Chapter 25, Problem 1Q

Interpretation Introduction

Interpretation:

$\text{n}\to {\text{π}}^{\text{*}}$ and $\text{π}\to {\text{π}}^{*}$ transition in acetaldehyde should be identified.

Concept introduction:

Organic compounds absorb in ultraviolet or visible part of electromagnetic spectrum. The UV and visible light have a wavelength of $\text{10-400nm}$ and $\text{400-800nm}$, respectively. When electrons undergo a transition from bonding and non-bonding to anti-bonding orbitals, absorption in UV and VIS region occurs.

In case of conjugated compounds, $\text{n}\to {\text{π}}^{\text{*}}$ and $\text{π}\to {\text{π}}^{*}$ transitions only occur.

UV and VIS spectroscopy help in the characterization of functional groups present in organic compounds and identification of the types of transition they show through wavelength of light absorbed.

In UV and VIS spectroscopy, a spectrum is obtained as a plot of absorbance versus wavelength in nanometers. Absorbance is calculated by Beers Lambert's law which is as follows:

  $A\text{=}\text{log}\left(\frac{I°}{I}\right)\text{=}\text{ε}lc$

Where,

• $I°$ is incident light intensity
• $I$ is transmitted light intensity.
• $\text{ε}$ is molar absorptivity coefficient.
• $l$ is length of the cuvette.
• $c$ is concentration.

Beers Lambert's law is utilized for estimation of the concentration of a solution as the absorbance of a solution is directly related to the concentration of the solution.

Explanation of Solution

According to Planck's law, energy absorbed is inversely related to the wavelength of the transition.

The energy gap in $\text{π}\to {\text{π}}^{*}$ is larger than $\text{n}\to {\text{π}}^{\text{*}}$ so $\text{π}\to {\text{π}}^{*}$ transition occurs at a shorter wavelength in the UV region than for $\text{π}\to {\text{π}}^{*}$ .

So, in the case of acetaldehyde shorter wavelength of $182\text{nm}$ corresponds to $\text{π}\to {\text{π}}^{*}$ transition and longer wavelength of $289\text{nm}$ corresponds to $\text{n}\to {\text{π}}^{\text{*}}$ transition.