Question

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.