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

Step 1: What is a chromophore?
A chromophore is a group in a molecule that absorbs UV or visible radiation. The type of electronic transition determines where absorption occurs.

Step 2: Types of electronic transitions.
There are four main types: sigma to sigma* (very high energy, far UV), n to sigma* (non-bonding electrons to sigma*), n to pi* (non-bonding electrons to pi*, needs a heteroatom with lone pair), and pi to pi* (pi electrons to pi*, needs unsaturation or aromatic rings).

Step 3: What is benzene's electronic structure?
Benzene contains a fully conjugated aromatic ring with delocalized pi electrons. It has no lone pairs on carbon (the ring carbons are sp² and carry no non-bonding electrons in the pi system). Therefore, n to pi* and n to sigma* transitions are not relevant to benzene's UV absorption band in the typical range.

Step 4: Identify the correct transition.
The aromatic ring of benzene has pi bonding molecular orbitals and pi* antibonding orbitals. UV light promotes a pi electron from the pi orbital to the pi* orbital. This pi to pi* transition is responsible for benzene's characteristic UV absorption around 254 nm (the fine structure band).

Step 5: Confirm the correct answer.
The chromophore responsible for UV absorption in benzene is the aromatic pi system, and the transition is pi to pi*.

Answer: Option (3) — π → π*