12.5:
IR and UV–Vis Spectroscopy of Aldehydes and Ketones
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.
12.5:
IR and UV–Vis Spectroscopy of Aldehydes and Ketones
Infrared spectroscopy, also known as vibrational spectroscopy, is mainly used to determine the types of bonds and functional groups in molecules. In aldehydes and ketones, the carbonyl (C=O) bond shows an absorption around 1710 cm-1. The C=O bond vibration of an aldehyde occurs at lower frequencies than that of a ketone. In addition to the C=O absorption in an aldehyde, the aldehydic C–H bond also gives two peaks in the 2700–2800 cm-1 range. This absorption, coupled with the C=O stretching, is characteristic of an aldehydic group.
Conjugation reduces the electron density in the C=O bond, thereby reducing its stretching frequency. The stretching frequencies of cyclic ketones vary depending on their ring sizes. The strain in larger rings is lower than in smaller cyclic ketones. Stretching frequency increases with increasing ring strain. Therefore, the smallest and highly strained cyclopropanone ring has the highest stretching frequency.
UV-Visible spectroscopy employs UV and visible light to transition between different electronic energy levels. Two major transitions in organic compounds are n to π* and π to π* transitions. The π–π* is a stronger transition but occurs below 200 nm, which is not detectable in UV-Vis spectrometers. Conjugation of these molecules with a double bond or an aromatic ring shifts the absorption wavelength to above 200 nm. Each double bond that is conjugated adds a value of 30 nm to the absorption wavelength of the molecule.
n–π* transition is weaker than π–π* transition. Since the non-bonding orbital on oxygen and the anti-bonding π* orbital on the C–O bond is perpendicular, no overlap occurs between these two orbitals. Hence the n–π* is a forbidden transition and occurs much less frequently.