitro group makes the chlorine replaceable by hydroxide.

The substance. The same resonance that strengthens the C-Cl bond also makes chlorine an ortho and para director — one effect explains both behaviours.

In an elimination reaction, a haloalkane heated with alcoholic potassium hydroxide loses a hydrogen from the neighbouring carbon together with the halide, forming an alkene, and Saytzeff's rule predicts the more substituted alkene as the major product.

$$\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{Br}+\mathrm{KOH}\stackrel{\text{ethanol}}{\to }\mathrm{C}{\mathrm{H}}_2=\mathrm{C}{\mathrm{H}}_2+\mathrm{KBr}+{\mathrm{H}}_2\mathrm{O}$$

- Also called dehydrohalogenation or beta-elimination
- The base removes a hydrogen from the carbon next to the one carrying the halogen

Saytzeff rule. When more than one alkene can form, the one with more alkyl groups on the double bond is the major product, so 2-bromobutane gives mainly but-2-ene.

Substitution versus elimination:

- Aqueous KOH favours substitution, giving an alcohol; alcoholic KOH favours elimination, giving an alkene
- Tertiary halides eliminate most readily, while primary halides tend to substitute
- Higher temperature and strong, bulky bases favour elimination

Worked example. 2-Bromo-2-methylbutane with alcoholic KOH can give 2-methylbut-2-ene or 2-methylbut-1-ene; the major product is 2-methylbut-2-ene, with three alkyl groups on its double bond.

Reactions with metals. Haloalkanes form Grignard reagents with magnesium in dry ether, $\mathrm{RX}+\mathrm{Mg}\to \mathrm{RMgX}$, and join together with sodium in the Wurtz reaction.

An everyday example. PVC pipes overheated in a fire release choking hydrogen chloride, because the polymer undergoes elimination, losing HCl from neighbouring carbons.

The substance. The same reagent gives different products depending on its solvent — KOH in water favours substitution, while KOH in ethanol favours elimination.

Chloroform is a volatile solvent that slowly oxidises in air and light to poisonous phosgene, iodoform is a yellow antiseptic solid with a strong smell, and DDT is a powerful but persistent insecticide whose use is now restricted.

**Chloroform, $\mathrm{CHC}{\mathrm{l}}_3$:**

- A dense, sweet-smelling liquid and a good solvent for fats, alkaloids and iodine
- Oxidised by air in light to phosgene: $2CHC{\mathrm{l}}_3+{\mathrm{O}}_2\to 2COC{\mathrm{l}}_2+2HCl$
- Stored in dark bottles filled to the top, often with a little ethanol to destroy any phosgene
- Used as a solvent and to make refrigerants; no longer used as an anaesthetic because it can damage the liver

**Iodoform, $\mathrm{CH}{\mathrm{I}}_3$:

- A yellow crystalline solid with a characteristic smell
- Its antiseptic action comes from the iodine it releases, though its smell has limited its use
- Formed in the iodoform test** when compounds containing $\mathrm{C}{\mathrm{H}}_3\mathrm{CO}-$ or $\mathrm{C}{\mathrm{H}}_3\mathrm{CH}(\mathrm{OH})-$ react with iodine and sodium hydroxide

DDT:

- A chlorinated insecticide that is very effective against mosquitoes that spread malaria
- Not biodegradable and soluble in fat, so it accumulates in fatty tissue and builds up along food chains
- Many insects have become resistant to it, and in India its use is now limited mainly to controlling disease-carrying mosquitoes

Worked example. Ethanol and propan-2-ol give yellow iodoform with iodine and sodium hydroxide; methanol and propan-1-ol do not, because they lack the $\mathrm{C}{\mathrm{H}}_3\mathrm{CH}(\mathrm{OH})-$ group.

An everyday example. Chloroform bottles in school laboratories are brown and filled to the brim, so that neither light nor air can turn the solvent into phosgene.

The substance. DDT's persistence made it both useful and harmful — the stability that keeps it working on sprayed walls also lets it build up in birds and fish.