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JEE Main2026Apr 4, Shift 2Chemistry
Q.

Consider the following molecules/species:

The correct order of carbon–oxygen double bond length is:

  1. A

    x y z

  2. B

    y z x

  3. C

    z x y

  4. D

    x z y

Solution

Compare the extent of C=O bond order reduction due to delocalization.

(x) Tropone: the carbonyl participates in a quasi-aromatic resonance — the alternative structure has a positively charged 6π aromatic tropylium ring and a negatively charged oxygen. This delocalization weakens the C=O bond substantially, making it the longest.

(z) Acetate: symmetric resonance distributes negative charge equally between the two oxygens, so the formal C=O acquires partial single-bond character. Bond order — intermediate length.

(y) Acetone: pure C=O double bond, bond order = 2 — shortest.

x z y.

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