A homologous series is a family of organic compounds that:1. have the same functional group;
2. have the same general formula;
3. differ from the next member by a CH2 group;
4. have similar chemical properties;
5. show a gradual trend in physical properties, such as boiling point.The chemical properties are similar because the functional group, which decides the reactions, is the same in every member. The physical properties change gradually because each extra CH2 adds electrons, so the London forces between molecules get stronger and the boiling point rises.General formulae of some series:Alkanes: CnH2n+2
Alkenes: CnH2n
Alcohols: CnH2n+1OH
Halogenoalkanes (one halogen X): CnH2n+1XWorked example: give the molecular formula of the alkane with eight carbon atoms. With n = 8, 2n + 2 = 18, so the formula is C8H18 (octane).Because each member differs by CH2, the relative molecular masses of successive members differ by 12.0 + 2 × 1.0 = 14.0. Methane has Mr 16.0, ethane 30.0 and propane 44.0.Knowing the series of a compound lets you predict how a new member will behave: pentan-1-ol will react in the same way as ethanol, but will have a higher boiling point.
2. have the same general formula;
3. differ from the next member by a CH2 group;
4. have similar chemical properties;
5. show a gradual trend in physical properties, such as boiling point.The chemical properties are similar because the functional group, which decides the reactions, is the same in every member. The physical properties change gradually because each extra CH2 adds electrons, so the London forces between molecules get stronger and the boiling point rises.General formulae of some series:Alkanes: CnH2n+2
Alkenes: CnH2n
Alcohols: CnH2n+1OH
Halogenoalkanes (one halogen X): CnH2n+1XWorked example: give the molecular formula of the alkane with eight carbon atoms. With n = 8, 2n + 2 = 18, so the formula is C8H18 (octane).Because each member differs by CH2, the relative molecular masses of successive members differ by 12.0 + 2 × 1.0 = 14.0. Methane has Mr 16.0, ethane 30.0 and propane 44.0.Knowing the series of a compound lets you predict how a new member will behave: pentan-1-ol will react in the same way as ethanol, but will have a higher boiling point.