Substituted Benzene Rings—The Rest of the Story, Part I: Peak Positions

In past columns, I introduced the spectra of mono- and di-substituted benzene rings. At that time, we found that the presence or absence of the ring bend peak at 690 cm^-1 and the position of the aryl C–H wag between 1000 and 700 cm^-1 together help distinguish mono-, ortho-, meta-, and para-substituted rings. But of course, benzene can have up to six substituents. In this column, we will introduce the spectra of tri-, tetra-, and penta-substituted benzene rings. We will find that many of the same concepts that applied to mono- and di-substituted rings can be extended to higher substitution patterns as well. We will summarize with the rules for determining the substitution pattern of all substituted benzene rings.

Aromatic rings constitute a large family of organic molecules. A common example of an aromatic ring is benzene, C₆H₆, whose molecular and electronic structures are shown in Figure 1.¹

The structure of benzene is that of a regular hexagon with a carbon atom at each vertex and one hydrogen attached. Each carbon atom contains a p orbital with a single electron in it, as seen at the top of Figure 1. What makes this ring aromatic is that the p electrons delocalize to form a cloud of electron density above and below the ring, as illustrated at the bottom of Figure 1. The cloud of electron density is called a π cloud, and the electrons that comprise it are called π electrons. A characteristic of many aromatic rings is the existence of delocalized π electrons.

There are many types of aromatic rings. So far in this column series, I have restricted discussion to mono- and di-substituted benzene rings because these have the simplest spectra and are probably the most common types of aromatic rings.² Here we will extend the discussion to tri-, tetra-, and penta-substituted rings.

Benzene Ring Substitution Patterns

Recall² that mono- and di-substituted benzene rings form the structures seen in Figure 2.

We will call the hydrogens attached to the aromatic carbons in a ring aryl C–Hs or aryl hydrogens. Note that the mono-substituted ring has five hydrogens, all adjacent to each other. Di-substituted rings can form three different structural isomers, as seen in Figure 2. Structural isomers are molecules with the same chemical formula but different chemical structures. The three di-substituted rings seen in Figure 2 are chemically distinct and have different reactivities, boiling points, and infrared spectra, as discussed previously.² In an ortho ring, the two substituents are on adjacent carbons, and there are four adjacent aryl hydrogens. In the meta-substituted ring illustrated, there is an unsubstituted carbon separating the two substituents with what we will call a lone hydrogen bonded to it, and then there are three adjacent hydrogens. In a para ring, the substituents are at opposite ends of the ring, separated by a pair of adjacent hydrogens on each side of the ring.

When we studied the spectra of these molecules,² we saw that the position of the aryl C–H wag, the presence or absence of the ring mode at 690 cm^-1 (going forward, all peak positions will be in cm^-1 units even if not so stated), and the pattern of the benzene fingers could be used to distinguish these molecules from each other. Table 1 summarizes some of this information.

For the benzene finger patterns for these molecules, I refer you to a previous column.³

Like di-substituted rings, when three substituents are attached to a benzene ring, three structural isomers are possible, as shown in Figure 3, using trimethylbenzenes as examples.

The isomers are named by numbering the carbon atoms from 1 through 6, and then stating the carbon number each substituent is attached to. For the 1,2,3-isomer as seen to the left in Figure 3, all substituents are on adjacent carbons, and there are three adjacent hydrogens. For the 1,2,4-isomer, two of the substituents are next to each other, but a carbon atom is skipped until the third substituent is found, giving a lone hydrogen and two adjacent hydrogens. The 1,3,5-isomer has substituents on every other carbon and thus three lone hydrogens. This structure is unique because of its high symmetry.

When four substituents are placed around a benzene ring 3 structural isomers are formed, as seen in Figure 4.

As above, the six carbons of the benzene ring are numbered 1 through 6, and then the isomer is named for the carbon numbers, which have substituents attached. Going left to right in Figure 4, there is the 1,2,3,5-isomer where three adjacent carbons contain a substituent, and there are two lone hydrogens. Next is the 1,2,4,5-isomer, which also contains two lone hydrogens. Lastly, to the right in Figure 4 is the 1,2,3,4-isomer, where all four substituents are on adjacent carbons, and there is a pair of adjacent hydrogens.

Benzene, of course, can also have five substituents, as illustrated by the structure of pentamethyl benzene seen in Figure 5.

Note that this molecule has one lone hydrogen only. There exist hexa-substituted benzene rings that contain no aryl hydrogens. These are hard to see spectroscopically because of the lack of aryl hydrogens and will not be discussed further.

There is a reason I have been pointing out the number of adjacent hydrogens in the structures seen in Figures 2 through 5. Recall from above, and as seen in Table 1, the position of the C–H wag is different for mono-, ortho-, meta-, and para-substituted rings. What I haven’t disclosed is why they are different. The key is the number of adjacent hydrogens that wag together in-phase with each other, as seen in Figure 6.

The + signs in Figure 6 represent arrow tails going behind the plane of the page, indicating that the hydrogens are all moving in phase in the same direction. They all move either below the plane of the page or above the plane of the page together. As it turns out, the position of the aryl C–H wagging peak in an infrared spectrum is determined by the number of adjacent hydrogens wagging in phase. For example, the structure of toluene is seen in the top left of Figure 6. Toluene contains five adjacent hydrogens because it is mono-substituted. The aryl C–H wagging vibration for toluene consists of all five hydrogens wagging in phase above and below the plane of the molecule as seen in the figure. The position for this peak is 770 to 710 cm⁻¹. Any substituted benzene ring with five adjacent hydrogens will have its C–H wag in this range.

The 4-adjacent C–H wag in Figure 6 is illustrated with ortho-xylene. The reason ortho molecules have their C–H wag from 770 to 735 cm⁻¹ is that they contain four adjacent hydrogens that wag in phase with each other. Any benzene ring with four adjacent hydrogens will exhibit a C-H wagging peak from 770 to 735 cm⁻¹. The 3-adjacent hydrogen case is illustrated in Figure 6 by 1,2,3-trimethyl benzene. Three adjacent hydrogens wagging in phase with each other give a peak from 810 to 750 cm⁻¹, and any benzene ring with three adjacent hydrogens should have a peak in this range. The 2-adjacent example in Figure 6 is 1,2,3,4-tetramethyl benzene. Generally, the 2-adjacent C–H wag is found from 860 to 790 cm⁻¹. Most benzene rings with two adjacent hydrogens will have a peak in this range. The lone hydrogen C–H wag example in Figure 6 is pentamethylbenzene. Lone hydrogen C–H wags usually fall from 900 to 860 cm⁻¹, and most benzene rings that contain a lone hydrogen will have a peak in this range. A summary of the peak positions for benzene ring aryl in-phase C–H wagging peaks is given in Table 2.

Note in Table 2 that as the number of adjacent wagging hydrogens decreases, the peak position increases. Table 2 is important because it allows us to understand all substituted benzene rings, not just the mono- and di-substituted variants discussed earlier, and Table 2 is generally universal. For example, para-substituted rings, such as the structure of para-xylene in Figure 2, have two sets of 2-adjacent hydrogens, and reading Table 2, we would then expect para molecules to have a C–H wag from 860 to 790 cm⁻¹. This is the case as seen in Table 1. Similarly, ortho-substituted molecules have 4-adjacent hydrogens and hence have their C-H wag from 770-735 cm⁻¹, as seen in Tables 2 and 1. We will see that Table 2 will be very useful in distinguishing the many substituted benzenes from each other.

Table 1 also shows that the presence or absence of the ring bend at 690±10 changes with the substitution pattern. Recall² that this vibration involves the carbon-carbon bonds in a benzene ring bending above and below the plane of the molecule. The reason for some substitution patterns is that this peak is intense, and for others, it is zero, which has to do with symmetry. Recall that one of the things that determines peak intensity in infrared spectra is (dμ/dx)² or the change in dipole moment with respect to bond distance during a vibration.⁴ For some substitution patterns (dμ/dx)² for the ring bend is large; for others, it is zero. In addition to this peak appearing in the spectra of mono- and di-substituted molecules, it also appears in some higher substituted benzene rings as well.

The moral of the story is that originally, we saw that for mono- and di-substituted benzene rings the position of the C–and H wag and the presence or absence of the ring bend was useful in distinguishing these four types of molecules from each other. What we have seen here is that these same concepts can be used to distinguish all the substituted benzene rings from each other. Table 3 summarizes all this information.

The middle column in Table 3 lists the position of the aryl C–H wag for every benzene ring but those with six substituents, since these rings contain no aryl C–H bonds. All the aryl C–H wags listed are determined by the number of adjacent hydrogens. One can use the structures listed in Figures 2 through 6, determine the number of adjacent hydrogens in each structure, consult Table 2 to find where the aryl C–H wag falls, and then use Table 3 to determine whether that substitution pattern has a ring bend peak or not. The C–H wag peak positions for mono, ortho, meta, and para rings we learned originally² and are listed in Table 1 are just a subset of Table 3.

Note that only four of the substitution patterns in Table 3 exhibit the ring bend. Previously² we learned for mono and di-substituted rings that this peak appears at 690±10. Note in Table 3 that this range is extended somewhat for the rings that exhibit this peak.

So now you know why the title of this column contains the quip, “the rest of the story.” Benzene contains not only one or two substituents, but up to six. The position of the aryl C–H wag and ring bend peaks we used previously for mono and di-substituted rings can be extended to all substituted benzene rings. I’m afraid we don’t have space in this column to look at any spectra. These will be discussed next time.

Conclusions

The position of the aryl C–H wagging peak and the presence or absence of the ring bend peak used to previously distinguish between mono-, ortho-, meta-, and para-substituted benzene rings can be extended to the rest of the family. The aryl C–H wagging peak position depends upon the number of adjacent hydrogens wagging in-phase. The appearance of the ring bend depends upon symmetry. Using these facts, the spectra of mono, di-, tri-, tetra-, and penta- substituted benzene rings can be understood.

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 July 6, 2026).

(2) 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 July 6, 2026).

(3) Smith, B. C. The benzene fingers, Part II: Let your fingers do the walking through the benzene fingers. Spectroscopy2016, 31 (9), 30–33. https://www.spectroscopyonline.com/view/benzene-fingers-part-ii-let-your-fingers-do-walking-through-benzene-fingers (accessed July 6, 2026).

(4) Smith, B. C. IR spectral interpretation workshop. Spectroscopy 2015, 30 (1), 16–23. https://www.spectroscopyonline.com/view/ir-spectral-interpretation-workshop (accessed July 6, 2026).