Homologous Series and Functional Groups

25 min

Millions of compounds, a handful of families

Cooking gas, petrol, candle wax and the oil in a car's engine are all alkanes, differing only in the length of their carbon chains. The ethanol in hand sanitiser and the methanol used as a solvent are both alcohols, and they react in similar ways because both carry the same $\mathrm{-OH}$ group. Chemists organise the millions of organic compounds into families like these, so that learning one member tells you a great deal about the rest.

Definition

Homologous series: a family of organic compounds with the same general formula and the same functional group, in which each member differs from the next by a $\mathrm{CH_2}$ unit. Example: methane $\mathrm{CH_4}$ , ethane $\mathrm{C_2H_6}$ , propane $\mathrm{C_3H_8}$ are the first three alkanes.

Functional group: the atom or group of atoms that gives a homologous series its typical chemical reactions. Example: the hydroxyl group, $\mathrm{-OH}$ , in alcohols.

General formula: a formula in $n$ , the number of carbon atoms, that fits every member of a series. Example: alkanes are $\mathrm{C_nH_{2n+2}}$ .

Features of a homologous series

All members fit the same general formula.
Neighbouring members differ by $\mathrm{CH_2}$ , so their relative molecular masses differ by $12 + 2 = 14$ .
They have the same functional group, so similar chemical properties.
Their physical properties change gradually along the series: boiling point and melting point rise as the chain gets longer (Figure 2.2).
They can be made by similar methods.

Table 2.1: Five homologous series

Series	General formula	Functional group	First member	Member with 2 carbons
alkanes	$\mathrm{C_nH_{2n+2}}$	none: only C–C and C–H single bonds	methane, $\mathrm{CH_4}$	ethane, $\mathrm{C_2H_6}$
alkenes	$\mathrm{C_nH_{2n}}$	C=C double bond	ethene, $\mathrm{C_2H_4}$	ethene, $\mathrm{C_2H_4}$
alkynes	$\mathrm{C_nH_{2n-2}}$	C≡C triple bond	ethyne, $\mathrm{C_2H_2}$	ethyne, $\mathrm{C_2H_2}$
alcohols	$\mathrm{C_nH_{2n+1}OH}$	$\mathrm{-OH}$ , hydroxyl	methanol, $\mathrm{CH_3OH}$	ethanol, $\mathrm{C_2H_5OH}$
carboxylic acids	$\mathrm{C_nH_{2n+1}COOH}$	$\mathrm{-COOH}$ , carboxyl	methanoic acid, HCOOH	ethanoic acid, $\mathrm{CH_3COOH}$

Definition

Saturated hydrocarbon: one with only single bonds between carbon atoms (alkanes). Unsaturated: one with at least one C=C or C≡C bond (alkenes, alkynes). Example: ethene, $\mathrm{C_2H_4}$ , is unsaturated.

Worked example

Problem. Use the general formulas to write the molecular formula of (a) the alkane with 7 carbon atoms; (b) the alkene with 5 carbon atoms; (c) the alkyne with 4 carbon atoms; (d) the alcohol with 3 carbon atoms. Find the relative molecular mass of (a). (C = 12, H = 1)

Solution. (a) $n = 7$ : $\mathrm{C_7H_{2(7)+2}} = \mathrm{C_7H_{16}}$ ; $M_r = 7 \times 12 + 16 \times 1 = 100$ .

(b) $\mathrm{C_5H_{10}}$ . (c) $\mathrm{C_4H_{6}}$ . (d) $\mathrm{C_3H_7OH}$ .

Worked example

Problem. To which homologous series does each compound belong? (a) $\mathrm{C_6H_{14}}$ ; (b) $\mathrm{C_3H_6}$ ; (c) $\mathrm{C_5H_8}$ ; (d) $\mathrm{C_4H_9OH}$ ; (e) $\mathrm{C_2H_5COOH}$ .

Solution. (a) $2 \times 6 + 2 = 14$ : alkane. (b) $2 \times 3 = 6$ : alkene. (c) $2 \times 5 - 2 = 8$ : alkyne. (d) contains $\mathrm{-OH}$ : alcohol. (e) contains $\mathrm{-COOH}$ : carboxylic acid (propanoic acid).

A trend in a physical property

Longer molecules attract one another more strongly, so more energy is needed to separate them: the boiling point rises steadily along the series. The boiling points below are standard reference values.

The first four alkanes are gases at room temperature (they are used as fuel gases); pentane and hexane are liquids.

Warning

Common mistakes. (1) Members of a series differ by $\mathrm{CH_2}$ , not by one carbon atom alone. (2) The alkene and alkyne series start at two carbons: there is no "methene", because a double or triple bond needs two carbon atoms. (3) In the alcohol formula $\mathrm{C_nH_{2n+1}OH}$ , the H of the OH is counted separately: ethanol has 6 hydrogen atoms in total.

Tip

Count the hydrogens: $2 n + 2$ ane, $2 n$ ene, $2 n - 2$ yne. Each extra bond between carbons removes two hydrogens.

FuseSchool: The Functional Group Concept Explained. Watch how the functional group decides how a compound reacts, while the carbon chain mostly affects physical properties.

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Practice

Define homologous series and give four of its features.
Write the general formula and draw the functional group of (a) alkenes, (b) alcohols, (c) carboxylic acids.
Write the molecular formula of the alkane with 10 carbon atoms and find its relative molecular mass.
Identify the series of: $\mathrm{C_8H_{18}}$ , $\mathrm{C_6H_{10}}$ , $\mathrm{C_4H_8}$ , $\mathrm{CH_3OH}$ .
Draw the displayed formulas of ethane and ethene. Which is saturated?
By how much does the relative molecular mass increase from one member of a series to the next? Explain.
Use Figure 2.2 to state which of the first six alkanes are liquids at 25 °C, and predict whether heptane, $\mathrm{C_7H_{16}}$ , boils above or below 69 °C.
A student says propane and propene belong to the same homologous series because both have three carbon atoms. Evaluate this.

Show answersHide answers

A family with the same general formula and functional group, each member differing by $\mathrm{CH_2}$ . Features: same general formula; differ by $\mathrm{CH_2}$ (14 in $M_{r}$ ); similar chemical properties; gradual change in physical properties; similar methods of preparation.
(a) $\mathrm{C_nH_{2n}}$ , C=C; (b) $\mathrm{C_nH_{2n+1}OH}$ , –O–H; (c) $\mathrm{C_nH_{2n+1}COOH}$ , a carbon with =O and –O–H.
$\mathrm{C_{10}H_{22}}$ ; $M_{r} = 120 + 22 = 142$ .
Alkane; alkyne; alkene; alcohol.
Ethane, $\mathrm{CH_3{-}CH_3}$ , has only single bonds; ethene, $\mathrm{CH_2{=}CH_2}$ , has a C=C double bond. Ethane is saturated.
By 14, the mass of one $\mathrm{CH_2}$ unit (12 + 2).
Pentane and hexane are liquids (boiling points above 25 °C). Heptane has a longer chain, so it boils above 69 °C.
Wrong. They have different general formulas ( $\mathrm{C_nH_{2n+2}}$ and $\mathrm{C_nH_{2n}}$ ) and different functional groups (propene has C=C), so propane is an alkane and propene an alkene, with different chemical properties.

Sources and credits

Video: The Functional Group Concept Explained, FuseSchool, embedded from YouTube.
Written by Ganzaa. The diagrams, graph, table, worked examples and Practice items were drawn and written by Ganzaa; the boiling points are standard reference values.

1Millions of compounds, a handful of families

2Features of a homologous series

3A trend in a physical property

Millions of compounds, a handful of families

Features of a homologous series

A trend in a physical property