12.5 : アルデヒドとケトンの IR および UV-Vis 分光法

The IR spectrum of a simple aldehyde shows a strong carbonyl stretch around 1725 cm^-1. In a simple ketone, the C=O stretching absorption shifts to a lower frequency.

Aldehydes also give two weak bands at higher wavenumbers, characteristic of the aldehydic C–H bond stretch.

Conjugating the carbonyl group with a double bond or an aromatic ring delocalizes the π electrons, thereby inducing a partial single bond character.

Therefore, the bond requires less energy to stretch and absorbs at a lower frequency than the nonconjugated carbonyl bond.

In cyclic ketones, decreasing ring size increases the ring strain and raises the frequency of carbonyl absorption.

Simple aldehydes and ketones exhibit n-π* and π-π* transitions.

Although strong, the π-π* UV absorption band falls below the observable range of typical UV spectrometers. Conjugation causes this absorption to shift to a higher wavelength within the observable range.

The n-π* absorption band is weak. The perpendicular arrangement of the nonbonding orbital of oxygen and the π* antibonding orbital forbids the n-π* transition, making it less probable to occur.