Hydrocarbons
Unit 11 tests the preparation, structure and characteristic reactions of alkanes, alkenes, alkynes and aromatic hydrocarbons.
Core rules
- Alkanes are sp³, saturated, and react mainly by free-radical substitution (halogenation under UV light).
- Alkenes are sp² with a π bond; they undergo electrophilic addition, and Markovnikov's rule places the hydrogen on the carbon with more hydrogens. In the presence of peroxide, HBr adds anti-Markovnikov.
- Alkynes are sp hybridised; terminal alkynes are acidic and form acetylides with sodium or ammoniacal silver nitrate. They add hydrogen in two stages.
- Ozonolysis cleaves C=C to carbonyl compounds and locates the double bond.
- Benzene is planar, with six delocalised π electrons and Hückel's 4n + 2 rule; it undergoes electrophilic substitution (nitration, sulphonation, halogenation, Friedel–Crafts) rather than addition.
Worked example
Propene (CH₃–CH=CH₂) reacts with HBr. Markovnikov addition puts H on C-1 and Br on C-2, giving 2-bromopropane. With benzoyl peroxide, the bromine radical adds first to C-1, giving the more stable secondary radical, so the product is 1-bromopropane.
Traps
- The peroxide effect works for HBr but not for HCl or HI.
- Benzene prefers substitution because the aromatic stabilisation energy would be lost on addition.
- cis/trans isomerism requires restricted rotation and two different groups on each double-bond carbon.
- Terminal alkynes react with ammoniacal AgNO₃; internal alkynes do not.
- Complete ozonolysis of a double bond gives two carbonyl fragments.