JEE Main & Advanced Chemistry Hydrocarbons / हाइड्रोकार्बन Individual members of alkanes

(1) Methane : Known as marsh gas.

(i) Industrial method of preparation : Mathane gas is obtained on a large scale from natural gas by liquefaction. It can also be obtained by the application of following methods,

(a) From carbon monoxide : A mixture of carbonmonoxide and hydrogen is passed over a catalyst containing nickel and carbon at \[{{250}^{o}}C\] when  methane is formed.

\[CO+3{{H}_{2}}\underset{{{250}^{o}}C}{\mathop{\xrightarrow{Ni+C}}}\,C{{H}_{4}}+{{H}_{2}}O\]

(b) Bacterial decomposition of cellulose material present in sewage water : This method is being used in England for production of methane.

\[\underset{\text{Cellulose}}{\mathop{{{({{C}_{6}}{{H}_{10}}{{O}_{5}})}_{n}}}}\,+n{{H}_{2}}O\xrightarrow{{}}3nC{{H}_{4}}+3nC{{O}_{2}}\]

(c) Synthesis :

• By striking an electric arc between carbon electrodes in an atmosphere of hydrogen at \[{{1200}^{o}}C\], methane is formed.

\[C+2{{H}_{2}}\xrightarrow{{{1200}^{o}}C}C{{H}_{4}}\]

By passing a mixture of hydrogen sulphide and carbon disulphide vapour through red hot copper, methane is formed. 

\[C{{S}_{2}}+2{{H}_{2}}S+8Cu\xrightarrow{High\,\,temperature}C{{H}_{4}}+4C{{u}_{2}}S\]

(ii) Physical properties

(a) It is a colourless, odourless, tasteless and non-poisonous gas.

(b) It is lighter than air. Its density at NTP is 0.71 g/L.

(c) It is slightly soluble in water but is fairly soluble in ether, alcohol and acetone.

(d) Its melting point is \[-{{182.5}^{o}}C\] and boiling point is \[-{{161.5}^{o}}C\].

(iii) Uses

(a) In the manufacture of compounds like methyl alcohol, formaldehyde, methyl chloride, chloroform, carbon tetrachloride, etc.

(b) In the manufacture of hydrogen, used for making ammonia.

(c) In the preparation of carbon black which is used for making printing ink, black paints and as a filler in rubber vulcanisation.

(d) As a fuel and illuminant.

(2) Ethane

(i) Methods of preparation

(a) Laboratory method of preparation : 

\[\underset{\text{Ethyl}\,\text{iodide}}{\mathop{{{C}_{2}}{{H}_{5}}I+}}\,2H\underset{{{C}_{2}}{{H}_{5}}OH}{\mathop{\xrightarrow{Zn-Cu\,couple}}}\,\underset{\text{Ethane}}{\mathop{{{C}_{2}}{{H}_{6}}}}\,+HI\]

(b) Industrial method of preparation :

\[\underset{\text{(ethene)}}{\mathop{\underset{\text{Ethylene}}{\mathop{C{{H}_{2}}=C{{H}_{2}}}}\,}}\,+{{H}_{2}}\underset{{{300}^{o}}C}{\mathop{\xrightarrow{Ni}}}\,\underset{\text{Ethane}}{\mathop{C{{H}_{3}}-C{{H}_{3}}}}\,\]

(ii) Physical properties

(a) It is a colourless, odourless, tasteless and non-poisonous gas.

(b) It is very slightly soluble in water but fairly soluble in alcohol, acetone, ether, etc.

(c) Its density at NTP is 1.34 g/L

(d) It boils at \[-{{89}^{o}}C\]. Its melting point is \[-{{172}^{o}}C\].

(iii) Uses

(a) As a fuel.  (b) For making hexachloroethane which is an artificial camphor.

(3) Interconversion of Alkanes

Ascent of alkane series,

(i) Methane to ethane :

\[\underset{\text{Methane}}{\mathop{C{{H}_{4}}}}\,\underset{UV}{\mathop{\xrightarrow{C{{l}_{2}}}}}\,C{{H}_{3}}Cl\underset{\text{Heat}\,\text{with}\,Na\,\text{in}\,\text{ether}}{\mathop{\xrightarrow{\text{Wurtz}\,\text{reaction}}}}\,\underset{\text{Ethane}}{\mathop{C{{H}_{3}}-C{{H}_{3}}}}\,\]

(ii) Butane from ethane :

\[\underset{(\text{excess)}}{\mathop{\underset{\text{Ethane}}{\mathop{{{C}_{2}}{{H}_{6}}}}\,}}\,\underset{UV}{\mathop{\xrightarrow{C{{l}_{2}}}}}\,\underset{\text{Ethyl chloride}}{\mathop{{{C}_{2}}{{H}_{5}}Cl}}\,\underset{\text{Heat}\,\text{with}\,Na\,\text{in}\,\text{ether}}{\mathop{\xrightarrow{\text{Wurtz}\,\text{reaction}}}}\,\underset{\text{Butane}}{\mathop{{{C}_{2}}{{H}_{5}}-{{C}_{2}}{{H}_{5}}}}\,\]

Descent of alkane series : Use of decarboxylation reaction is made. It is a multistep conversion.

Ethane to methane

\[\underset{\text{(excess)}}{\mathop{\underset{\text{Ethane}}{\mathop{{{C}_{2}}{{H}_{6}}}}\,}}\,\underset{UV}{\mathop{\xrightarrow{C{{l}_{2}}}}}\,\underset{\text{Ethyl}\,\text{chloride}}{\mathop{{{C}_{2}}{{H}_{5}}Cl}}\,\xrightarrow{Aq.\,KOH}\underset{\text{Ethyl}\,\text{alcohol}}{\mathop{{{C}_{2}}{{H}_{5}}OH}}\,\xrightarrow{[O]}\underset{\text{Acetaldehyde}}{\mathop{C{{H}_{3}}CHO}}\,\xrightarrow{[O]}\underset{\text{Acetic}\,\text{acid}}{\mathop{C{{H}_{3}}COOH}}\,\xrightarrow{NaOH}\underset{\text{Sodium}\,\text{acetate}}{\mathop{C{{H}_{3}}COONa}}\,\underset{\text{heat}}{\mathop{\xrightarrow{NaOH/CaO}}}\,\underset{\text{Methane}}{\mathop{C{{H}_{4}}}}\,\]

\[\xrightarrow{[O]}\underset{\text{Acetic}\,\text{acid}}{\mathop{C{{H}_{3}}COOH}}\,\xrightarrow{NaOH}\underset{\text{Sodium}\,\text{acetate}}{\mathop{C{{H}_{3}}COONa}}\,\underset{\text{heat}}{\mathop{\xrightarrow{NaOH/CaO}}}\,\underset{\text{Methane}}{\mathop{C{{H}_{4}}}}\,\]

\[\underset{\text{alkane}}{\mathop{\text{Higher}}}\,\underset{UV}{\mathop{\xrightarrow{C{{l}_{2}}}}}\,\underset{\text{halide}}{\mathop{\text{Alkyl}}}\,\underset{KOH}{\mathop{\xrightarrow{Aq.}}}\,\text{Alcohol}\xrightarrow{[O]}\text{Aldehyde}\xrightarrow{[O]}\]

\[\text{Acid}\xrightarrow{NaOH}\underset{\text{the}\,\text{acid}}{\mathop{\text{Sodium}\,\text{salt}\,\text{of}}}\,\underset{\text{heat}}{\mathop{\xrightarrow{NaOH/CaO}}}\,\text{Lower}\,\text{alkane}\]