(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 $C_{18} H_{36} .$ Name its homologous series.
(d) Select the hydrocarbons which belong to the same homologous series. Give the name of each series.
$C H_{4}, C_{2} H_{2}, C_{2} H_{4}, C_{2} H_{6}, C_{4} H_{10}, C_{3} H_{4}, C_{3} H_{6}$
(e) What is meant by 'heteroatom'? Give examples. Write the names and formulae of two organic compounds containing different heteroatoms.

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Solution

(a) Homologous series: A homologous series is a series of carbon compounds that have different numbers of carbon atoms but contain the same functional group. A homologue of $C H_{3} O H$ will differ by the $- C H 2$ group; therefore, methanol's successive homologue will be ethanol $(C_{2} H_{5} O H)$ .

(b) Charecterstics include a general formula and neighbouring members differing by CH2, with similar chemical properties and with a gradation in physical properties. Homologous series are 'families' of organic compounds. They share common characteristics: They all contain the same functional group.

(c) The general formula of the compound is $C_{n} H_{2} n$ . Thus it's an alkene.

(d) Alkane - $C H_{4}$ , $C_{2} H_{6}$ , $C_{4} H_{1} 0$

Alkene - $C_{2} H_{4}$ , $C_{3} H_{6}$

Alkyne - $C_{2} H_{2}$ , $C_{3} H_{4}$

(e) A heteroatom is any atom that is not carbon or hydrogen. Usually, the term is used to indicate that non-carbon atoms have replaced carbon in the backbone of the molecular structure. Typical heteroatoms are nitrogen (N), oxygen (O), sulfur(S), phosphorus (P), chlorine (Cl)etc.

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Similar questions

(a) Give the names and structural formulae of the next two higher homologues of methane.
(b) The molecular formula of a hydrocarbon is $C_{10} H_{18} .$ Name its homologous series.
(c) Select the hydrocarbons which are members of the same homologous series. Give the name of each series.
$C_{5} H_{10}; C_{3} H_{8}; C_{6} H_{10}; C_{4} H_{10}; C_{7} H_{12}; C_{8} H_{16}$

Q. A colourless organic liquid X of molecular formula C₂H₄O₂ turns blue litmus to red. Another colourless organic liquid Y of molecular formula C₃H₆O has no action on any litmus but it is used as a nail polish remover. A yet another colourless organic liquid Z of molecular formula C₂H₆O has also no action on litmus but it is used in tincture of iodine.

(a) Name the liquid X. To which homologous series does it belong? Give the name of another member of this homologous series.
(b) Name the liquid Y. To which homologous series does it belong? Write the name of another member of this homologous series.
(c) Can you name an organic compound having the same molecular formula as liquid Y but which belongs to a different homologous series? What is this homologous series?
(d) Name the liquid Z. To which homologous series does it belong? Write the name of another member of this homologous series.

Q. What is meant by homologous series of organic compounds? Write the chemical formulae of two members of a homologous series and state which part determines the (i) physical properties, (ii) chemical properties, of these compunds.

Q. What is a homologous series? Which two of the following organic compounds belong to the same homologous series?

C_{2} H_{6}, C_{2} H_{6} O, C_{2} H_{6} O_{2}, C H_{4} O

Q. (a) Define a homologous series. Give the name and structural formula of one homologue of the following:
CH₃OH
(b) Write the molecular formula of the third member of the homologous series of carbon compounds with general formula C_nH₂_n₊₁OH.
(c) Name any two fossil fuels.

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