Question

The chromophore responsible for UV absorption in benzene is

• A. σ → σ*
• B. n → π*
• C. π → π*
• D. n → σ*

Hint:

UV absorption happens when electrons jump from a lower energy molecular orbital to a higher one. The type of jump (electronic transition) depends on the kind of bonds and electrons present in the molecule.

Answer 1

The Correct Option is C

Concept:
UV absorption happens when electrons jump from a lower energy molecular orbital to a higher one. The type of jump (electronic transition) depends on the kind of bonds and electrons present in the molecule. This question is about which transition is responsible in benzene.

Step 1: Benzene is an aromatic ring with a system of delocalised π (pi) electrons spread over its double bonds. These π electrons are the ones that absorb UV light.

Step 2: When benzene absorbs UV radiation, an electron is promoted from a filled bonding π orbital to an empty anti-bonding π* orbital. This is the π → π* transition, and it is the chromophore responsible for benzene's characteristic UV absorption.

Step 3: The other transitions need different features: σ → σ* requires very high energy (seen in saturated compounds), while n → π* and n → σ* need non-bonding (lone-pair) electrons, which the simple benzene ring does not have. So the answer is π → π*.

Answer: Option (3) — π → π*.