Question

Which statement is incorrect about the given reaction?

$HC \equiv CH \xrightarrow[+Hg^{2+}]{Dil.H_2SO_4} CH_3CH = O$

• A. C atom accepting the H is reduced, and the C atom forming a bond with OH is oxidised.
• B. The final product results from tautomerism of an enol.
• C. The average oxidation number of the two C atoms in each compound is the same (-1).
• D. The average oxidation number of the two C atoms in each compound is the same (-2). The net effect is no change in average oxidation state.

Answer 1

Answer: (D)

The average oxidation number of the two C atoms in each compound is the same (-2). The net effect is no change in average oxidation state.

Answer 2

The reaction given is the hydration of acetylene ($HC \equiv CH$) catalyzed by $Hg^{2+}$ and dilute $H_2SO_4$, which produces acetaldehyde ($CH_3CHO$). This reaction proceeds via the formation of an unstable enol intermediate, vinyl alcohol ($CH_2=CH-OH$), which rapidly tautomerizes to acetaldehyde.

Let's analyze each statement:

1. "C atom accepting the H is reduced, and the C atom forming a bond with OH is oxidised."

Let's assign oxidation states to the carbon atoms in the reactant and product. We assign +1 to H, -2 to O, and 0 for C-C bonds.

In $HC \equiv CH$: Let the oxidation state of each C be $x$. $2x + 2(+1) = 0 \implies 2x = -2 \implies x = -1$. Both carbons have an oxidation state of -1.

In $CH_3CHO$:

For the $CH_3$ carbon: Let its oxidation state be $x_1$. $x_1 + 3(+1) + 0 = 0 \implies x_1 = -3$.

For the $CHO$ carbon: Let its oxidation state be $x_2$. $x_2 + 1(+1) + 1(-2) + 0 = 0 \implies x_2 - 1 = 0 \implies x_2 = +1$.

Comparing the oxidation states from reactant to product: One carbon goes from -1 to -3 (reduction), and the other goes from -1 to +1 (oxidation). The carbon that becomes part of the $CH_3$ group has gained hydrogens and is reduced. The carbon that becomes part of the $CHO$ group is bonded to oxygen and is oxidized. The statement correctly identifies which carbon is reduced (gains H) and which is oxidized (bonds to O/OH). This statement is correct.

2. "The final product results from tautomerism of an enol."

The hydration of alkynes first forms an enol intermediate ($CH_2=CH-OH$ in this case). Enols are generally unstable and rapidly tautomerize to the more stable keto form (aldehyde or ketone). So, acetaldehyde is formed by the tautomerization of vinyl alcohol. This statement is correct.

3. "The average oxidation number of the two C atoms in each compound is the same (-1)."

For $HC \equiv CH$: Average oxidation number = $(-1 + -1) / 2 = -1$.

For $CH_3CHO$: Average oxidation number = $(-3 + +1) / 2 = -2 / 2 = -1$.

The average oxidation number of the two C atoms is -1 in both acetylene and acetaldehyde. This statement is correct.

4. "The average oxidation number of the two C atoms in each compound is the same (-2). The net effect is no change in average oxidation state."

From the calculation in statement 3, the average oxidation number of the two C atoms in each compound is -1, not -2. The second part of the statement, "The net effect is no change in average oxidation state," is correct because the average oxidation state remains -1. However, the claim that the average oxidation state is -2 is incorrect. This statement is incorrect.

The question asks for the incorrect statement. Statement 4 is incorrect.