CBSE · Class 10 · Science · Carbon and its CompoundsExplain the concept of homologous series in organic chemistry. Write any four characteristics of a homologous series and give consecutive examples of the alcohol homologous series.

Definition of Homologous Series\nA homologous series is a group or family of organic compounds having similar structures and similar chemical properties in which the successive members differ by a $-CH_2-$ group (methylene group) or a molecular mass increase of $14\text{ u}$ (unified atomic mass units).

Characteristics of a Homologous Series

1. General Formula: All members of a homologous series can be represented by the same general chemical formula. For example, alkanes are represented by $C_n{H}{2n+2}$, alkenes by $C_n{H}{2n}$, and alcohols by $C_n{H}_{2n+1}OH$.
2. Molecular Mass Difference: Any two adjacent (consecutive) members in a series differ by a $-CH_2-$ group in their molecular formula. In terms of mass, the difference between two consecutive members is $14\text{ u}$ (12 u for carbon and 2 u for two hydrogen atoms).
3. Chemical Properties: All members of a homologous series show similar chemical properties because they have the same functional group. For instance, all lower alcohols react with sodium to evolve hydrogen gas.
4. Gradation in Physical Properties: The physical properties of the members show a gradual change (gradation) with an increase in molecular mass. Properties such as melting point, boiling point, and solubility increase with an increase in molecular weight due to increased intermolecular forces.

Examples of Alcohol Homologous Series\nThe functional group for alcohols is $-OH$ (hydroxyl group), and their general formula is $C_n{H}{2n+1}OH$ or $C_n{H}{2n+2}O$. The first four consecutive members are:

1. Methanol (Methyl Alcohol): $CH_3OH$ (Molecular mass = $12 + (3\times1) + 16 + 1 = 32\text{ u}$)
2. Ethanol (Ethyl Alcohol): $C_2H_5OH$ (Molecular mass = $(2\times12) + (5\times1) + 16 + 1 = 46\text{ u}$, difference from methanol = $46 - 32 = 14\text{ u}$)
3. Propanol (Propyl Alcohol): $C_3H_7OH$ (Molecular mass = $(3\times12) + (7\times1) + 16 + 1 = 60\text{ u}$, difference from ethanol = $60 - 46 = 14\text{ u}$)
4. Butanol (Butyl Alcohol): $C_4H_9OH$ (Molecular mass = $(4\times12) + (9\times1) + 16 + 1 = 74\text{ u}$, difference from propanol = $74 - 60 = 14\text{ u}$)