Question

Pure methane can be produced by ?

• A. Wurtz reaction
• B. Kolbe’s electrolytic method
• C. Soda-lime decarboxylation
• D. Reduction with ${\mathrm{H}}_2$

Answer 1

The correct option is C

Soda-lime decarboxylation

The explanation for correct answer:-

Option: (C) Soda-lime decarboxylation

• In a decarboxylation reaction, there is a reduction of one carbon atom by the release of Carbon dioxide.
• Herein, an acid is decarboxylated through a carbanion intermediate to produce a hydrocarbon, which is done with the help of soda lime $(\mathrm{NaOH}+\mathrm{CaO}).$
• ${\mathrm{OH}}^{-}$acts as a nucleophile and attacks on carbonyl group of carboxylic acid and the carbon of the carboxylic group is released out as ${\mathrm{CO}}_2$
• Therefore, a hydrocarbon with 1 Carbon less than the carboxylic acid is produced
• When Acetic acid reacts with the soda-lime $(\mathrm{NaOH}+\mathrm{CaO}).$ it produces Methane along with the release of ${\mathrm{CO}}_2$, which can be shown as:
• $\underset{AceticAcid}{C{H}_{3}COOH}\underset{∆}{\overset{NaOH+CaO}{\to }}\underset{Methane}{C{H}_4}+\underset{CarbonDioxide}{C{O}_2}$

The explanation for incorrect answer:-

Option: (A) Wurtz reaction

• Wurtz reaction increases and doubles the number of carbon in a haloalkane.
• The general form of the Wurtz reaction equation can be written as follows:

$2\mathrm{R}-\mathrm{X}+2\mathrm{Na}\to \mathrm{R}-\mathrm{R}+2\mathrm{Na}+{\mathrm{X}}^{–}$

• Methane cannot be obtained from this reaction.
• Hence it is the incorrect option.

Option: (B) Kolbe’s electrolytic method

• Kolbe's electrolysis reaction increases the number of carbon in a sodium salt of carboxylic acid.
• The general form of Kolbe's electrolysis can be given as:

$2\mathrm{RCOOK}+2{\mathrm{H}}_2\mathrm{O}\to \mathrm{R}-\mathrm{R}+2{\mathrm{CO}}_2+2\mathrm{KOH}+{\mathrm{H}}_2$

• Methane cannot be obtained from this reaction.
• Hence it is the incorrect option.

Option: (D) Reduction with ${\mathrm{H}}_2$

• In the Reduction with ${\mathrm{H}}_2$ , the addition of Hydrogen occurs by adsorption in π orbitals of unsaturated carbon.
• It converts unsaturated Carbon bonds to saturated, unsaturated Oxygen bonds to alcohol, and unsaturated Nitrogen bonds to amine
• Therefore, it doesn’t produce methane.

Thus, Pure methane can be produced by Soda-lime decarboxylation .