Question

What is the name of the product $B$ in the following reaction?
$Ca{{C}_{2}}\xrightarrow{hydrolysis}A\xrightarrow{HgS{{O}_{4}}+dil.{{H}_{2}}S{{O}_{4}}}B$
(A). acetylene
(B). acetaldehyde
(C). acetone
(D). acetic acid

Answer 1

Hint: Think about what would happen if we carried out the hydrolysis of calcium carbide. When the product is formed, consider what would happen if the water is added to the obtained molecule.

Complete step by step solution:
The commercial method for the production of acetylene is mentioned here. We use calcium carbide and carry out hydrolysis to obtain one molecule of acetylene and one molecule of calcium hydroxide. The reaction for this is:
\[Ca{{C}_{2}}+2{{H}_{2}}O\to H-C\equiv C-H+Ca{{(OH)}_{2}}\]
Here, calcium carbide is found in its ionic state. $C{{a}^{2+}}$ is presence with ${{[:C\equiv C:]}^{2-}}$ and we can see how the hydrogen atom combines with the carbon atoms by forming a bond through the lone pair present, and calcium combines with the hydroxide ions to form an ionic compound. From this, we can deduce that the main product $A$ formed is acetylene.
Now, let us see what happens when acetylene reacts with dilute ${{H}_{2}}S{{O}_{4}}$ in the presence of $HgS{{O}_{4}}$. So, we know that when acetylene reacts with dilute sulphuric acid, it reacts with the aqueous composition that is present in the acid and not the acid itself. It also requires the presence of $H{{g}^{2+}}$ ions to react, which are provided by the $HgS{{O}_{4}}$. So, when acetylene reacts with dilute ${{H}_{2}}S{{O}_{4}}$, one hydrogen proton attaches itself to one of the carbon atoms and the hydroxide ion attaches itself to the other carbon atom. This molecule then undergoes tautomerization to form acetaldehyde. The reaction is as follows:


Hence,, the correct answer is option (B).

Note: Remember that this enol is not that stable and hence tautomerizes into an aldehyde so that the energy levels remain low. Periodic tautomerization back into an enol also occurs. Electrons distributed over a wider area are more stable.