Laboratory Techniques in Organic Chemistry
Laboratory Techniques in Organic Chemistry
4th Edition
ISBN: 9781464134227
Author: Jerry R. Mohrig, David Alberg, Gretchen Hofmeister, Paul F. Schatz, Christina Noring Hammond
Publisher: W. H. Freeman
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Chapter 25, Problem 1Q
Interpretation Introduction

Interpretation:

nπ* and ππ* 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 10-400nm and 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, nπ* and ππ* 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=log(I°I)=εlc

Where,

  • I° is incident light intensity
  • I is transmitted light intensity.
  • 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.

Expert Solution & Answer
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Explanation of Solution

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

The energy gap in ππ* is larger than nπ* so ππ* transition occurs at a shorter wavelength in the UV region than for ππ* .

So, in the case of acetaldehyde shorter wavelength of 182nm corresponds to ππ* transition and longer wavelength of 289nm corresponds to nπ* transition.

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Chapter 25 Solutions

Laboratory Techniques in Organic Chemistry

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