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Which one of the following molecules exhibits all three electronic effects together, namely inductive, mesomeric (resonance) and hyperconjugation?

1. CH3Cl
2. CH3-CH=CH2
3. CH3-CH=CH-C(=O)-CH3
4. CH2=CH-CH=CH2

Correct answer: CH3-CH=CH-C(=O)-CH3

Solution

In CH3-CH=CH-C(=O)-CH3 (an alpha,beta-unsaturated ketone): the C=O group is polar giving an inductive effect; the conjugated C=C-C=O system allows mesomeric (resonance) delocalization; and the CH3 attached to the C=C double bond provides alpha C-H bonds for hyperconjugation. CH3Cl shows only inductive; CH3-CH=CH2 shows inductive and hyperconjugation but no mesomeric (with a heteroatom/carbonyl) in the classic sense; CH2=CH-CH=CH2 (butadiene) shows mesomeric only (no strong inductive, symmetric, terminal). Hence option C has all three.

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