Chapter 13: Q 36P (page 605)

Predict the $λ_{m a x}$ of the following compound:

Short Answer

Expert verified

The given compound has $λ_{m a x}$ value of 219 nm.

Step by step solution

Step-by-step Solution Step 1: Ultraviolet spectroscopy

Ultraviolet spectroscopy refers to absorption spectroscopy in part of the ultraviolet and the full adjacent visible region of the electromagnetic spectrum.

The UV region of the electromagnetic spectrum extends from 100-400 nm.
The visible region extends from 400-800 nm.

Factor affecting value of wavelength in UV spectroscopy

The absorption of light in UV spectroscopy promotes the $π - π *$ electron transition. For this transition occur, the energy gap between HOMO and LUMO has to correspond to energies of the UV radiation.

Generally, the most probable transition is from the highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO).

The most important factor that affects the HOMO and LUMO energy gap, and, therefore, the energy and the wavelength of absorbed radiation is the extent of conjugation. The conjugation lowers the difference between HOMO and LUMO and the more extended it is, the lower the energy difference will be.
The value of in the ultraviolet spectrum of dienes becomes larger with increase in alkyl substitution.

Calculate λmax for the given compound

The wavelength which occurs at the transition increases as the number of conjugated double bonds in the compound increases. In the given compound, we have two conjugated double bonds like in methyl vinyl ketone. So it must have approximately the same value for its , which is 219 nm, because the compound has the same chromophore as methyl vinyl ketone.