The Big Review VII: More Carbonyl Compounds

In this part of our ongoing review of the infrared spectra of carbonyl-containing functional groups, we will study the spectra of esters and carbonates. Esters are ubiquitous in our food and medicines, and polymeric carbonates form an important part of the materials around us. As always, concepts will be illustrated with reference spectra.

We started our discussion of carbonyl compounds in the last column (1), but I will give a brief review of the carbonyl group and its spectroscopy here for convenience. 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.

Carbonyl bonds are highly polar because of the large electronegativity difference between the carbon and oxygen. As a result, the carbonyl carbon has a large partial positive charge, as denoted by the (delta +) d+ in Figure 1, and the oxygen has a large partial negative charge, as denoted by the (delta –) d- in the figure. Recall that in a chemical bond that then two charges are separated by a distance a dipole moment is formed (2). Additionally, recall 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 (2). 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.

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 of an infrared spectrum is sometimes referred to as the carbonyl stretching region.

Carbonyl bonds can be divided into two classes depending upon what types 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.

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 because of a phenomenon called conjugation. Aromatic rings, such as benzene, contain p-type orbitals with electron density that sticks up out of the plane of the molecule (3), as illustrated to the left in Figure 3. The carbonyl group also contains a p-type orbital that points through space directly towards the orbitals on the aromatic ring, as is also illustrated. The orbitals are close enough that they overlap somewhat, allowing some of the electron density from the carbonyl bond to be pulled off into the benzene ring via 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 reduced by about 30 cm^-1 (3). Thus, for almost every carbonyl-containing functional group we discuss, there will be two carbonyl stretching regions, one for the saturated and one for the aromatic versions.

The Infrared Spectroscopy of Saturated and Aromatic Esters

Saturated Esters

The structural framework of the ester functional group is seen in Figure 4.

Note that in addition to containing a C=O bond and an alpha carbon, esters also contain two C-O bonds. For esters, if the alpha carbon is saturated the ester is saturated, and if the alpha carbon is aromatic the ester is aromatic.

The infrared spectra of esters follow what I like to call the “Rule of 3.” Their spectra exhibit a trio of strong peaks at ~1700, ~1200, and ~1100 from C=O and C-O stretching vibrations. These are present in the spectrum of ethyl acetate as seen in Figure 5.

The carbonyl stretch is the biggest peak in the spectrum and is labeled A at 1742. For saturated esters, these peaks fall from 1755 to 1735. The second of the Rule of 3 peaks is a C-C-O stretch involving the alpha and carbonyl carbons, as seen in Figure 6.

In general, this peak falls from 1210 to 1160 for saturated esters. Ethyl acetate exhibits this peak at 1241, which is typical of acetate esters (3).

The third peak in Figure 5, labeled C at 1047, is from the stretching of the second C-O bond in the ester, which is the one to the right of the ester oxygen. This vibration also involves any carbon attached to the right of this bond, forming an O-C-C moiety. I call this vibration the asymmetric O-C-C stretch, and it is illustrated in Figure 7.

For saturated esters in general, the O-C-C stretch appears from 1100-1030. To be clear, a linkage such as C=O(O-CH3), which has an O-C bond rather than an O-C-C moiety, will still exhibit this peak.

Note in Figure 5 that as we go from left to right to peaks A, B, and C, that the third peak is a little less intense than the other two. This intensity pattern is typical of esters and can be useful in identifying them. A summary of the group wavenumbers for saturated esters is found in Table I.

Aromatic Esters

Aromatic esters (4) also follow the Rule of 3, but all three peaks for aromatic esters fall in different wavenumber ranges than for saturated esters do, as seen in Figure 8.

The peak labeled A at 1725 in Figure 8 is the C=O stretch, almost always the most intense peak in a spectrum. In general, for aromatic esters, this peak falls between 1730 and 1715, and is lower than that of saturated esters because of conjugation. Peak B at 1280 is the C-C-O stretch, which is normally found from 1310 to 1250 for aromatic esters. Lastly, peak C is the O-C-C stretch, which is seen from 1130 to 1100. These three peaks follow the intensity pattern we have seen before for esters, where the first two Rule of 3 peaks are more intense than the third. Table II lists the Rule of 3 peak positions for aromatic esters and compares them to those for saturated esters.

Note in Table II that for each of the three vibrations listed the ranges for saturated and aromatic esters are different. This means that each ester type has a unique set of Rule of 3 peaks, and any of these peaks can be used to distinguish saturated and aromatic esters from each other.

Organic Carbonates

The chemical term carbonate is a little confusing. In general, it refers to a functional group with the formula CO₃. There exist inorganic carbonates that contain the CO₃^-2 ion where each carbon-oxygen bond has the same bond order (5). Although this functional group contains a carbon atom it acts inorganic since it forms ionic bonds and makes up rocks such as limestone. Organic carbonates contain a carbon atom with three oxygens bonded to it, but the bonding is covalent and consists of C=O and C-O bonds as seen in Figure 9.

Note that organic carbonates contain two alpha carbons. If both alpha carbons are saturated, the carbonate is saturated;, if one is aromatic and one is saturated the carbonate is said to be mixed, and if both alpha carbons are aromatic the carbonate is aromatic.

As you can see in Figure 9, carbonates are symmetrical, and each side can be thought of as an ester, with each half having a carbonyl carbon, ester oxygen, and an alpha carbon. Note that the carbonyl carbon has not one oxygen attached to it as in an ester but two. This means there are two ester oxygens and two alpha carbons, as seen.

Given the structural similarity between esters and organic carbonates, we might expect carbonates to follow the Rule of 3 like esters do. Carbonates do have three intense peaks like esters, but the vibrations responsible are a little different than because the structures of esters and carbonates are not the same. Carbonates have C=O and O-C-C stretches like esters, but unlike esters have an O-C-O linkage and hence an O-C-O asymmetric stretching peak instead of a C-C-O peak as in esters. Table III shows the group wavenumbers for the three varieties of organic carbonates, saturated, mixed, and aromatic.

Note that the C=O stretch is different for all three types of carbonates, and that, for mixed and aromatic carbonates, the C=O stretch is much higher than any ester C=O stretch, making these two types of organic carbonate easy to spot. Table III also shows that the organic carbonate O-C-O stretching peak position goes down as we move from the saturated, mixed, and aromatic versions of the functional group. The O-C-C stretching peak range is the same for all three types of carbonates. The saturated carbonate C=O stretch falls in the same range as that of saturated esters. However, saturated carbonates have their O-C-O peak from 1280 to 1240, whereas saturated esters have their C-C-O peak lower from 1210 to 1160. It is the relative position of these two peaks that allows one to distinguish saturated esters and carbonates.

The infrared spectrum of the common polymer and aromatic carbonate Lexan (2,2-Bis(4-hydroxyphenyl)propane polycarbonate) is seen in Figure 10.

Lexan is an aromatic carbonate because the two alpha carbons are part of benzene rings. The peak labeled A at 1777 is a C=O stretch based on its strength and position. The carbonyl stretch of aromatic carbonates in general falls from 1820 to 1775, unusually high for a carbonyl stretch. The C=O stretch of mixed carbonates falls from 1790 to 1760, and for saturated carbonates this peak is seen at 1740±10. In Figure 10, the O-C-O stretch is labeled B at 1230. For aromatic carbonates this peak is typically found from 1230-1205, whereas for mixed carbonates it falls between 1250 and 1210 and for saturated carbonates from 1280-1240. The O-C-C stretch in Figure 10 is at 1015, and, for all carbonates, this peak falls from 1060 to 1000.

Conclusions

We did a general review of the structure and spectroscopy of carbonyl groups, and then focused on the spectroscopy of esters and organic carbonates. Esters form a trio of peaks, the “Rule of 3,” which fall at ~1700, ~1200, and ~1100 from C=O, C-C-O, and O-C-C vibrations, respectively. All three of these peaks are sensitive as to whether the ester is saturated or aromatic, resulting in two sets of Rule of 3 peaks. There are three types of organic carbonates, depending on the nature of the alpha carbons, saturated, mixed, and aromatic. Organic carbonates are similar to esters, and have C=O, O-C-O, and O-C-C vibrations. The C=O peaks of some organic carbonates fall at an unusually high wavenumber and are easy to spot. The trio of carbonate peaks discussed can generally be used to distinguish the different types of carbonate from each other.

References

1. Smith, B. C. The Big Review VI: Carbonyl Compounds. Spectroscopy 2025, 40 (4), 12-18. DOI: 10.56530/spectroscopy.zd9677w3
2. Smith, B. C. IR Spectral Interpretation Workshop. Spectroscopy 2015, 30 (1), 16-25. Available at: https://www.spectroscopyonline.com/view/ir-spectral-interpretation-workshop
3. Smith, B. C. The Carbonyl Group, Part I: Introduction. Spectroscopy 2017, 32 (9), 31-36. Available at: https://www.spectroscopyonline.com/view/carbonyl-group-part-i-introduction
4. Smith, B. C. The C=O Bond, Part VII: Aromatic Esters, Organic Carbonates, and More of the Rule of Three. Spectroscopy 2018, 33 (9), 24–28. Available at: https://www.spectroscopyonline.com/view/co-bond-part-vii-aromatic-esters-organic-carbonates-and-more-rule-three
5. Smith, B. C. Inorganics II: The Spectra. Spectroscopy 2024, 39 (1), 14–17. DOI: 10.56530/spectroscopy.gh3585q6