Which of the following compounds does not exhibit resonance?

Solution

Chemistry ma’am · 3 steps

Resonance needs a lone pair or π bond right next to another π bond, with no sp³ carbon between.

1. The rule. Resonance happens when p orbitals on adjacent atoms overlap. A lone pair next to a π bond, or alternating π bonds, will do. An sp³ carbon in between breaks the chain because it has no free p orbital.

   Compound	What is conjugated	Resonance?
   (a) CH₃CH₂O-CH=CH₂	Lone pair on O sits directly next to C=C	Yes
   (b) Benzyl alcohol, C₆H₅CH₂OH	The benzene ring itself has alternating π bonds	Yes
   (c) CH₃CH₂CH₂CONH₂	Lone pair on N sits directly next to C=O (amide)	Yes
   (d) CH₃CH₂CH=CH-CH₂-NH₂	CH₂ (sp³) separates the N lone pair from C=C	No

2. Check option (d). The only π bond is one isolated C=C. The NH₂ lone pair is separated from it by a saturated CH₂, so it cannot delocalise into the double bond.
3. So. Option (d) is the compound that shows no resonance.

The idea to keep. Before calling something resonance-stabilised, check that the lone pair or π bond is directly attached to the next π system. One sp³ carbon in between switches resonance off.

Answer from the NTA final answer key. Solution by PrepWiser.