Question

Why are homologous series of carbon compounds so called? Write the chemical formula of two consecutive members of a homologous series and state the part of these compounds that determines their i) physical properties, and ii) chemical properties?

Answer 1

Organic compounds can be classified into various families. The members of a family possess similar or almost similar chemical properties. Their molecular formulae can be obtained by a general molecular formula. When the members of a family are arranged in the order of increasing molecular weights, we get a series known as homologous series.

Complete answer:
A series of organic compounds that are similarly constituted in which the members possess the same functional group, have similar or almost similar chemical characteristics, can be represented by the same general formula and the two consecutive members differ by $- C{H_2}$ group in their molecular formula is known as homologous series.
Step 2
Homologues are the various members of a particular homologous series. Here we can find a few homologues of alcohol series (containing straight chain alcohols) are as follows:

$$C{H_3}OH - methylalcohol \\\ C{H_3}C{H_2}OH - ethylalcohol \\\ C{H_3}C{H_2}C{H_2}OH - propylalcohol \\\ C{H_3}C{H_2}C{H_2}C{H_2}OH - butylalcohol \\\ C{H_3}C{H_2}C{H_2}C{H_2}C{H_2}OH - pentylalcohol \\\$$

Step 3
All the members of a series can be represented by the same molecular formula. For example, all the members of the alcohol family can be represented by the general formula ${C_n}{H_{2n + 1}}OH$, where n may have the values 1, 2, 3, ……….., etc.
Step 4
The molecular mass of a compound in the series differs by 14 amu $( - C{H_2} = 12 + 2 \times 1 = 14)$ from that its neighbour.
Step 5
All the members of a series possess the same functional group.
Hence,
The chemical properties of all the members of a series are similar or almost similar and is dependent on the functional group attached to the carbon chain. However, the first member may differ considerably from others.
The physical properties of an organic compound such as density, melting point, boiling point, solubility, etc., of the members of a homologous series show almost regular variation in ascending or descending the series.

Note:
An atom or a group of atoms that largely decides the mode of activity of a given organic compound is known as a functional group. In this example, we have shown the different homologous of alcohol where the functional group is $- OH$.