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Magazine Chapter 13: Q 61. (page 526)
Question: Explain why a ketone carbonyl typically absorbs at a lower wavenumber than an aldehyde carbonyl (1715 vs. 1730cm-1)
Short Answer
Answer
Weaker bonds would vibrate at a lower frequency.
Step by step solution
Inductive effect
The inductive effect refers to the electron-withdrawing effect of an electronegative atom, increasing the polarity of the bond.
The two electron-donating (EDG) methyl groups in ketone would increase the electron density on the carbonyl carbon.
The electronegative oxygen would pull electrons towards itself and would increase the polarity of the C=O bond. This would weaken the carbonyl bond giving a lower IR stretching frequency.
In aldehydes, the hydrogen being less electron-donating than the alkyl group would not depolarise the C=O bond.
Inductive effect of ketone and aldehyde
Resonance effect
Ketones have more polarity and higher dipole moment than aldehydes because they have more resonating structures compared to aldehydes.
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