Question

$C_2H_2\xrightarrow[1% \ HgOH]{H_2SO_4}A\overset{[O]}{\rightarrow}B$, B is
A. An acid
B. An aldehyde
C. A ketone
D. Ethanol

Answer 1

Hint: In the presence of hot dilute sulfuric acid and mercury (I) hydroxide, alkynes add a molecule of water. Alkyne forms aldehydes or ketones as a product in this reaction. Aldehydes on oxidation will form carboxylic acids.

Complete Step by Step Solution:
Alkynes are unsaturated hydrocarbons having the general formula \[{{\rm{C}}_{\rm{n}}}{{\rm{H}}_{{\rm{2n - 2}}}}\].
Such compounds are more unsaturated than alkenes as they contain a triple bond in a molecule.
 \[{{\rm{C}}_{\rm{2}}}{{\rm{H}}_{\rm{2}}}\] or acetylene is the first member of this series with two carbon atoms joined by one triple bond.
The IUPAC name of acetylene is ethyne.
Ethyne when treated with hot dilute sulfuric acid and mercury (I) hydroxide undergoes hydration.

A molecule of water gets added to it.
Vinyl alcohol is formed as the resulting product.
Vinyl alcohol being unstable undergoes tautomerization to form acetaldehyde or ethanal.
So, A in the given reaction is acetaldehyde.

Oxidation is the reaction by which the addition of oxygen to a molecule happens.
Acetaldehyde on oxidation leads to the formation of acetic acid or ethanoic acid.
So, B is acetic acid.

The reaction happens as follows:

Image: hydration of acetylene followed by hydrolysis

So, option A is correct.

Additional Information: Compounds whose structure differs in the arrangement of atoms but exists simultaneously in dynamic equilibrium with each other are called tautomers. This phenomenon is called tautomerism.

Note: Acetylene in the reaction with HgOH and dilute sulfuric acid leads to the formation of vinyl alcohol. Vinyl alcohol tautomerizes to form acetaldehyde. Acetaldehyde on oxidation forms acetic acid.