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Homologous Series
In this video, we will focus on homologous series in Organic Chemistry.
A homologous series is a family in Organic Chemistry.
At secondary level Chemistry, you are to be familiar with the following homologous series: alkanes, alkenes, alcohols and carboxylic acids.
Homologous series have the following general characteristics:
1. All members of a homologous series have the same general formula.
2. The consecutive member of a homologous series differ by a -CH2– group.
3. All compounds in a homologous series have the same functional group. Hence, they have similar chemical properties. Functional group is basically the atom or group of atoms that take part in chemical reaction.
4. The physical properties of the members of a homologous series change gradually with an increase in size and mass of the molecules.
General characteristics of homologous series
1. All members of a homologous series have the same general formula.
Members of a homologous series have the same general formula
The general formula of alkane is CnH2n+2, where n = 1, 2, …
The first member is methane, n=1, hence the formula is CH4.The second member is ethane, n=2, hence the formula is C2H6.The third member is propane, n=3, hence the formula is C3H8.The fourth member is butane, n=4, hence the formula is C4H10.
The general formula of alkene is CnH2n, where n = 2, 3, … (Do note, alkene requires at least 2 carbons to form. Hence, n starts with 2)
The first member is ethene, n=2, hence the formula is C2H4.The second member is propene, n=3, hence the formula is C3H6.The third member is butene, n=4, hence the formula is C4H8.
The general formula of alcohol is CnH2n+1OH, where n = 1, 2, …
The first member is methanol, n=1, hence the formula is CH3OH.The second member is ethanol, n=2, hence the formula is C2H5OH.The third member is propanol, n=3, hence the formula is C3H7OH.The fourth member is butanol, n=4, hence the formula is C4H9OH.
The general formula of carboxylic acid is CnH2n+1COOH, where n = 0, 1, …
The first member is methanoic acid, n=0, hence the formula is HCOOH.The second member is ethanoic acid, n=1, hence the formula is CH3COOH.The third member is propanoic acid, n=2, hence the formula is C2H5COOH.The fourth member is butanoic acid, n=3, hence the formula is C3H7COOH.
(b) The consecutive member of a homologous series differ by a -CH2– group.
(b) The consecutive member of a homologous series differ by a -CH2– group.
Consecutive member of a homologous series differ by a CH2 group
By stating the formula down in a table, it is very clear that in each homologous series, the next member is formed by addition of -CH2– group.
(c) All compounds in a homologous series have the same functional group. Hence, they have similar chemical properties.
(c) All compounds in a homologous series have the same functional group. Hence, they have similar chemical properties.
Members in a homologous series have the same functional group
You may be wondering what is ‘functional group’. A functional group is an atom or group of atoms that gives a molecule its characteristic properties. It is basically the group of atoms that take part in chemical reaction. Functional group is the same for all members in the same homologous series.
Let’s identify the functional group for each homologous series.
Alkane does not have functional group. Alkanes are generally unreactive except in terms of combustion and substitution reaction, which we will look into in upcoming videos.
The functional group of alkene is carbon-carbon double bond (C=C). Alkenes are very reactive.
The functional group of alcohol is hydroxyl group (-OH).
The functional group of carboxylic acid is carboxyl group (-COOH).
(d) The physical properties of the members of a homologous series change gradually with an increase in size and mass of the molecules.
Let’s work out the molecular mass of each member in the homologous series. As we go down each homologous series, the molecular size of the members increases. With bigger size and mass, the melting and boiling points increases, alkanes and alkenes become more viscous and less flammable.
Table below is the summary of the general characteristics of a homologous series.
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