Infrared Spectral Interpretation Advanced Topics: Branched Alkanes

Now that our Grand Review is over it is time to move on to what I call “advanced topics.” This will be a series of articles diving into more detail on functional groups previously studied and will introduce new functional groups not yet discussed. In this column we will briefly review the C-H stretching and bending vibrations of methyl and methylene groups and then discuss how to use infrared spectroscopy to determine some of the branch points found in alkanes. We will see that the key is that some branch points give rise to what is known as a “split umbrella mode.”

Alkanes are molecules that contain C-C and C-H bonds only. The original articles on the spectroscopy of alkanes are here,^1-3 and we recently reviewed this topic.⁴ As a convenience to readers new to this series, I will briefly summarize these articles:

• Alkanes contain CH₃ units called methyl groups, and CH₂ or methylene groups.
• Both methyl and methylene groups have asymmetric and symmetric stretching peaks that appear between 3000 cm^-1 and 2850 cm^-1 (going forward, all peak positions and ranges will be in cm^-1 units unless otherwise stated). A pattern of 3 or 4 peaks between 3000 and 2850 means both CH₂ and CH₃ are present in a sample, whereas if there are one or two peaks in this region, it means the sample contains just CH₂ or CH₃. The individual peak positions will disclose which functional group is present.
• The diagnostic pattern for methyl groups is an asymmetric C-H stretch at 2926±10, a symmetric stretch at 2872±10, and an umbrella mode at 1375±10.
• The diagnostic pattern of peaks for methylene groups is an asymmetric stretch at 2926±10, a symmetric stretch at 2855±10, and a CH₂ rocking peak at 720±10. However, this latter peak appears only when there are 4 or more methylenes in a row.
• Methyl and methylene groups contain saturated carbons and, as a result, all their C-H stretching peaks fall below 3000.

Introduction to Branch Points

Previous articles^1-3 focused on what are called straight-chain alkanes, whose generic chemical structure is seen in Figure 1.

Note in Figures 1 and 2 that every carbon has at least two hydrogen atoms attached to it, and each carbon atom has only 1 or two other carbons bonded to it. So far in this column series we have restricted our discussion to straight-chain alkanes because they are common and their spectral interpretation is straightforward.

In an alkane a branch point occurs when there is a carbon atom with three or four other carbon atoms attached to it, as illustrated by the structure of isooctane seen in Figure 3.

A methine carbon as seen in Figure 3 is a carbon atom that has three carbon atoms and one hydrogen atom attached to it. A quaternary carbon has four carbon atoms and no hydrogen bonded to it. These carbon atoms are said to form branch points because they change the shape of straight-chain alkanes, producing more than one carbon-carbon chain, forming a structure akin to the branches of a tree. The chain “changes directions” at a branch point.

There are many types of branch points possible in alkanes. Unfortunately, many of them do not have a unique spectral signature, and their infrared spectra look like those of straight-chain alkanes. For example, the structures of tetraethyl methane and 5-ethyl-3-methyloctane seen in Figure 4 contain methyl and methylene groups whose spectra are not much different from those in straight-chain alkanes. So, unfortunately, branch points like these are hard to see by infrared spectroscopy.

However, a group of alkane branch points has a unique infrared spectral signature, and their structures all include carbon atoms that have two or three methyl groups attached to them, as illustrated in Figure 5.

Note that an isopropyl group contains a methine carbon with two methyls attached, a gem-dimethyl moiety consists of a quaternary carbon with two methyl groups bonded to it, a tertiary or t-butyl group consists of a quaternary carbon with the three methyl groups attached, and an isobutyl group consists of a methine carbon with two methyls and a methylene group. The spectral signature of these four branch points is a split umbrella mode peak. That is, rather than having one umbrella mode peak at 1375±10 as in normal methyl groups, there are two peaks in the 1395 to 1365 region. This is illustrated by the spectrum of isopropyl alcohol in Figure 6.

We have seen the spectrum of this molecule before when we originally discussed alcohols.⁵ Its structure contains a methine carbon with two methyls attached i.e., an isopropyl group, hence why we call this molecule isopropyl alcohol. The spectral signature of the isopropyl group is the pair of umbrella mode peaks at 1379 and 1369, as labeled in the figure.

The reason the group of four branch points illustrated in Figure 5 has a split umbrella mode is that when the umbrella mode vibrations of the two or three methyls attached to the same carbon are excited, they vibrationally couple i.e., the vibrations affect each other. One of the peaks of the split umbrella mode doublet is seen when the two methyl groups’ umbrella modes bend in phase with each other, and the other occurs when the two umbrella modes bend out of phase with each other. The reason there are two peaks is that the two vibrations have slightly different force constants.⁶ The reason that other branch points do not exhibit a split umbrella mode is that they don’t have carbons with two or more methyl groups on the same carbon and hence do not vibrationally interact.

The Infrared Spectra of Branched Alkanes

The spectrum of a molecule that consists of two isopropyl groups bonded together, 2,3-dimethylbutane, is shown in Figure 7.

Note first that there are only two C-H stretching peaks between 2850 and 3000 because there are only methyls in this structure. Secondly, the split umbrella mode peaks are labeled C at 1380 and 1371. For isopropyl and gem-dimethyl groups, generally, these two peaks fall between 1385 and 1365. Thirdly and importantly, note that the peaks at 1380 and 1371 in Figure 7 are the same size, or in other words, the two peaks have a 1:1 intensity ratio.

The spectrum of isooctane is seen in Figure 8.

Note that the structure of isooctane can be thought of as an isobutyl group attached to a t-butyl group. The split umbrella mode peaks from these two branch points are seen at 1393 and 1366. Generally, for both the isobutyl and t-butyl groups, this pair of peaks is seen between 1395 and 1365. Note in Figure 8 that these two peaks are different heights, and that their intensity ratio is approximately 1:2, which contrasts with the 1:1 intensity ratio seen in the spectra of isopropyl and gem-dimethyl groups. Figure 9 helps illustrate this point.

To the left in Figure 9 is the 1:1 relative intensity split umbrella mode peak typical of isopropyl and gem-dimethyl groups (ignore the lines above the peaks, these are peak markings added by the computer program). To the right in Figure 9 is the 1:2 relative intensity split umbrella mode peaks typical of the t-butyl and isobutyl groups. These differences in intensity ratios then allow us to split these four branch points into two different classes. If the umbrella mode peak intensities are about the same, the branch point is isopropyl or gem-dimethyl only. If the umbrella mode peak intensities are in about a 1:2 ratio, the branch point is either t-butyl or isobutyl. However, if you see two peaks in this region and their intensity ratio is different than 1:1 or 1:2, let’s say 2:1, the peaks do not comprise a split umbrella mode.

The methine functional group, which is a carbon atom in an alkane with a single C-H bond, has a C-H stretch ~2900. This is a weak band and is not always seen in the spectra of branched alkanes. For example, the structure of 2,3-dimethylbutane contains two methine groups and yet there is no C-H stretch at 2900 in Figure 7. On the other hand, the spectrum of isooctane in Figure 8 has an easily seen methine C-H stretching peak labeled B at 2901.

The group wavenumbers for the alkane branch points that exhibit a split umbrella mode are listed in Table 1.

Conclusions

Alkanes contain many types of branch points. The only ones with unique spectral signatures are isopropyl, gem-dimethyl, isobutyl, and t-butyl. What these functional groups have in common is two or more methyl groups attached to the same carbon atom. Their spectral signature is a “split” umbrella mode where there is a pair of peaks where we would normally expect a single umbrella mode peak. For isopropyl and gem-dimethyl, the two peaks appear between 1385 and 1365, and their intensity ratio is 1:1. For t-butyl and isobutyl, the two peaks are found from 1395 to 1365, and their intensity ratio is 1:2.

The Return of Infrared Spectral Interpretation Workshop Problems!

In many of my early columns I featured problem spectra that readers could work on, and then we would go over the solution to these problems in the next column. It has been a while since you have seen one of these problems because, frankly, I ran out of them. But never fear, please find below your latest Infrared Spectral Interpretation Workshop problem. Do your best to assign the peaks, determine what functional groups are present, and put them together to make up a complete chemical structure. Hint No. 1: Read the caption. Hint No. 2: Now that we are in the Advanced Topics universe, any functional group we have ever studied is now fair game.

References

1. Smith, B. C. Electromagnetic Radiation, Spectral Units, and Alkanes. Spectroscopy 2015, 30 (4), 18-23.
2. Smith, B. C. More Theory and Practice: The Thorny Problem of Mixtures and More on Straight Chain Alkanes. Spectroscopy 2015, 30 (7), 28-33, 48.
3. Smith, B. C. How to Properly Compare Spectra, and Determining Alkane Chain Length From Infrared Spectra. Spectroscopy 2015, 30 (9), 40-45.
4. Smith, B. C. The Big Review IV: Hydrocarbons. Spectroscopy 2025, 40 (1), 16-19. DOI: 10.56530/spectroscopy.vt7783b7
5. Smith, B. C. Alcohols—The Rest of the Story. Spectroscopy 2017, 32 (4), 19-23.
6. Smith, B. C. Why Spectral Interpretation Needs To Be Taught. Spectroscopy 2015, 30 (1), 16-25.