Organic chemistry: hydrocarbons and halogenoalkanes

Basics

Homologous series, functional groups, IUPAC naming and isomerism: structural (chain, position, functional group) and stereoisomerism (geometric cis/trans around C=C; optical, with a chiral carbon bonded to four different groups).

Alkanes

Saturated; burn in excess oxygen to CO₂ and H₂O (incomplete combustion gives CO). Free-radical substitution with chlorine in UV light: initiation (Cl₂ → 2Cl•), propagation (Cl• + CH₄ → HCl + •CH₃; •CH₃ + Cl₂ → CH₃Cl + Cl•), termination (radicals combine). Cracking produces smaller alkanes and alkenes.

Alkenes

The C=C has a σ and a π bond. Electrophilic addition: with Br₂ (test: decolourises), HBr, H₂O (steam with H₃PO₄ catalyst, making ethanol), H₂ (Ni catalyst). Markovnikov's rule: H adds to the carbon with more H atoms, via the more stable carbocation. Addition polymerisation makes poly(ethene) and PVC.

Arenes (benzene)

Delocalised π electrons give extra stability; benzene undergoes electrophilic substitution (nitration with concentrated HNO₃/H₂SO₄; halogenation with an AlCl₃ or FeBr₃ catalyst) rather than addition.

Halogenoalkanes

Nucleophilic substitution: with OH⁻ (warm aqueous NaOH) → alcohol; with CN⁻ (ethanol) → nitrile (adds a carbon); with NH₃ → amine. Primary halogenoalkanes react by SN2; tertiary by SN1. Rate of hydrolysis: C–I > C–Br > C–Cl (bond strength). Elimination with ethanolic KOH gives alkenes. CFCs damage the ozone layer.

Check yourself

1. What type of reaction occurs between ethene and bromine?
2. Name the mechanism by which methane reacts with chlorine in UV light.
3. Which halogenoalkane hydrolyses fastest: chloro-, bromo- or iodoethane?