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In which of the following pairs is the C-O bond length shorter in the FIRST compound (compared to the second)?

1. (A) o-Methyl substituted benzoic acid compared with benzoic acid
2. (B) Cyclohexadienyl carbonyl compound compared with cyclohexadienyl aldehyde
3. (C) p-Aminobenzaldehyde compared with benzaldehyde
4. (D) Cyclohexadienone compared with cyclohexanone

Correct answer: (D) Cyclohexadienone compared with cyclohexanone

Solution

C-O bond length increases as the C=O acquires more single-bond character through resonance/conjugation that pushes electron density onto oxygen. In cyclohexadienone, the carbonyl is conjugated with the diene (cross-conjugation/extended conjugation typical of dienone systems) but the comparison hinges on the relative C=O character. In option D the first compound retains more double-bond (shorter) C=O character relative to its partner among the given choices, making the C-O bond shorter in the first compound. The other comparisons involve electron-donating groups (p-amino, o-methyl) or conjugation that lengthen the C-O of the first compound.

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