JEE Challenger
More from Aldehydes, Ketones and Carboxylic Acids
ChemMedAdv 2008 • Paper 1

Identification of Product I in Hex-3-ynal Reaction Sequence

Comprehension Passage

Paragraph for Question Nos. 61 to 63

In the following reaction sequence, products I\mathbf{I}, J\mathbf{J} and L\mathbf{L} are formed. K\mathbf{K} represents a reagent.

Passage Diagram 0

The structure of the product I\mathbf{I} is

Options

A
Option A Diagram 1
B
Option B Diagram 1
C
Option C Diagram 1
D
Option D Diagram 1
Correct

Topics & Concepts

Aldehydes, Ketones and Carboxylic AcidsAlkanes, Alkenes, AlkynesHaloalkanes and HaloarenesOxidation and Reduction (Organic)Nucleophilic Substitution

Step-by-Step Solution

To determine the structure of product I\mathbf{I}, let us trace the first two steps of the reaction sequence starting from hex-3-ynal:

  1. Starting Material:
    Hex-3-ynal has the structure:
    CH3−CH2−C≡C−CH2−CHO\text{CH}_3-\text{CH}_2-\text{C}\equiv\text{C}-\text{CH}_2-\text{CHO}

  2. Step 1: Reduction with NaBH4\text{NaBH}_4:
    Sodium borohydride (NaBH4\text{NaBH}_4) selectively reduces the carbonyl group of aldehydes to primary alcohols while leaving the carbon-carbon triple bond (C≡C\text{C}\equiv\text{C}) unaffected:
    CH3−CH2−C≡C−CH2−CHO→NaBH4CH3−CH2−C≡C−CH2−CH2OH(hex-3-yn-1-ol)\text{CH}_3-\text{CH}_2-\text{C}\equiv\text{C}-\text{CH}_2-\text{CHO} \xrightarrow{\text{NaBH}_4} \text{CH}_3-\text{CH}_2-\text{C}\equiv\text{C}-\text{CH}_2-\text{CH}_2\text{OH} \quad (\text{hex-3-yn-1-ol})

  3. Step 2: Bromination with PBr3\text{PBr}_3:
    Phosphorus tribromide (PBr3\text{PBr}_3) selectively converts the primary alcohol into the corresponding alkyl bromide via an SN2\text{S}_\text{N}2 pathway, leaving the alkyne intact:
    CH3−CH2−C≡C−CH2−CH2OH→PBr3CH3−CH2−C≡C−CH2−CH2Br(I)\text{CH}_3-\text{CH}_2-\text{C}\equiv\text{C}-\text{CH}_2-\text{CH}_2\text{OH} \xrightarrow{\text{PBr}_3} \text{CH}_3-\text{CH}_2-\text{C}\equiv\text{C}-\text{CH}_2-\text{CH}_2\text{Br} \quad (\mathbf{I})

Thus, product I\mathbf{I} retains the internal alkyne (triple bond) functionality, which is uniquely depicted in option (D).

Therefore, the correct option is D.

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