Homologous Series and Functional Group Overview
A homologous series is a family of organic compounds that have the same functional group and general formula and differ from one another by a CH2 unit, and a functional group is the atom or group of atoms that is responsible for the characteristic reactions of the compound. In brief: methanol, ethanol, propan-1-ol and butan-1-ol all contain the –OH group and belong to the alcohols, with the general formula CnH2n+1OH; each is 14 g/mol heavier than the one before.
Organic chemistry has millions of compounds, but they can be sorted into a small number of families by their functional groups. Once you know the properties of the group, you can predict the behavior of every member of the family. This article explains the idea of homologous series, lists the main functional groups with their names and examples, shows the trends in physical properties and works through some identification exercises.
What is a homologous series?
The members of a homologous series have four things in common:
- the same functional group,
- the same general formula (for alkanes CnH2n+2, for alkenes CnH2n, for alcohols CnH2n+1OH),
- a regular change in physical properties such as boiling point and density,
- similar chemical properties, and similar methods of preparation.
Each member differs from the next by a CH2 group, which corresponds to a difference of 14.03 g/mol in molar mass. Knowing one member of a series therefore makes it possible to predict the properties of the others.
What is a functional group?
A functional group is a specific arrangement of atoms in a molecule that determines how the molecule reacts. The rest of the molecule, the hydrocarbon part, is often written as R. For example, any alcohol, R–OH, can be oxidized, can react with sodium to release hydrogen and can form esters, whatever the size of R. The functional group also determines the name: the suffix or prefix in the IUPAC name tells you which group is present.
Hydrocarbons and oxygen-containing groups
| Family | Functional group | Suffix (or prefix) | Example |
|---|---|---|---|
| Alkane | C–C single bonds | -ane | Ethane, CH3CH3 |
| Alkene | C=C | -ene | Ethene, CH2=CH2 |
| Alkyne | C≡C | -yne | Ethyne, HC≡CH |
| Alcohol | –OH | -ol (hydroxy-) | Ethanol, CH3CH2OH |
| Ether | –O– (between two carbons) | alkoxy- | Methoxymethane, CH3OCH3 |
| Aldehyde | –CHO | -al | Ethanal, CH3CHO |
| Ketone | C=O inside a chain | -one | Propanone, CH3COCH3 |
| Carboxylic acid | –COOH | -oic acid | Ethanoic acid, CH3COOH |
| Ester | –COO– (between two carbons) | -oate | Methyl ethanoate, CH3COOCH3 |
Nitrogen, halogen and other groups
| Family | Functional group | Suffix (or prefix) | Example |
|---|---|---|---|
| Amine | –NH2 (or –NHR, –NR2) | -amine (amino-) | Methylamine, CH3NH2 |
| Amide | –CONH2 | -amide | Ethanamide, CH3CONH2 |
| Nitrile | –C≡N | -nitrile | Ethanenitrile, CH3CN |
| Haloalkane | –F, –Cl, –Br, –I | fluoro-, chloro-, bromo-, iodo- | Chloromethane, CH3Cl |
| Thiol | –SH | -thiol | Methanethiol, CH3SH |
| Nitro compound | –NO2 | nitro- | Nitromethane, CH3NO2 |
How do properties change along a series?
| Name | Formula | Molar mass (g/mol) | Boiling point (°C) | Solubility in water |
|---|---|---|---|---|
| Methanol | CH3OH | 32.04 | 64.7 | Miscible |
| Ethanol | C2H5OH | 46.07 | 78.4 | Miscible |
| Propan-1-ol | C3H7OH | 60.10 | 97.2 | Miscible |
| Butan-1-ol | C4H9OH | 74.12 | 117.7 | About 7 g/100 mL |
| Pentan-1-ol | C5H11OH | 88.15 | 138 | About 2 g/100 mL |
The molar mass rises by 14 g/mol for each step. The boiling point rises by about 14–20 °C per carbon atom in this range, because dispersion forces between the longer chains are stronger. Solubility in water falls, since the nonpolar hydrocarbon part becomes dominant. The chemical properties remain essentially the same, so every member is oxidized by acidified dichromate and forms esters with carboxylic acids.
How do you identify functional groups?
Example 1: aspirin
Aspirin (2-acetoxybenzoic acid, C9H8O4) has a benzene ring, a carboxylic acid group (–COOH) and an ester group (–OCOCH3). It belongs to two families at the same time.
Example 2: paracetamol
Paracetamol (C8H9NO2) has a benzene ring with a phenol –OH group and an amide group (–NHCOCH3).
Example 3: naming the family from a formula
Which family has the formula CnH2n+1OH, and what is the formula when n = 6? These are alcohols, and the formula is C6H13OH, hexanol, with the molar mass 6 × 12.011 + 14 × 1.008 + 15.999 = 102.2 g/mol.
Example 4: predicting a property
Given that the boiling points of ethanol and propan-1-ol are 78.4 and 97.2 °C, estimate the boiling point of butan-1-ol. The step is about 19 °C, so the estimate is 97.2 + 19 = about 116 °C. The measured value is 117.7 °C.
How did the concept develop?
In the 1830s and 1840s, chemists such as Justus von Liebig, Jean-Baptiste Dumas and Charles Gerhardt tried to classify organic compounds into groups according to their reactions. Gerhardt introduced the concept of homologous series in 1843, in which compounds differ by CH2 and show regular changes in properties. The structural theory of the 1850s and 1860s then linked the behavior of each family to a specific atomic arrangement. Modern IUPAC nomenclature is built on the idea that the main functional group determines the suffix of the name.
Which mistakes are common?
- Confusing the ester and carboxylic acid groups: an acid has –COOH, and an ester has –COOR.
- Confusing aldehydes and ketones: in aldehydes the C=O is at the end of the chain (with an H), and in ketones it is inside the chain.
- Calling any compound with N an amine: amides and nitriles contain nitrogen but behave differently.
- Thinking that alkanes have a functional group: they are the parent hydrocarbons with only single bonds.
- Forgetting that a molecule may have more than one group: use the priority rules to name it.