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 CnH2n, 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.
- Calculate the difference in the formulae and molecular masses for (a) CH3OH and C2H5OH (b) C2H5OH and C3H7OH, and (c) C3H7OH and C4H9OH.
- Is there any similarity in these three?
- Arrange these alcohols in the order of increasing carbon atoms to get a family. Can we call this family a homologous series?
- Generate the homologous series for compounds containing up to four carbons for the other functional groups given in Table 4.3.