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

This query seeks the count of oxygen atoms within the ultimate major product, denoted as 'P', derived from a two-stage reaction sequence initiating with but-2-yne.

Concept Utilized:

The resolution necessitates the application of two sequential, named organic transformations:

1. Birch Reduction (Stage i): The reductive process applied to a non-terminal alkyne using sodium (Na) dissolved in liquid ammonia (liq. NH₃) is a stereoselective reaction yielding a trans-alkene. This reaction mechanism involves a radical anion intermediate.
2. Syn-Dihydroxylation (Stage ii): The reaction between an alkene and cold, dilute, alkaline potassium permanganate (KMnO₄), also recognized as Baeyer's reagent, results in the addition of two hydroxyl (-OH) moieties across the double bond. This addition exhibits stereospecificity and proceeds in a syn manner, indicating that both -OH groups attach to the identical face of the double bond.

Step-by-Step Resolution:

Step 1: Examination of the initial reaction stage.

The reactant is but-2-yne (\( \text{CH}_3\text{C} \equiv \text{C}-\text{CH}_3 \)). The reacting agent is Na/liq. NH₃, indicative of a Birch reduction of an alkyne.

This transformation reduces the alkyne to an alkene characterized by 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 structural representation of the intermediate product, trans-but-2-ene, is as follows:

Step 2: Analysis of the subsequent reaction stage.

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

Two hydroxyl (-OH) groups are appended across the double bond on the same plane. This reaction effects the conversion of the alkene into a vicinal diol (glycol).

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

The reaction involves the cleavage of the alkene's pi bond and the formation of two new C-O sigma bonds.

The structural depiction 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 yields a racemic mixture of (2R,3R) and (2S,3S) enantiomers, but the chemical composition remains butane-2,3-diol).

Final Calculation & Outcome:

The objective of this inquiry is to determine the quantity of oxygen atoms present in the final product 'P'.

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

A review of the structure or the chemical formula reveals the presence of two hydroxyl (-OH) groups within the molecule. Each hydroxyl group contains a single oxygen atom.

Consequently, the total number of oxygen atoms in product 'P' is calculated to be 2.