a. Which compound has the stretching vibration for its carbonyl group at the highest frequency: acetyl chloride, methyl acetate, or acetamide?

b. Which one has the stretching vibration for its carbonyl group at the lowest frequency?

In infrared spectroscopy, the stronger the bond between two atoms, the higher will be its frequency. Also, if a similar type of bond between two atoms is considered in different compounds which have different substituents attached to it then due to the change in the electronic environment of the bond both bonds show different frequencies in infrared spectra.

The carbonyl stretching frequency in different compounds such as acetyl chloride, methyl acetate, and acetamide depends on the substituent attached to the carbonyl compound. The higher the electron-withdrawing group attached to the carbon atom of the carbonyl group, the stronger will be the carbonyl bond thus, the higher will be the stretching frequency.

Among the given three types of compounds, in acetyl chloride, the highly electronegative chlorine atom attached to the carbonyl group is more electron-withdrawing due to its inductive effect and hence its stretching frequency is high.

In methyl acetate, an electronegative oxygen atom attached directly to the carbonyl group will increase the stretching frequency through an inductive effect but simultaneously lowers the stretching frequency through its resonance effect which provides electrons to a carbonyl group. In one resonance form, there is a double bond between carbonyl carbon and oxygen, in another resonance structure single bond character between carbon and oxygen. Therefore, the net bond order between carbonyl carbon and oxygen is 1.5. As the resonance effect is more pronounced than the inductive effect thus, it shows comparatively lower stretching frequency.

Resonance effect of methyl acetate

In acetamide, the nitrogen atom attached to the carbonyl group shows a resonance effect and increases the electron density on the carbon atom of the carbonyl group thus, lowers the stretching frequency of the carbonyl group.

Thus, the stretching vibration for the carbonyl group of acetyl chloride is at the highest frequency.

b)

Due to the presence of -I.E of the chloro group in acetyl chloride, the stretching vibration for the carbonyl group is at the highest frequency.

Both methyl acetate and acetamide show resonance effects. But besides of the mesomeric effect, oxygen and nitrogen show the inductive effect on the carbonyl group. Here in oxygen inductive effect is more compared with the nitrogen inductive effect in amide. Therefore, the stretching vibration for the carbonyl group of acetamide is at the lowest frequency than the ester group.