GCSE-readyAlkanes
🎬 The doodle video for this lesson is coming soon. Subscribe on YouTube to see it first.
Alkanes are saturated hydrocarbons. They are quite unreactive but do react with halogens in ultraviolet light.
General formula and naming
General formula: C_nH2n+2
Methane CH4, ethane C2H6, propane C3H8, butane C4H10, pentane C5H12.
Alkanes are saturated: they contain only single carbon-carbon bonds.
Methane CH4, ethane C2H6, propane C3H8, butane C4H10, pentane C5H12.
Alkanes are saturated: they contain only single carbon-carbon bonds.
Isomers
From four carbons, alkanes can have isomers.
C4H10: butane (straight chain) and a branched isomer (methylpropane).
C5H12: three isomers: pentane (straight chain) and two branched ones.
In displayed formulae, every carbon has four bonds and every hydrogen has one.
C4H10: butane (straight chain) and a branched isomer (methylpropane).
C5H12: three isomers: pentane (straight chain) and two branched ones.
In displayed formulae, every carbon has four bonds and every hydrogen has one.
Substitution with halogens
In ultraviolet light, an alkane reacts with a halogen. One hydrogen atom is substituted by a halogen atom.
CH4 + Br2 → CH3Br + HBr
The products are bromomethane and hydrogen bromide. Without UV light there is no reaction, so bromine water stays orange with an alkane.
CH4 + Br2 → CH3Br + HBr
The products are bromomethane and hydrogen bromide. Without UV light there is no reaction, so bromine water stays orange with an alkane.
Write the formula of the alkane with 6 carbon atoms.
- n = 6
- 2n + 2 = 14
Answer: C6H14
Alkanes have the general formula CnH2n+2 and are saturated (single bonds only). C4H10 has two isomers and C5H12 has three. In UV light, alkanes undergo substitution with halogens: CH4 + Br2 → CH3Br + HBr.
Check you have got it
Answer 6 quick questions with instant marking. If you get one wrong, GCSE-ready shows you why and gives you another go. It is free, and you do not need an account.