Question

The major product of the following reaction is P.

Number of oxygen atoms present in product 'P' is _______ (nearest integer).

Answer 1

Correct Answer: 2

The problem asks for the number of oxygen atoms in the final major product 'P' of a two-step reaction sequence starting from but-2-yne.

Concept Used:

The solution involves two sequential named organic reactions:

1. Birch Reduction (Step i): The reduction of a non-terminal alkyne with sodium (Na) in liquid ammonia (liq. NH₃) is a stereoselective reaction that produces a trans-alkene. The reaction proceeds via a radical anion mechanism.
2. Syn-Dihydroxylation (Step ii): The reaction of an alkene with cold, dilute, alkaline potassium permanganate (KMnO₄), also known as Baeyer's reagent, results in the addition of two hydroxyl (-OH) groups across the double bond. This addition is stereospecific and occurs in a syn fashion, meaning both -OH groups add to the same face of the double bond.

Step-by-Step Solution:

Step 1: Analyze the first step of the reaction sequence.

The starting material is but-2-yne (\( \text{CH}_3\text{C} \equiv \text{C}-\text{CH}_3 \)). The reagent is Na/liq. NH₃. This is the Birch reduction of an alkyne.

This reaction reduces the alkyne to an alkene with trans stereochemistry.

\[ \text{CH}_3\text{C} \equiv \text{C}-\text{CH}_3 \xrightarrow{\text{(i) Na/liq.NH}_3} \text{trans-But-2-ene} \]

The structure of the intermediate product, trans-but-2-ene, is:

 

Step 2: Analyze the second step of the reaction sequence.

The intermediate, trans-but-2-ene, is treated with dilute KMnO₄ at 273K (0 °C). This is a syn-dihydroxylation reaction (Baeyer's test).

Two hydroxyl (-OH) groups are added across the double bond on the same side. This reaction converts the alkene into a vicinal diol (a glycol).

\[ \text{trans-But-2-ene} \xrightarrow{\text{(ii) dil. KMnO}_4, 273\text{K}} \text{Butane-2,3-diol} \]

The reaction breaks the pi bond of the alkene and forms two new C-O sigma bonds.

The structure of the final product 'P', butane-2,3-diol, is:

\[ \text{CH}_3-\underset{\text{OH}}{\underset{|}{\text{CH}}}-\underset{\text{OH}}{\underset{|}{\text{CH}}}-\text{CH}_3 \]

(Note: The reaction of a trans-alkene with syn-addition results in a racemic mixture of (2R,3R) and (2S,3S) enantiomers, but the chemical constitution is butane-2,3-diol).

Final Computation & Result:

The question asks for the number of oxygen atoms present in the final product 'P'.

The final product is butane-2,3-diol, with the chemical formula \( \text{C}_4\text{H}_{10}\text{O}_2 \).

By inspecting the structure or the formula, we can see that the molecule contains two hydroxyl (-OH) groups. Each hydroxyl group has one oxygen atom.

Therefore, the total number of oxygen atoms in product 'P' is 2.

Answer 2

The reaction proceeds as follows:
Step 1: The triple bond in CH$_3$C$\equiv$C-CH$_3$ is reduced to a trans-alkene using Na/liq. NH$_3$:
\[\text{CH}_3\text{C} \equiv \text{C-CH}_3 \xrightarrow{\text{Na/liq. NH}_3} \text{CH}_3\text{CH} = \text{CHCH}_3\]
Step 2: The trans-alkene undergoes oxidation with dilute KMnO$_4$ at 273 K. This leads to the formation of a vicinal diol:
\[\text{CH}_3\text{CH} = \text{CHCH}_3 \xrightarrow{\text{dil. KMnO}_4} \text{CH}_3\text{CH(OH)CH(OH)CH}_3\]
The final product `P' contains two hydroxyl (-OH) groups, contributing two oxygen atoms.
Number of oxygen atoms in product `P':
\[2 \, (\text{from hydroxyl groups})\]