Characteristic reactions
Each homologous series has typical reactions. Alkanes undergo combustion and substitution (with halogens in UV light). Alkenes are unsaturated and undergo addition reactions that decolourise bromine water. Alcohols can be oxidised to carboxylic acids and undergo esterification. Carboxylic acids react like weak acids with metals, bases and carbonates.
Worked example
Complete combustion of ethanol:
C2H5OH + 3O2 → 2CO2 + 3H2O
Balance C first (2 CO2), then H (3 H2O), then count O on the right (4 + 3 = 7); the alcohol supplies 1 O, so 3 O2 supply the other 6.
Interchange between series
- Alkene → alkane: addition of hydrogen (hydrogenation) with a nickel catalyst: C2H4 + H2 → C2H6.
- Alkene → alcohol: addition of steam (hydration): C2H4 + H2O → C2H5OH.
- Alcohol → alkene: dehydration by hot concentrated H2SO4 or hot Al2O3.
- Alcohol → carboxylic acid: oxidation by acidified KMnO4 or K2Cr2O7.
- Alcohol + acid → ester: esterification with concentrated H2SO4 catalyst.
These reactions form a connected cycle. Starting from an alkene, hydration gives an alcohol; oxidation of that alcohol gives a carboxylic acid; the acid with another alcohol gives an ester; and dehydration of an alcohol returns an alkene. Knowing the reagent and conditions for each arrow lets you plan a route from any one family to another. The reaction of an alkene with bromine water, turning it from brown to colourless, is the standard laboratory test used to tell an alkene apart from an alkane.
Remember
- Alkenes decolourise bromine water; alkanes do not (a key test).
- Oxidation adds oxygen; addition/hydrogenation removes the C=C bond.
- Esterification: acid + alcohol ⇌ ester + water.