Key ideas
Hydrocarbons contain only carbon and hydrogen. Alkanes are fairly unreactive, while alkenes and alkynes take part in addition reactions at their pi bonds.
- Alkanes (CₙH₂ₙ₊₂) burn and undergo radical substitution with halogens in light. Alkenes (CₙH₂ₙ) and alkynes (CₙH₂ₙ₋₂) add H₂, X₂ and HX across the multiple bond.
- Markovnikov’s rule: in HX addition to an alkene, the hydrogen goes to the carbon that already has more hydrogens, and X to the more substituted carbon.
- Bromine water is decolorized by alkenes and alkynes but not by alkanes. Hydrogenation uses one H₂ per pi bond.
- cis/trans isomers exist only when each carbon of the double bond has two different groups. Branching lowers the boiling point of alkane isomers.
Key equations
alkane CₙH₂ₙ₊₂alkene CₙH₂ₙalkyne CₙH₂ₙ₋₂CₓH_y + (x + y ÷ 4) O₂ → x CO₂ + (y ÷ 2) H₂O
Worked example
Propene + HBr: H goes to CH₂ (the carbon with two hydrogens) and Br to the middle carbon, giving 2-bromopropane as the major product. But-1-yne CH≡C–CH₂CH₃ takes up 2 mol H₂ to become butane. Pent-2-ene has cis/trans isomers, but propene (CH₂=CHCH₃) does not, because one carbon carries two identical hydrogens.
Common mistakes
- Putting the bromine on the carbon that already has more hydrogens, which is the opposite of Markovnikov addition.
- Assuming every alkene has cis/trans isomers. A carbon with two identical groups rules them out.
- Expecting an alkane to decolorize bromine water. It reacts only slowly, and only in light.
Practice
Test yourself with practice questions on this topic: Hydrocarbons: alkanes, alkenes and alkynes