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Consider the central carbon-carbon bond in each compound: (I) CH3O-CH=CH-NO2, (II) CH2=CH-NO2, (III) CH2=CH-Cl, (IV) CH2=CH2. Arrange these compounds in decreasing order of their C=C (carbon-carbon) bond length.

1. I > II > III > IV
2. IV > III > II > I
3. I > III > II > IV
4. II > III > I > IV

Correct answer: I > II > III > IV

Solution

Ethylene (IV) has a pure C=C double bond, the shortest. Conjugation with a group that withdraws/accepts pi density introduces single-bond character into the C=C, lengthening it. Vinyl chloride (III) has weak conjugation (Cl lone pair donation), nitroethylene (II) has stronger withdrawal by NO2 giving more single-bond character, and compound I is a push-pull alkene (OCH3 donor on one carbon, NO2 acceptor on the other) which maximizes single-bond character of the central C-C, making it the longest. Order of decreasing length: I > II > III > IV.

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