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P-1 Carbon Compounds — Science STD 10 — Question

Maharashtra BoardEnglish MediumSTD 10ScienceP-1 Carbon Compounds5 Marks
Question
Write short notes: Homologous series.
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Answer

The length of the carbon chains in carbon compounds is different their chemical properties are very much similar due to the presence of the same functional group in them. The series of compounds formed by joining the same functional group in place of a particular hydrogen atom on the chains having sequentially increasing length is called homologous series. Two adjacent members of the series differ by only one $-\mathrm{CH}_2-$ (methylene) unit and their mass differ by 14 units. The homologous series of straight chain alkanes can be represented by the general formula $\mathrm{C}_n \mathrm{H}_{2 n+2}$
The members of this series are as follows:
Methane - $\mathrm{CH}_4$ Ethane $-\mathrm{C}_2 \mathrm{H}_6$ - These differ by $-\mathrm{CH}_2$ units
Ethane - $\mathrm{C}_2 \mathrm{H}_6$ Propane $-\mathrm{C}_3 \mathrm{H}_8$ - These differ by $-\mathrm{CH}_2$ units
Butane $-\mathrm{C}_4 \mathrm{H}_{10}$ Pentane - $\mathrm{C}_5 \mathrm{H}_{12}$ - These differ by $-\mathrm{CH}_2$ units
Characteristics:
(1) In homologous series while going in an increasing order of the length of carbon chain
(a) one methylene unit $\left(-\mathrm{CH}_2-\right)$ gets added
(b) molecular mass increases by 14 u
(c) number of carbon atoms increases by one.
(2) Chemical properties of members of a homologous series show similarity due to the presence of the same functional group in them.
(3) Each member of the homologous series can be represented by the same general molecular formula.
(4) while going in an increasing order of the length there is gradation in the physical properties i.e., the boiling and melting points.

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