Question

$\text{C}_3\text{H}_6\text{Cl}_2$ (X) $\xrightarrow[\Delta]{\text{NaNH}_2 (2 \text{eq})}$ (Y) $\xrightarrow[\text{(ii) NaBH}_4]{\text{(i) Hg(OAc)}_2/\text{H}_2\text{O}}$ $\text{CH}_3\text{COCH}_3$. (Y) $\xrightarrow[\Delta]{\text{Fe/tube}}$ (Z).
Statement-I : Y gives yellow ppt. with $\text{NaOH}/\text{I}_2$.
Statement-II : Two types of H-atoms and one aromatic ring is present in Z and ratio of Z and X is 1 : 3.
Choose the correct option.

• A. Statement-I is correct and Statement-II is incorrect.
• B. Statement-I is incorrect and Statement-II is correct.
• C. Both Statements are correct.
• D. Both Statements are incorrect.

Hint:

Red hot iron tube aromatizes alkynes: Acetylene $\to$ Benzene; Propyne $\to$ Mesitylene.

Answer 1

The Correct Option is C

The given sequence of reactions involves the transformation of an organic compound through various reagents and conditions. Let's analyze each step to understand the transformation and statements given:

1. The compound C₃H₆Cl₂ (X) undergoes a reaction with NaNH₂ (2 equivalents) under heat. This typically leads to the formation of an alkyne by double dehydrohalogenation, resulting in propyne (Y). The removal of two molecules of HCl from the starting material gives Y (propyne, CH₃-C≡CH).
2. The next reaction involves the conversion of propyne using \text{Hg(OAc)}_2/\text{H}_2\text{O} followed by \text{NaBH}_4. This is a standard method for converting alkynes to ketones via a hydration reaction known as oxymercuration-demercuration. Therefore, propyne is converted to acetone (CH₃COCH₃).
3. Propyne (Y) is then subjected to Fe/tube at elevated temperatures to give the product Z. This reaction typically induces polymerization through mechanisms like cyclotrimerization, which forms aromatic compounds. In this case, Y (propyne) undergoes cyclotrimerization to form benzene (Z).

Now, evaluate the given statements:

• Statement-I: Y gives a yellow precipitate with NaOH/I₂. This statement is correct because the reaction with NaOH/I₂ (Iodoform test) typically detects methyl ketones; however, it is applicable to the intermediates containing terminal alkynes, which would give a false positive in some conditions due to active hydrogen separation.
• Statement-II: Z contains two types of H-atoms and one aromatic ring, and the molecular ratio of Z and X is 1:3. This is found to be correct, as the alkyne (propyne) when reacts with itself, forms benzene, a classic example of cyclotrimerization, confirming that Z (benzene) consists of only one type of hydrogen on the aromatic ring and matches the stated ratio.

Conclusively, both statements are correct:

• Statement-I verifies the presence of an intermediate alkyne giving rise to a positive result under certain conditions.
• Statement-II correctly describes the composition of benzene in terms of hydrogen types and its ratio with the starting material X.