11.3 - Homologous Series
- 1What homologous series are
- 2Examples of homologous series
- 3Trends in physical properties within homologous series
Defining homologous series
A homologous series is a family of organic compounds that have similar chemical structures and properties.
The members of a homologous series:
- Differ from one another by a CH2 group.
- Can be represented by the same general formula.
- Have similar chemical properties as they contain the same functional group.
- Show gradual changes in physical properties as the carbon chain length increases.
Knowing the general formula allows you to identify the homologous series a compound belongs to without drawing out its structure.
Examples of homologous series
The table below summarises the general formulae and examples for some key homologous series:
| Homologous series | General formula | Defining feature | C_3_ example |
|---|---|---|---|
| Alkanes | C_n_H_2n+2_ | Contain only carbon-carbon single bonds | Propane, CH_3_CH_2_CH_3_ |
| Alkenes | C_n_H_2n_ | Contain a carbon-carbon double bond | Propene, CH_2_=CHCH_3_ |
| Alkynes | C_n_H_2n-2_ | Contain a carbon-carbon triple bond | Propyne, HC≡CCH_3_ |
| Halogenoalkanes | C_n_H_2n+1_X | Contain a carbon-halogen bond (X = F, Cl, Br, or I) | 1-chloropropane, CH_3_CH_2_CH_2_Cl |
| Alcohols | C_n_H_2n+1_OH | Contain a hydroxyl (-OH) group | Propanol, CH_3_CH_2_CH_2_OH |
| Aldehydes | C_n_H_2n_O | Contain a carbonyl (C=O) group at the end of the carbon chain | Propanal, CH_3_CH_2_CHO |
| Ketones | C_n_H_2n_O | Contain a carbonyl (C=O) group within the carbon chain | Propanone, CH_3_COCH_3_ |
| Carboxylic acids | C_n_H_2n+1_COOH | Contain a carboxyl (-COOH) group | Propanoic acid, CH_3_CH_2_COOH |
| Ethers | ROR' | Contain an oxygen atom bonded to two alkyl (R) groups | Methoxyethane, CH_3_OCH_2_CH_3_ |
| Amines | C_n_H_2n+3_N | Contain an amine (-NH_2_, -NHR or -NR_2_) group | Propanamine, CH_3_CH_2_CH_2_NH_2_ |
| Amides | RCONH_2_ | Derived from carboxylic acids; contain an amide (-CONH_2_) group | Propanamide, CH_3_CH_2_CONH_2_ |
| Esters | RCOOR' | Derived from carboxylic acids; contain a -COO- linkage | Methyl ethanoate, CH_3_COOCH_3_ |
Physical property trends in homologous series
While members of a homologous series have similar chemical reactivities due to their common functional group, their physical properties like melting and boiling points show clear trends.
As the carbon chain length increases by a CH2 each time:
- Surface area and electron count increases, creating stronger London dispersion forces between molecules.
- More energy is needed to overcome these stronger intermolecular forces.
- Therefore boiling and melting points increase with increasing molar mass.
The table shows this trend for the boiling and melting points of the straight-chain alkanes:
| Name | Formula | Boiling point (°C) | Melting point (°C) |
|---|---|---|---|
| Methane | CH_4_ | -161.5 | -182.5 |
| Ethane | C_2_H_6_ | -88.5 | -183.2 |
| Propane | C_3_H_8_ | -42.1 | -187.7 |
| Butane | C_4_H_10_ | -0.5 | -138.4 |
| Pentane | C_5_H_12_ | 36.1 | -129.7 |
The smooth trend in boiling points allows for reasonable predictions of the boiling point of the next member in the series. However, the melting point trend is less predictable due to the influence of molecular shape and bond polarity, making melting point estimates less reliable.