The Big Review V: The C-O Bond

In the fifth installment of “The Big Review” of infrared (IR) spectral interpretation, we review the spectroscopy of functional groups containing C-O bonds, discuss alcohols and phenols, and see how to use IR spectroscopy to distinguish these alcohols from each other. We then discuss ethers and see how to use IR spectroscopy to distinguish the three different types from each other.

The purpose of this series, which I call “The Big Review,” is to summarize the 10-plus years of columns I have written on infrared (IR) spectral interpretation in preparation for a series of advanced topics to be covered soon. The review articles up until now have been structured around the spectroscopy of functional groups containing specific types of chemical bonds. The last column (1) was a review of the spectroscopy of the C-H bond; future “Big Review” columns will summarize the C=O and N-H bonds. Here, we discuss functional groups containing the C-O bond.

The original articles where the interpretation of spectra of molecules containing the C-O bond are here (2–4). Before we review the spectra of C-O containing molecules, it will be helpful to reintroduce the concept of the dipole moment (4). Recall that a dipole is two charges separated by a distance. The dipole moment of a chemical bond is called a bond dipole, and its magnitude is given by equation 1.

where µ equals dipole moment; q equals charge; and r equals distance. Thus, the dipole moment of a chemical bond depends upon the size of the charges and the distance they are apart. The electronic structure of the C-O chemical bond is seen in Figure 1.

Figure 1: The electronic structure of the C-O bond.

Because of the electronegativity differences between carbon and oxygen, there is a partial positive charge, δ⁺, on the carbon atom, and a partial negative charge, δ^-, on the oxygen atom. This means that C-O bonds contain a bond dipole, and in fact, it is large because of the size of the partial charges on each of the atoms in the bond.

Also recall (4) that in IR spectroscopy, the intensity of the peaks depends on the dipole moment as seen in equation 2.

where I equals peak intensity; dµ equals change in dipole moment during a vibration; and dx equals change in bond length during a vibration. The stretching vibration of the C-O bond is seen in Figure 2.

Figure 2: The stretching vibration of the C-O bond.

When we take a large dipole moment like the one contained in the C-O bond shown in Figure 1 and stretch it and contract it as seen in Figure 2, the resultant value of dµ/dx is large, and hence the associated IR peak is large.

The spectral signature then of C-O stretching vibrations is a large peak generally found between 1300 and 1000 cm^-1 (going forward, all peak positions will be in cm^-1 units even if not noted). Now, there are many other functional groups that absorb between 1300 and 1000 because this is part of the frequently congested fingerprint region. However, you can typically identify a C-O stretching peak because it will be the biggest peak between 1300 and 1000. An example of a C-O stretching peak is seen in the IR spectrum of ethanol in Figure 3 labeled C at 1050.

Figure 3: The infrared spectrum of ethyl alcohol, C₂H₆O.

If we apply the criterion to Figure 3 that the C-O stretching peak will be the biggest one between 1300 and 1000, then we assign the peak at 1050 as the C-O stretch, which is correct. As we will see, the number and position of C-O stretching peaks will enable us to identify and distinguish alcohols and ethers.

Alcohols

Alcohols are characterized by the presence of an OH, or hydroxyl group, attached to a carbon. The carbon to which the hydroxyl group is attached is called the hydroxyl carbon. This is all illustrated in Figure 4.

Figure 4: The chemical structure of ethyl alcohol.

There are three types of alcohol depending upon the number of carbons attached to the hydroxyl carbon as seen in Figure 5.

Figure 5: The chemical structures of primary, secondary, and tertiary alcohols.

In the figure, the “R” stands for a carbon atom that has other things attached to it. Note that, for a primary alcohol, there is one carbon atom attached to the hydroxyl carbon, and that the OH is attached to a CH₂ or methylene group. By definition, then, a primary alcohol contains a methylene group. Note also from Figure 4 that ethyl alcohol is an example of a primary alcohol. As seen in Figure 5, a secondary alcohol has two carbons attached to the hydroxyl carbon, and a tertiary alcohol has three carbons attached to the hydroxyl carbon. So, in the nomenclature of alcohols, what we are counting is the number of carbons attached to the hydroxyl carbon.

An aromatic alcohol is called a phenol, named after the parent molecule of the group, whose structure is seen in Figure 6. Note that phenol consists simply of an OH group attached to a benzene ring.

Figure 6: The chemical structure of phenol.

As discussed previously (2), alcohols engage in hydrogen bonding, as seen in Figure 7.

Figure 7: Two alcohol molecules engaging in hydrogen bonding.

Because the O-H bond is polar, the partial positive charge on the oxygen of one alcohol molecule coordinates with the partial positive charge on the OH group of its neighbor, forming a hydrogen bond. The spectroscopic affect of hydrogen bonding is a significant broadening of the peaks associated with the OH group. Note in Figure 3 that the OH stretch at 3342 labeled A is approximately 1000 cm^-1 wide, whereas the OH wag at 667 is approximately 500 cm^-1 wide. These broad peaks make it easy to spot the presence of OH groups in spectra. For alcohols in general, the OH stretch falls at 3350±50 and the OH wag at 650±50. These peak position regions are the same for all types of alcohols and phenols.

We can use the position of the C-O stretching peak in the spectra of alcohols to distinguish them from each other. In the case of ethyl alcohol in Figure 3, the C-O stretching peak is labeled C at 1050. For primary alcohols, this peak generally falls from 1075 to 1000, so the fact that ethyl alcohol’s C-O stretch is at 1050 proves it is a primary alcohol. For secondary alcohols the C-O stretch is found between 1150 and 1075, for tertiary alcohols from 1210 to 1100, and for aromatic alcohols from 1260 to 1200. The group wavenumbers for alcohols are summarized in Table I.

Primary and secondary alcohols are easy to distinguish because the C-O stretching peak for primaries falls below 1075, and for secondary alcohols this peak falls above 1075. Unfortunately, the C-O stretching range for secondary and tertiary alcohols overlaps some; namely, if the peak falls from 1100 to 1150, it may be difficult to determine whether a sample contains a secondary or tertiary alcohol. Aromatic alcohol C-O stretching peaks are, for the most part, off by themselves above 1200.

Ethers

My original article on ethers can be found here (4). The ether functional group contains a central oxygen atom with two carbons attached to it. The oxygen atom is called the ether oxygen, and the carbons attached to the oxygen are called the ether oxygen. This is all illustrated in Figure 8.

Figure 8: The structure of the ether functional group with the ether carbons and ether oxygen labeled.

If both ether carbons are saturated, it is called a saturated ether; if one carbon is saturated and one is aromatic, we have a mixed ether. If both carbons are aromatic, it gives an aromatic ether.

As you can see from Figure 8, ethers, unlike alcohols, do not contain an O-H group. Thus, ethers do not exhibit broadened peaks because they are not hydrogen bonded, and there are no O-H stretching or bending peaks. Like alcohols, ethers contain C-O bonds and thus will exhibit one or more strong peaks between 1300 and 1000, which again is the signature of C-O stretching peaks.

In saturated ethers, both C-O bonds are equivalent, and hence there is only one C-O stretching peak, typically between 1140 and 1070. Mixed ethers exhibit two C-O stretching peaks because there are two different types of C-O bond; one involves a saturated carbon and the other an aromatic carbon. This gives rise to two intense C-O stretching peaks in the spectra of mixed ether. The saturated C-O stretch from 1050 to 1010, and the aromatic C-O stretch from 1300 to 1200. An example of the IR spectrum of a mixed ether, anisole, is seen in Figure 9.

Figure 9: The infrared spectrum of anisole, a mixed ether. Note the pair intense C-O stretching peaks between 1300 and 1000 (labeled A and B, respectively).

The spectrum of anisole is complex because it contains a benzene ring, a methyl group, and a mixed ether, so the region from 1300 to 1000 contains many peaks. Peak A at 1247 is from the stretching vibration of the aromatic C-O bond, and peak B at 1040 is the saturated C-O stretch .Note these two peaks are the biggest ones in this region and are about the same intensity. This is the signature of an aromatic ether. If the two peaks were of different intensity, it would be hard to assign both peaks as C-O stretches. For aromatic ethers, where both ether carbons are aromatic, there is one C-O stretching peak because the two carbon-oxygen bonds are equivalent, and this peak falls between 1300 and 1200.

The group wavenumber table for ethers is seen in Table II. Note from the table that we can use the number and position of the C-O stretching peaks to distinguish these three types of ethers from each other, and that the number of peaks as we go down the table goes from one, to two, to one.

Conclusions

The C-O bond is found in alcohols and ethers. The C-O bond has a large dipole moment which gives rise to an intense C-O stretching peak between 1300 and 1000. Alcohols also contain an OH bond, which leads to hydrogen bonding and broadened peaks. The position of the C-O stretching peak is useful in distinguishing primary, secondary, and tertiary alcohols and phenols from each other. For ethers, the number and position of the C-O stretching peaks helps distinguish between the saturated, mixed, and aromatic varieties.

References

(1) Smith, B.C. The C-O Bond, Part I: Introduction and the Infrared Spectroscopy of Alcohols. Spectroscopy 2017, 32 (1), 14–21. Available at: https://www.spectroscopyonline.com/view/c-o-bond-i-introduction-and-infrared-spectra-alcohols-0

(2) Smith, B.C. Alcohols – The Rest of the Story. Spectroscopy 2017, 32 (4), 19–23. Available at: https://www.spectroscopyonline.com/view/alcohols-rest-story-alf3

(3) Smith, B.C. The C-O Bond III: Ethers By a Knockout. Spectroscopy 2017, 32 (5), 22–26. Available at: https://www.spectroscopyonline.com/view/c-o-bond-iii-ethers-knockout

(4) Smith, B.C. Infrared Spectral Interpretation, In The Beginning I: The Meaning of Peak Positions, Heights, and Widths. Spectroscopy 2024, 39 (4), 18–24. DOI: 10.56530/spectroscopy.fi6379n1

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. He can be reached at: SpectroscopyEdit@MMHGroup.com ●