Question

What is a homologous series and what are its characteristics? Explain with the help of alkane series.

Answer 1

• Homologous series represent the class or group of organic compounds with same general chemical formula and chemical properties.
• For alkanes the general formula is ${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}+2}$.They contains single bonds only, that is between carbon - hydrogen and carbon - carbon atoms.
• Consider the first four members of the group viz., Methane$\mathrm{\left(}{\mathrm{CH}}_{\mathrm{4}}\mathrm{\right)}$, Ethane$\mathrm{\left(}{\mathrm{C}}_{\mathrm{2}}{\mathrm{H}}_{\mathrm{6}}\mathrm{\right)}$, Propane$\mathrm{\left(}{\mathrm{C}}_{\mathrm{3}}{\mathrm{H}}_{\mathrm{8}}\mathrm{\right)}$ and Butane$\mathrm{\left(}{\mathrm{C}}_{\mathrm{4}}{\mathrm{H}}_{\mathrm{10}}\mathrm{\right)}$.
• Methane has single carbon and four hydrogens. Hence for $n=1$ the formula we get is ${\mathrm{CH}}_4$ from the above general formula.
• Ethane has two carbon and six hydrogens. Hence for $n=2$, the formula we get is ${\mathrm{C}}_{\mathrm{2}}{\mathrm{H}}_{\mathrm{6}}$ from the general formula.
• Propane has three carbons and eight hydrogens. Hence for $n=3$,the formula we get is ${\mathrm{C}}_3{\mathrm{H}}_8$ from the general formula.
• Butane has four carbons and ten hydrogens. Hence for $n=4$,the formula we get is ${\mathrm{C}}_4{\mathrm{H}}_{10}$ from the general formula.
• Therefore Methane, Ethane, Propane and Butane are homologous compounds.