Question

Which one of the following represents hyperconjugation effect?

• A. \( \text{CH}_3 - \text{CH} = \text{CH}_2 \leftrightarrow \text{H}^{+} \dots \) (Sigma electrons shifting to double bond)
• B. \( \text{CH}_2 = \text{CH} - \stackrel{\bullet\bullet}{\text{Cl}}: \leftrightarrow \dots \) (Lone pair resonance)
• C. \( \text{CH}_3 \rightarrow \text{CH}_2 \rightarrow \text{NO}_2 \) (Inductive effect)
• D. \( \text{CH}_3 - \text{CH} = \text{CH}_2 + \text{H}^+ \rightarrow \dots \) (Electromeric effect)

Hint:

Hyperconjugation requires at least one hydrogen atom on the carbon adjacent (alpha) to a double bond or carbocation. Look for the "no-bond" \( \text{H}^+ \) structure.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:

Hyperconjugation involves the delocalization of \( \sigma \)-electrons of an \( \alpha \)-C-H bond into an adjacent empty or partially filled p-orbital or \( \pi \)-orbital. It is also known as "no-bond resonance".
Step 2: Detailed Explanation:

• Option 1:
  Depicts the transfer of electron density from the C-H sigma bond of the methyl group to the adjacent C-C pi bond, creating a double bond character and leaving \( \text{H}^+ \) with no bond (hyperconjugation).
• Option 2:
  Shows the delocalization of lone pair electrons from Chlorine to the pi system. This is the Mesomeric (Resonance) Effect (+M).
• Option 3:
  Shows the shifting of sigma electrons along a saturated chain due to electronegativity difference. This is the Inductive Effect.
• Option 4:
  Shows the movement of pi electrons at the demand of an attacking reagent. This is the Electromeric Effect.

Step 3: Final Answer:

Option (A) represents hyperconjugation.