Which of the following compounds would you expect to have a

UV absorption in the 200 to 400 nm range?

a)

b)

c)

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.

Energy equation is as follow:

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

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.

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

Transitions in which $\mathrm{\pi }$electron is excited to an antibonding ${\mathrm{\pi }}^{*}$MO are called transitions.

This type of transition occur in unsaturated compounds containing double and triple bonds and also in aromatic compounds.

Organic compounds containing unconjugated unsaturated system, the absorption due to $\mathrm{\pi }-{\mathrm{\pi }}^{*}$transition occur below 200 nm while in conjugated system, absorption occur above 200 nm.

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Only compounds having alternating multiple bonds show ultraviolet absorptions in the 200-400 nm range. In the following given compounds only pyridine (b) absorbs in this range.

(a) Since, compound (a) is an unconjugated unsaturated system, then the absorption due to $\mathrm{\pi }-{\mathrm{\pi }}^{*}$transition occur below 200 nm, hence this compound does not show ultraviolet absorptions in the 200-400 nm range

a)

(b) Since, pyridine $(\mathrm{\lambda }=254\mathrm{nm})$is a conjugated unsaturated system, the absorption due to$\mathrm{\pi }-{\mathrm{\pi }}^{*}$transition occur above 200 nm, hence this compound show ultraviolet absorptions in the 200-400 nm range.

b)

(c) Since, ketene is a unconjugated unsaturated system, the absorption due to $\mathrm{\pi }-{\mathrm{\pi }}^{*}$transition occur below 200 nm, hence this compound does not show ultraviolet absorptions in the 200-400 nm range.