How alkanes, alkenes, alkynes and arenes react
Hydrocarbons contain only carbon and hydrogen. Alkanes (CₙH₂ₙ₊₂) have single bonds and undergo substitution reactions such as free-radical halogenation in sunlight. They show conformations; the staggered form of ethane is more stable than the eclipsed form. Alkenes (CₙH₂ₙ) contain a C=C bond, show geometrical isomerism and undergo addition. Addition of HBr follows Markovnikov’s rule, except in the presence of peroxide, where the anti-Markovnikov product forms. Ozonolysis splits alkenes into carbonyl compounds and helps locate the double bond.
Alkynes (CₙH₂ₙ₋₂) have a C≡C bond; terminal alkynes are weakly acidic and react with sodium. Benzene is aromatic: it is planar, cyclic, fully conjugated and has 6 π electrons, fitting Hückel’s (4n + 2) rule. Because of its stability, it prefers electrophilic substitution, including nitration, halogenation, sulphonation and Friedel–Crafts reactions, over addition. Substituents direct incoming groups; –OH and –CH₃ direct to ortho and para positions, while –NO₂ directs to meta.
Why it matters
Petrol, diesel, LPG and kerosene are mixtures of hydrocarbons, and polythene and PVC are made from alkenes. Aromatic compounds are starting materials for dyes, drugs and explosives. Some aromatic hydrocarbons, such as benzopyrene in smoke, are carcinogenic, which is one reason tobacco smoke is harmful.
Key terms
- Markovnikov’s rule
- In adding HX to an unsymmetrical alkene, hydrogen attaches to the carbon already holding more hydrogens.
- Aromaticity
- The special stability of planar cyclic molecules with (4n + 2) delocalised π electrons.
- Electrophilic substitution
- A reaction in which an electrophile replaces a hydrogen atom on an aromatic ring.
- Ozonolysis
- Cleaving an alkene with ozone to form carbonyl compounds, used to find double bond position.