Organic Chemistry: Hydrocarbons
Core concepts
Understand homologous series, functional-group families and isomerism. For hydrocarbons, know alkanes (saturated, substitution reactions, combustion), alkenes and alkynes (unsaturation, addition reactions, Markovnikov-type orientation), and aromatic hydrocarbons such as benzene (delocalised ring, substitution rather than addition). Recognise structural isomerism, cis–trans isomerism in alkenes, and the naming rules of IUPAC.
Traps that appear in questions
- Substitution vs addition: alkanes undergo substitution with halogens under light; alkenes and alkynes undergo addition.
- Test for unsaturation: bromine water decolourises with alkenes and alkynes but not with alkanes under normal conditions.
- Markovnikov orientation: in addition of HX to an alkene, hydrogen adds to the carbon with more hydrogens, giving the more substituted product.
- Benzene stability: the delocalised ring makes benzene resist addition; it prefers substitution.
- Naming: choose the longest chain, number to give the lowest locants, and name substituents alphabetically.
Worked example
Name the product of HBr addition to propene. Markovnikov's rule places H on the terminal carbon (more hydrogens), so Br attaches to the middle carbon, giving 2-bromopropane.
Rule of thumb
Identify saturation first: saturated means substitution, unsaturated means addition.