(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.
(a) A homologous series is a group of organic compounds having similar structures and similar chemical properties in which the successive compounds differ by CH2 group.
Example of Homologous series: All the alkanes have similar structures with single covalent bonds and show similar chemical properties, so they can be grouped together in the form of a homologous series.
Homologous series of alkanes: Methane, CH4; Ethane, C2H6; Propane, C3H8; Butane, C4H10; Pentane, C5H12
(b) (i) All the members of the homologous series can be represented by the same general formula.
(ii) Any two adjacent homologues differ by 1 carbon atom and 2 hydrogen atoms in their molecular formulae.
(c) Alkene, CnH2n
(d) Alkanes: CH4, C2H6, C4H10
Alkenes :C2H4, C3H6
Alkynes : C2H2, C3H4
(e) In an organic compound, any atom other than carbon and hydrogen is called a heteroatom.
Example: Chlorine (Cl), Bromine (Br), Oxygen (O)
Chloromethane – CH3Cl and methanol – CH3OH