Chapter 2, Problem 28PP

Interpretation Introduction

Interpretation:

Based on the resonance and the electronegativity effects, the trends in carbonyl IR stretching frequencies from higher frequency for esters and carboxylic acids to lower frequency for amides are to be explained. Also, the way the nitrogen atom influences the distribution of electrons in an amide carbonyl group is to be suggested.

Concept Introduction:

▸ Infrared spectroscopy is a simple, instrumental technique, which helps to determine the presence of various functional groups.

▸ It depends on the interactions of atoms or molecules with the electromagnetic radiation.

▸ The molecules which have dipole moment are IR active and the molecules which do not have dipole moment are IR inactive.

▸ The change in the absorption frequency of a particular group takes place by changing the substituents in the neighborhood of that particular group.

▸ An electronegative atom or group causes –-I effect, which results in the bond order to increase.

▸ The lengthening and weakening of a bond leads to lower absorption frequency.

▸ The double bond character of carbonyl increases as the electronegativity of the atom bonded to carbonyl group increases.

▸ In amides the electron pair of nitrogen atom contributes to the resonance hybrid which decreases the double bond character.

▸ In case of carboxylic acid and esters, the carbonyl group is attached to the electronegative oxygen atom, which results in the increase of carbonyl absorption frequency.