Question

(a) What is a homologous series? Explain with an example.
(b) State two characteristics of a homologous series.
(c) The molecular formula of an organic compound is C₁₈H₃₆. Name its homologous series.
(d) Select the hydrocarbons which belong to the same homologous series. Give the name of each series.
CH₄, C₂H₂, C₂H₄, C₂H₆, C₄H₁₀, C₃H₄, C₃H₆
(e) What is meant by 'heteroatom'? Give examples. Write the names and formulae of two organic compounds containing different heteroatoms.

Answer 1

(a) A group of compounds having similar structures and chemical properties but their successive compounds differ by a CH₂ group is called a homologous series.
Example: Alkanes have similar structures and similar chemical properties, so they can be grouped together to form a homologous series. The general formula of the homologous series of alkanes is C_nH_2n+2, where n is the number of carbon atoms in each molecule.

The first five members of the homologous series of alkanes are as follows:
1. Methane CH₄ ( First member, contains 1 carbon atom)
2. Ethane C₂H₆ (Second member, contains 2 carbon atoms)
3. Propane C₃H₈ (Third member, contains 3 carbon atoms)
4. Butane C₄H₁₀ (Fourth member, contains 4 carbon atoms)
5. Pentane C₅H₁₂ (Fifth member, contains 5 carbon atoms)

(b) Characteristics of a homologous series are as follows:

(i) All the members of a homologous series can be represented by the same general formula.
For example, all the members of the alkene series are represented by the same formula C_nH_2n.
(ii) The members of a homologous series show a gradual change in their physical properties like melting point, boiling point and density with the increase in their molecular masses.

(c) The organic compound C₁₈H₃₆ is a member of the homologous series of alkenes having the general formula C_nH_2n.

(d)

Name of the series	Alkanes	Alkenes	Alkynes
Molecular formula of hydrocarbons	CH4, C2H6, C4H10	C2H4, C3H6	C2H2 , C3H4

(e) Heteroatom is any atom other than carbon and hydrogen. Examples: Nitrogen, sulphur, chlorine and bromine are heteroatoms. The name and formula of two organic compounds containing different heteroatoms are (i) chloromethane, CH₃Cl, and (ii) formaldehyde HCHO.