Question

Arrange the molecules according to where you would expect to find

their wavelength of maximum absorption in UV spectroscopy, from shortest to longest wavelength.

a)

b)

c)

Short answer

a)

b)

c)

Step-by-step solution

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 extend from 400-800 nm.

When UV or visible radiation is passed through the solution of an organic compound, it is absorbed by the molecule. As a result of absorption of energy, an electron can be promoted from the ground state orbital to an excited state orbital.

Energy and the wavelength are inversely proportional to each other as shown below in the equation.

$$\begin{gathered} E=hv \\ E=\frac{hc}{\lambda } \end{gathered}$$

Where E is the energy of the absorption, h is the Planck’s constant, c the velocity of light and 𝛌 the wavelength of absorption.

Factor affecting value of wavelength in UV spectroscopy

The absorption of light in UV spectroscopy promotes the electron transition. In order 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 will affect 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 lower the energy, the longer the wavelength of absorption is.

Arrange the molecules from shortest to longest wavelength in UV spectroscopy

a) The compound of the shortest ${\mathrm{\lambda }}_{\max }$has no conjugated double bonds.

b) The compound of the shortest ${\mathrm{\lambda }}_{\max }$has no conjugated double bonds

c) The compound of the shortest ${\mathrm{\lambda }}_{\max }$has both aromatic system and conjugated double bond of diene in a side chain, but those are not connected in a way that could enable extended conjugation. The second compound has the extended conjugation, therefore it have longer wavelength value than the first compound as shown below: