Question

What is X in the following sequence of reactions?
\[X\mathop \to \limits_{ - {H_2}}^{Na} Y\mathop \to \limits_{CaO}^{NaOH} C{H_4}\]
(A) Methanoic acid
(B) Ethanoic acid
(C) Propane
(D) Methane

Answer 1

Hint: It is a significant reaction that when carboxylic acids react with strong alkali metals, a strong basic salt is formed with the release of hydrogen gas. When calcium oxide is added to sodium hydroxide, soda lime is formed which results in decarboxylation of the salt.

Complete step-by-step solution:
We know that acids react with strong alkali metals like sodium and potassium that react with organic acids to produce strong basic salts along with release of hydrogen gas. In the reaction given above, ethanoic acid reacts with sodium metal that results in the formation of sodium acetate or sodium ethanoate and hydrogen gas is released. This reaction is highly exothermic in nature.
\[C{H_3}COOH + Na \to C{H_3}COONa + {H_2}\]
In decarboxylation, the – COOH or – COONa group is removed and replaced by a hydrogen atom and carbonate ion. This is performed by soda lime which is manufactured by adding sodium hydroxide solution to quicklime (CaO). Sometimes, this mixture might include calcium hydroxide as well.
The sodium salt of carboxylic acid when mixed with soda lime at high temperature results in methane gas and sodium carbonate. Thus, we get Y as sodium acetate or sodium ethanoate. This process is called decarboxylation of carboxylic acid and their salts.
\[C{H_3}COONa + NaOH + CaO \to C{H_4} + N{a_2}C{O_3}\]
Methanoic acid is also an organic acid, but on reaction with sodium metal it forms sodium methoxide and hydrogen gas, which can’t produce methane on reaction with soda lime.

Hence, the correct option is (B).

Note: In some reactions, only sodium hydroxide is used in place of soda lime. But solid sodium hydroxide absorbs water from the surrounding and ends up with puddles of extremely corrosive hydroxide solution when exposed to air. Therefore, soda lime is used as it is easy to handle and does not absorb water.