Homologous Series

You have seen that carbon atoms can be linked together to form chains of varying lengths. In addition, hydrogen atom or atoms on these carbon chains can be replaced by any of the functional groups that we saw above. The presence of a functional group such as alcohol dictates the properties of the carbon compound, regardless of the length of the carbon chain. For example, the chemical properties of CH₃OH, C₂H₅OH, C₃H₇OH and C₄H₉OH are all very similar. Hence, such a series of compounds in which the same functional group substitutes for hydrogen in a carbon chain is called a homologous series.

Let us look at the homologous series that we saw earlier in Table 4.2. If we look at the formulae of successive compounds, say –

CH₄ and C₂H₆ — these differ by a – CH₂– unit

C₂H₆ and C₃H₈ — these differ by a – CH₂– unit

What is the difference between the next pair – propane and butane (C₄H₁₀)? Can you find out the difference in molecular masses between these pairs (the atomic mass of carbon is 12 u and the atomic mass of hydrogen is 1 u)?

Similarly, take the homologous series for alkenes. The first member of the series is ethene which we have already come across in Section 4.2.1. What is the formula for ethene? The succeeding members have the formula C₃H₆, C₄H₈ and C₅H₁₀. Do these also differ by a –CH₂– unit? Do you see any relation between the number of carbon and hydrogen atoms in these compounds? The general formula for alkenes can be written as C_nH_2n, where n = 2, 3, 4. Can you similarly generate the general formula for alkanes and alkynes?

As the molecular mass increases in any homologous series, a gradation in physical properties is seen. This is because the melting points and boiling points increase with increasing molecular mass. Other physical properties such as solubility in a particular solvent also show a similar gradation. But the chemical properties, which are determined solely by the functional group, remain similar in a homologous series.

Activity 4.2

1. Calculate the difference in the formulae and molecular masses for (a) CH₃OH and C₂H₅OH (b) C₂H₅OH and C₃H₇OH, and (c) C₃H₇OH and C₄H₉OH.
2. Is there any similarity in these three?
3. Arrange these alcohols in the order of increasing carbon atoms to get a family. Can we call this family a homologous series?
4. Generate the homologous series for compounds containing up to four carbons for the other functional groups given in Table 4.3.