Question

Describe different types of electronic transitions in a molecule. Which type of transition is responsible for a band at max = 279 nm in absorption spectrum of acetone.

Answer 1

there are four types electronic transition possible in molecules:
1. σ to σ*
2. n to σ*
3. n to π*
4. π to π*

In case of n to σ* the range of lambda max is 150-160 nm
                π to π* the range of lambda max is 180-190 nm
                n to π* the range of lambda max is 275-295 nm in case of general carbonyl compounds.
so lambda max = 279 nm is due to n to π* transition.