Question

An unsaturated organic compound ($\text{C}_3\text{H}_6$), undergoes the following series of reactions: $\text{C}_3\text{H}_6$ $\xrightarrow{\text{dil. H}_2\text{SO}_4}$ [B] $\xrightarrow{\text{+HCl/ZnCl}_2}$ [C] $\xrightarrow{\text{Na / dry ether}}$ [D] (Major Product)} Identify compound [D].

• A. Cyclohexane
• B. Hexane
• C. 2,3-dimethylbutane
• D. 2-methyl pentane

Hint:

The final step is a classic Wurtz reaction, which couples alkyl radicals symmetrically. Since intermediate [C] is an isopropyl group, joining two isopropyl units together head-to-head forms a 6-carbon branched chain named 2,3-dimethylbutane.

Answer 1

The Correct Option is C

Concept: This question traces an addition reaction followed by substitution and a coupling chain mechanism:
• Acid-catalyzed hydration of propene ($\text{C}_3\text{H}_6$) following Markovnikov's rule to yield an alcohol.
• Conversion of the alcohol to an alkyl halide using Lucas reagent ($\text{HCl/ZnCl}_2$).
• Wurtz coupling reaction where two molecules of the alkyl halide react with sodium in dry ether to form a symmetrical alkane.

Step 1: Identify intermediate [B] via hydration.
Hydration of propene ($\text{CH}_3-\text{CH}=\text{CH}_2$) with dilute $\text{H}_2\text{SO}_4$ adds $-\text{OH}$ to the more substituted carbon atom, producing propan-2-ol [B]: \[ \text{CH}_3-\text{CH}=\text{CH}_2 + \text{H}_2\text{O} \xrightarrow{\text{H}^+} \text{CH}_3-\text{CH(OH)}-\text{CH}_3\quad \text{[B]} \]

Step 2: Identify intermediate [C] via substitution.
Treating propan-2-ol [B] with Lucas reagent ($\text{HCl/ZnCl}_2$) replaces the hydroxyl group with chlorine, yielding 2-chloropropane [C]: \[ \text{CH}_3-\text{CH(OH)}-\text{CH}_3 \xrightarrow{\text{HCl/ZnCl}_2} \text{CH}_3-\text{CH(Cl)}-\text{CH}_3\quad \text{[C]} \]

Step 3: Perform Wurtz coupling to determine final product [D].
When 2-chloropropane reacts with sodium metal in dry ether, two isopropyl radicals couple at their central secondary carbons: \[ 2\,\text{CH}_3-\text{CH(Cl)}-\text{CH}_3 + 2\text{Na} \xrightarrow{\text{dry ether}} \text{CH}_3-\text{CH(CH}_3)-\text{CH(CH}_3)-\text{CH}_3 + 2\text{NaCl} \] The resulting coupled alkane structure is structurally named 2,3-dimethylbutane [D].