The Infrared Spectra of Cyclic Alkanes

So far in this column series, we have assumed that all saturated hydrocarbon molecules were alkanes or alkyl chains. No more! In this installment, we will consider the spectra of saturated rings such as cyclopentane and cyclohexane. We will see, unfortunately, that not all saturated rings are easily detectable by infrared spectroscopy. We will also discuss the interesting phenomenon of ring strain and how it affects the infrared spectra of these molecules.

One of the fascinating things about the element carbon is how wishy-washy it is, it can’t make up its mind what type of bonds to form. Carbon forms single bonds, double bonds, triple bonds, and even bonds and half such as in the benzene rings discussed earlier.¹ Benzene rings are a type of aromatic ring, and the carbons in these rings are unsaturated.

However, aromatic rings are not the only type of cyclic compounds carbon can form. This element is also capable of forming saturated rings, examples of which are shown in Figure 1. These rings are saturated because they consist solely of methylene (CH₂) groups, which as we have discussed previously contain saturated carbons because no more hydrogens can be added to these carbons.² Figure 1 shows different ways of representing cyclohexane, C₆H₁₂, which is the cyclic form of hexane, and cyclopentane, C₅H₁₀ which is the cyclic form of pentane. To be clear, none of the depictions in Figure 1 is completely accurate. The left most structures show the number of methylenes in each ring, and the other figures make it look like cyclopentane and cyclohexane are planar polygons with a carbon at each vertex.

Nothing could be further from the truth. The bond angles around methylene groups are tetrahedral with a bond angle of about 109°. This means cycloalkanes cannot be planar. A more realistic structure of cyclohexane in seen in Figure 2.

Note in the leftmost structure the bent nature of the ring. The other two depictions show two configurations of cyclohexane, chair and boat, named after the shapes they are reminiscent of. Unfortunately, infrared (IR) spectroscopy cannot distinguish between these two structures.

The tricky part of synthesizing cyclic alkanes is that to form these rings sometimes carbon departs from its preferred tetrahedral geometry. This is seen in the diagrams of cyclopropane and cyclobutane seen in Figure 3.

The bond angles in cyclopropane are nominally 60° and in cyclobutane they are 90°, this is a far cry from the preferred tetrahedral bonding angle for carbon of 109.5°. As result the C-C bonds in these and similar rings are energetically unfavorable, giving rise to what is known as ring strain. Ring strain causes some cycloalkanes to be unstable and be not that common. As result, the most common cyclic alkanes are the ones with the least ring strain, which would be cyclopentane and cyclohexane as seen in Figure 1. Cyclohexane has little ring strain because as it turns out in a 6 membered ring the bond angles close to the ideal of 109.5°, and cyclopentane has some ring strain but is relatively stable. As a result we will restrict our discussion to cyclohexyl and cyclopentyl rings. We will also see how ring strain affects the infrared spectra of these molecules.

The Infrared Spectrum of Cyclohexane

The spectra of straight chain hexane and cyclohexane are shown in Figure 4.

The spectrum of straight chain hexane was one of the first reference spectra we studied in this column series.² Recall that straight chain alkanes and alkyl chains must end with a methyl group to avoid dangling bonds. Since hexane has both methyl and methylene groups there are 4 C-H stretches between 3000 and 2850 cm^-1 (going forward all peak positions will be in cm^-1 units even if not so stated), and an umbrella mode at 1379. Structurally the difference between hexane and cyclohexane is that the former contains CH₃ and CH₂ groups, while the latter contains methylene groups only. This is reflected in the spectrum of cyclohexane. Note that there are only two C-H stretching peaks at 2929 and 2954 from the asymmetric and symmetric stretching of the CH₂ groups respectively. Note that these are in the same range as the methylenes in a straight chain alkane or alkyl chain because the bond angles in cyclohexane are close to the ideal of 109.5° and there is little ring strain. So, chemically and spectroscopically the methylene groups in hexane and cyclohexane are pretty much the same.

Also note there is no methyl umbrella mode in the spectrum of cyclohexane as denoted by the dashed line in Figure 4, so hexane and cyclohexane can be distinguished. If there are methyl and methylene peaks it is straight chain, if there are methylenes only it is cyclic. Note that there is no diagnostic peak for a cyclohexyl ring, it is the absence of an umbrella mode peak here that is diagnostic.

However, life of course is not always so easy for us. It is entirely possible for a molecule to contain both a cyclohexyl ring and a methyl group as shown by the structure of methyl cyclohexane seen in Figure 5.

The problem here is that the methylene spectra in this molecule look like those of a straight chain, and the methyl group will add two C-H stretches and an umbrella mode making the spectrum of this molecule very similar to straight chain hexane. Unfortunately then there is no way from the spectra of a molecule like this to tell whether the methylenes are in a chain or a ring.

The Infrared Spectrum of Cyclopentane

The infrared spectrum of cyclopentane is seen in Figure 6.

The CH₂ asymmetric stretch is seen at 2956, and the symmetric stretch is found at 2868. Unfortunately these peaks fall smack dab in the middle of the C-H stretching regions for methyl groups! Recall that these peaks for CH₃ groups fall in the ranges 2962±10 and 2860±10.² This distortion in peak positions for methylene groups is due to the ring strain mentioned above. The bond angles in a cyclopentyl ring are less than the ideal of about 109° distorting the CH₂ stretching peak positions.

If you didn’t know any better, you would think the sample whose spectrum is shown in Figure 6 contains methyl groups not methylene groups. However, note the absence of a methyl umbrella mode here. The key here is in cyclopentane the C-H stretches look like a methyl group, but the lack of an umbrella mode tells us otherwise. However, for cyclohexane if there were a methyl group anywhere in the sample there would be an umbrella mode, C-H stretches from the methyl group, and C-H stretches from the cyclopentyl ring that also look like a methyl group, and an umbrella mode. So, like with cyclohexyl rings there is no diagnostic peak for this moiety. Instead, it is the C-H stretches and the absence of an umbrella mode that is useful.

Conclusions

Cyclic alkanes have chemical structures consisting solely of CH₂ groups. The non-ideal bond angles in some of these structures cause ring strain, making these structures energetically unfavorable. The common cyclic alkane structures are cyclohexane and cyclopentane because they have relatively little ring strain. The bond angles in cyclohexane are close to the ideal of about 109°, much like straight chain hexane, meaning the spectra of the methylene groups in both molecules are similar. However, the lack of a methyl group in cyclohexane means it is missing a CH₃ umbrella mode.

Because of ring strain, the CH₂ stretching peaks in cyclopentane fall in the same range as a normal CH₃ group. However, cyclopentane does not contain a methyl group hence no umbrella mode. So, if there are CH₃ looking C-H stretching peaks but no umbrella mode, you may have a cyclopentyl ring.

Infrared Spectral Interpretation Workshop Answer

At this point in our IR spectral interpretation workshop journey, we have covered dozens of functional groups which means they are all in play for this problem. The spectrum of this workshop problem is seen in Figure 7. Note that we have C-H stretches above and below 3000 which means we have saturated and unsaturated carbons present. The peaks between 3100 and 3000 could mean an alkene or aromatic ring is present. Recall that C=C stretches fall between 1680 and 1620 while the carbon-carbon bond-and-a-half stretches of benzene rings fall from 1620 to 1400.³ There are no peaks in Figure 7 between 1680 and 1620 ruling out a C=C, but there are ring mode peaks at 1602 and 1497 proving our source of unsaturation is a benzene ring.

Recall that the substitution pattern on mono- and di-substituted benzene rings can be determined based on the presence or absence of the ring bend at about 690 and the position of the aromatic C-H wag.⁴ There is clearly a peak at 698 for the ring bend meaning the substitution pattern is either mono- or meta. The C-H wag is at 763 which unfortunately means the substitution pattern may still be mono or meta. Recall that the benzene fingers, a series of small overtone and combination bands between 2000 and 1650, can be used to distinguish ortho, meta, and para substituted rings from each other.⁵ The pattern of four evenly spaced bands in this region indicate the aromatic ring is mono-substituted.

Now, what is that substituent? There are only two saturated C-H stretching bands at 2965 and 2870. The fact there are two peaks means there are methyl OR methylene units present, the peak positions show they are methyl groups. The third useful group wavenumber peak for methyl groups is the umbrella mode usually found as a single peak at 1365±10. However if we look closely there is a split umbrella mode instead with peaks at 1394 and 1365 and note their intensity ratio is 1:2. Recall from last time that a split umbrella mode means there is a branch point, and the 1:2 intensity ratio means tert-butyl or isobutyl.⁶ Isobutyl groups have methyls and a methylene, tert-butyl groups have only methylenes. Our substituent then must be a tert-butyl group, and since we know our benzene ring is mono-substitued, we can put the two parts together to determine the answer, t-butyl benzene whose structure is seen in Figure 7.

Table 1 shows the proper peak assignments for the successful completion of this problem.

References

1. Smith, B. C. Group Wavenumbers and an Introduction to the Spectroscopy of Benzene Rings. Spectroscopy 2016, 31 (3), 34–37. https://www.spectroscopyonline.com/view/group-wavenumbers-and-introduction-spectroscopy-benzene-rings (accessed 2026-05-26).
2. Smith, B. C. Electromagnetic Radiation, Spectral Units, and Alkanes. Spectroscopy 2015, 30 (4), 18–23. https://www.spectroscopyonline.com/view/electromagnetic-radiation-spectral-units-and-alkanes (accessed 2026-05-26).
3. Smith, B. C. The Infrared Spectroscopy of Alkenes. Spectroscopy 2016, 31 (11), 28–34. https://www.spectroscopyonline.com/view/infrared-spectroscopy-alkenes (accessed 2026-05-26).
4. Smith, B. C. Distinguishing Structural Isomers: Mono- and Disubstituted Benzene Rings. Spectroscopy 2016, 31 (5), 36–39. https://www.spectroscopyonline.com/view/distinguishing-structural-isomers-mono-and-di-substituted-benzene-rings (accessed 2026-05-26).
5. Smith, B. C. The Benzene Fingers, Part II: Let Your Fingers Do the Walking Through the Benzene Fingers. Spectroscopy 2016, 31 (9), 30–33. https://www.spectroscopyonline.com/view/benzene-fingers-part-ii-let-your-fingers-do-walking-through-benzene-fingers (accessed 2026-05-26).
6. Smith, B. C. Infrared Spectral Interpretation Advanced Topics: Branched Alkanes. Spectroscopy 2026, 41 (2), 6–10. https://doi.org/10.56530/spectroscopy.ve3683l6