Question

Explain, from a molecular point of view, why a conjugated diene absorbs UV radiation while an alkene and cyclohexane do not.

Answer 1

Alkenes also absorb uv radiation but compared to conjugated dienes alkenes absorb high energy radiation that uv radiation of low wavelength. Alkanes absorb higher radiation than alkenes that means at very low wavelength below 120nm which is called as far uv region.

When uv radiation is absorbed by a molecule it undergoes some electronic transitions between sigma bonding and anti bonding moleculer orbitals and between pi bonding and anti bonding moleculer orbitals.

Since sigma bonds are strong they require high energy to break the sigma bond. So low wavelength.

But in alkenes pi bonds are present which are broken with less energy than that compared with alkanes. So alkenes require less energy uv radiation than for alkanes. That means alkenes require uv radiation of high wavelength than alkanes. It is around 180nm.

But when conjugated dienes or trienes are considered they require low energy than alkenes. That means high wavelength.

This is because in conjugated dienes the pi bond is already broken by conjugation so they require less energy than expected.

In terms of moleculer orbitals, the energy gap between bonding and anti bonding moleculer orbitals is very low. So with less energy radiation transitions take place. But in case of isolated doublebond the energy gap between bonding and anti bonding moleculer orbitals is high. So they require high energy for electronic transitions.

For example let us consider 1,3 butadiene.

In this two bonding moleculer orbitals and two anti bonding moleculer orbitals. We can understand why energy gap is low from this diagram

1,3 butadiene absorbs uv radiation at 214nm where as ethylene absorbs at 182nm.