Question

Which carbonyl group in each pair absorbs at a higher frequency?

a.

b.

Short answer

a. (E)-but-2-enal absorbs at a higher frequency.

b. Cyclopropanone absorbs at a higher frequency.

Step-by-step solution

Effect of conjugation on wavenumber

If conjugation is present in a compound, it possesses a lower wavenumber. This is because the carbonyl group loses its double-bond character and shifts frequency closer to C-O.

Explanation for higher wavenumber 

a.

The given compounds are shown below:

Given structures

In the first compound, the double bond is in conjugation with the carbonyl group. Due to this, it has a lower wavenumber as compared to the second compound.

b.

The given compounds are shown below:

Given structures

As the ring size increases, the strain of the ring decreases. Thus, the smaller the ring, the higher will be the wavenumber.

Cyclopropanone has more strain, due to which it possesses a higher wavenumber.