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)?