Question

Question: Which compound in each pair absorbs UV light at a longer wavelength?

a.

b.

Short answer

Answer

a.

b.

Step-by-step solution

Conjugated and non-conjugated dienes

Dienes in which a single bond separates two double bonds are conjugated dienes.

Dienes in which two double bonds are set apart by more than one single bond are non-conjugated dienes

Conjugated diene and ultraviolet radiation

The promotion of electrons in the σ bonds and unconjugated $\mathrm{\pi }$bonds requires light having a shorter wavelength and higher energy than the light in the UV region.

Whereas for the conjugated dienes, the promotion of electrons occurs by absorbing a longer wavelength of light as the energy difference between the ground and excited state decreases.

Find which compound absorbs UV light at a longer wavelength

Molecules in conjugation absorb light at a longer wavelength than the molecules not in conjugation.

Representation of A

The first compound is conjugated, and the second one is non-conjugated. So, the first compound absorbs light at a longer wavelength compared to the second one.

Representation of B

In the first compound, all the double bonds are in conjugation, and in the second compound, there is only one set of conjugated dienes. So, the first compound absorbs light at a longer wavelength compared to the second.