The Big Review VI: Carbonyl Compounds

Carbonyl containing functional groups are strong infrared absorbers and a number of them are easy to identify by infrared spectroscopy. Here, we will review the spectra of ketone and carboxylic acids.

The carbonyl, or C=O functional, group contains a carbon atom doubly bonded to an oxygen atom. The carbon in the C=O bond is referred to as the “carbonyl carbon,” as seen in Figure 1.

FIGURE 1: The chemical structure and charge distribution in a carbonyl bond.

Carbonyl bonds are highly polar because of the large electronegativity difference between carbon and oxygen. As a result, the carbonyl carbon has a large partial positive charge (as denoted by the d⁺ in Figure 1), and the oxygen has a large partial negative charge (as denoted by the d^- in Figure 1). Recall that when, in a chemical bond, there are two charges separated by a distance, it forms what is called a dipole moment (1). Additionally, remember that one of the things that determines the intensity of infrared peaks is the change in dipole moment with respect to bond length, dµ/dx, during a molecular vibration (1). Since the carbonyl group has a large dipole moment, when it stretches and contracts, the value of dµ/dx is large, giving an intense peak. This vibration is illustrated in Figure 2.

FIGURE 2: The stretching vibration of a carbonyl bond.

Carbonyl stretching peaks generally fall between 1900 and 1600 cm^-1 (assume all peak positions hereafter are in wavenumber units), a relatively unique part of the infrared spectrum. This area is sometimes referred to as the carbonyl stretching region. The carbonyl stretching peak is perhaps the perfect example of a group wavenumber, a peak that shows up intensely in a unique and relatively narrow wavenumber range. This makes C=O stretches one of the easiest infrared peaks to spot and assign. Because of this, an infrared spectrometer is in some respects the perfect carbonyl detector.

Carbonyl bonds can be divided into two classes, depending upon what type of carbons are attached to the carbonyl carbon. A saturated carbonyl group has two saturated carbons attached to the carbonyl carbon. An aromatic carbonyl group has one or more aromatic carbons attached to the carbonyl carbon. An example of an aromatic carbonyl group is seen in Figure 3.

FIGURE 3: The p-type orbitals and conjugation within an aromatic carbonyl group.

Saturated and aromatic carbonyl groups can be distinguished from each other using infrared spectroscopy. In general, aromatic C=O stretching peaks fall ~30 cm^-1 lower than saturated C=O stretching peaks. Aromatic rings, such as benzene, contain p-type orbitals with electron density that sticks up out of the plane of the molecule (2), as illustrated to the left in Figure 3. The carbonyl group also contains a p-type orbital that points through space towards the orbitals on the aromatic ring, as is also illustrated. The orbitals are close enough that they somewhat overlap, allowing some of the electron density from the carbonyl bond to be pulled off into the benzene ring in a phenomenon known as conjugation, which is illustrated by the dashed line at right in Figure 3. Conjugation weakens the C=O bond, lowers its force constant, and its peak position is hence reduced, on average about 30 cm^-1 (2). Thus, for almost every carbonyl containing functional group we discuss, I will quote two carbonyl stretching regions, one for the saturated and one for the aromatic versions of the functional group.

Ketones

Perhaps one of the most common chemical structures to contain a carbonyl group are ketones. These molecules consist of a C=O with two what are called alpha carbons attached, one to the left and one to the right, as illustrated in Figure 4.

FIGURE 4: The structural framework of the ketone functional group.

If both alpha carbons are saturated, the ketone is saturated. If one or both alpha carbons are aromatic, the ketone is aromatic.

The diagnostic pattern of peaks that define the presence of a ketone in a sample are the carbonyl stretch, which is illustrated in Figure 2, and the C-C-C stretch, seen in Figure 5. Note the C-C-C stretch is an asymmetric stretch involving the carbonyl carbon and the two alpha carbons.

FIGURE 5: The C-C-C stretching vibration of ketones.

A hallmark of the infrared spectra of carbonyls is the intense, easy to spot C=O stretching vibration peak. This peak is clearly seen labeled A at 1716 in the spectrum of acetone shown in Figure 6.

FIGURE 6: The infrared spectrum of acetone, C₃H₆O.

For saturated ketones, the C=O stretch falls at 1715±10, while, in aromatic ketones, this peak appears from 1700 to 1640 due to conjugation. Note that, for saturated ketones, the C=O stretch is at 1705 or above, and, for aromatic ketones, it is 1700 or below, meaning 1700 is a good dividing line between the two. Note in Figure 6 that the C-C-C stretch (labeled B) is the second biggest peak in the spectrum and falls at 1222. For saturated ketones, this peak falls between 1230 and 1100. For aromatic ketones, the C-C-C stretch is found from 1300 to 1230. Thus, the 1230 mark in a spectrum is the dividing line for the C-C-C stretch between saturated and aromatic ketones.

To summarize then, ketones have a pair of intense peaks that comprise their diagnostic pattern, an intense C=O stretch around 1700, and an almost as intense C-C-C stretch around 1200. Both of these bands are sensitive as to whether the ketone is saturated or aromatic. The group wavenumbers for ketones are summarized in Table I.

Carboxylic Acids

The structural framework of carboxylic acids is seen in Figure 7. Note that these molecules contain a C=O bond and an alpha carbon like ketones, but, instead of a second alpha carbon, there is an OH or hydroxyl group attached to the carbonyl carbon. For acids, if the alpha carbon is saturated, the acid is saturated, and if the alpha carbon is aromatic, the acid is aromatic (3).

FIGURE 7: The structural framework of the carboxylic acid functional group. Note the locations of the alpha carbon and the hydroxyl group.

Carboxylic acids are acids because some of them are water soluble and will then dissociate into a proton and a negatively charged anion. However, these are not strong acids. A solution of acetic acid in water, otherwise known as vinegar, has a pH of about 5, and we eat it. On the other hand, it would not be an good idea to consume concentrated sulfuric acid with a pH of 1 or lower.

As we have discussed previously (1), strong intermolecular interactions cause infrared peaks to be significantly broadened. This is particularly true with carboxylic acids, since in the solid and liquid states they are never found as individual molecules, but are bound up in the dimer form, as seen in Figure 8.

FIGURE 8: An example of a carboxylic acid dimer, a result of the strong hydrogen bonding that takes place in the solid and liquid forms of these molecules.

As a result of the strong hydrogen bonding in carboxylic acid dimers, all peaks associated with this functional group are unusually broadened. This is well illustrated in the spectrum of octanoic acid seen in Figure 9. The broad peak labeled A in the spectrum is from the OH stretching vibration of the acid. This peak is over 1000 cm^-1 wide, and is one of the broadest peaks seen in infrared spectra. It often times does not have an obvious peak top, and we simply say that this OH stretching envelope falls from 3500 to 2500. The carbonyl stretch of octanoic acid, labeled B in Figure 9, falls at 1711 and is the most intense peak in the spectrum, as is often the case with C=O peaks. For saturated acids, this peak falls from 1730 to 1700, and, for aromatic acids, from 1710 to 1680 due to conjugation.

FIGURE 9: The infrared spectrum of octanoic acid.

We discussed functional groups containing the C-O bond in our last article (4). Among other bonds, carboxylic acids have a C-O bond; hence we would expect a large C-O stretching peak between 1300 and 1000. This peak is seen in Figure 9 (labeled C at 1282), and for carboxylic acids generally falls from 1320 to 1210. In the spectra of alcohols and ethers, we are used to seeing C-O stretching peaks as the biggest in the spectrum. It is a testament to the intensity of C=O stretching peaks that the 1710 peak in Figure 9 towers over that one at 1282.

Lastly, carboxylic acids display a broadened O-H wag peak at 960–900. In the spectrum of octanoic acid, this peak is labeled D and falls at 938. This peak is a useful group wavenumber because there are not many peaks this broad that fall around 900.

In summary, carboxylic acids have four useful group wavenumbers. The OH stretching envelope from 3500 to 2500, the C=O stretch around 1700, the C-O stretch near 1250, and the OH wag in the vicinity of 900. Note that of all these peaks the only one that is sensitive as to whether a carboxylic acid is saturated or aromatic is the C=O stretch. The group wavenumbers for carboxylic acids are seen in Table II.

Summary

The carbonyl or C=O bond consists of a carbon atom with a large partial charge on it, and an oxygen atom with a large partial negative charge. The resulting large dipole moment means that du/dx for the carbonyl stretching vibration is large, giving rise to an intense peak in the 1900 to 1600 region. Ketones contain two alpha carbons attached to the carbonyl group, and their diagnostic peak pattern consists of an intense C=O stretch and a C-C-C stretch, both of which are sensitive as to whether the ketone is saturated or aromatic. Carboxylic acids contain a carbonyl bond with an alpha carbon and OH group attached. There are four useful group wavenumbers for carboxylic acids: the broad OH stretch; carbonyl stretch; C-O stretch; and broadened OH wag.

References

1. Smith, B. C. Why Spectral Interpretation Needs to Be Taught. Spectroscopy. 2015, 30 (1), 16–23.
2. Smith, B. C. The Carbonyl Group, Part I: Introduction. Spectroscopy, 2017, 32 (9), 31–36.
3. Smith, B. C. The C=O Bond, Part III: Carboxylic Acids. Spectroscopy, 2018, 33 (1), 14–20.
4. Smith, B. C. The Big Review V: The C-O Bond. Spectroscopy 2025, 40 (3), 10–13. DOI: 10.56530/spectroscopy.mq4273p5

Brian C. Smith, PhD, is the founder and CEO of Big Sur Scientific, a maker of portable mid-infrared cannabis analyzers. He has over 30 years experience as an industrial infrared spectroscopist, has published numerous peer-reviewed papers, and has written three books on spectroscopy. As a trainer, he has helped thousands of people around the world improve their infrared analyses. In addition to writing for Spectroscopy, Dr. Smith writes a regular column for its sister publication Cannabis Science and Technology and sits on its editorial board. He earned his PhD in physical chemistry from Dartmouth College. ●