Question

The compound which does not lead to nitrile by substitution with NaCN/DMSO is

• A. benzyl chloride
• B. ethyl chloride
• C. isopropyl chloride
• D. chlorobenzene
• E. isobutyl chloride

Hint:

Aryl halides and vinyl halides are notoriously inert toward nucleophilic substitution because of the partial double bond character of the C-X bond.

Answer 1

The Correct Option is D

Concept: Substitution with NaCN in DMSO (a polar aprotic solvent) typically proceeds via the \(S_N2\) mechanism. This requires the carbon-leaving group bond to be easily breakable by a nucleophile (\(CN^-\)).

Step 1: Evaluate aliphatic vs. aromatic halides.
Options (A), (B), (C), and (E) are all alkyl or aralkyl halides. In these compounds, the Carbon-Chlorine bond is a standard single bond, and the carbon is \(sp^3\) hybridized, allowing for nucleophilic attack.

Step 2: Analyze Chlorobenzene (D).
In chlorobenzene, the chlorine atom is directly attached to an \(sp^2\) hybridized carbon of the benzene ring.
• Resonance: The lone pair of electrons on Chlorine is in conjugation with the \(\pi\) system of the ring, giving the C-Cl bond partial double bond character.
• Hybridization: \(sp^2\) carbons are more electronegative than \(sp^3\) carbons, holding the chlorine more tightly.
• Steric Hindrance: The benzene ring prevents the nucleophile from attacking from the back (\(S_N2\)).

Step 3: Conclusion.
Due to these factors, aryl halides like chlorobenzene do not undergo simple nucleophilic substitution under standard conditions.