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Question 18.35: Explain why the IR absorption of the C=C bond of 1-butene (1......

Explain why the IR absorption of the C=C bond of 1-butene (1642 cm^{−1}) is at higher wavenumber than the C=C bond absorption of 3-buten-2-one (1613 cm^{−1}).

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The double bond of 3-buten-2-one is conjugated to the carbonyl group. One of the resonance structures has a double bond between C-3 and C-4. Therefore, the C-3-C-4 bond has some double bond character, which decreases the energy required for bond-stretching.

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