The Big Review VIII: Organic Nitrogen Compounds

In our ongoing review of infrared spectra, we will study organic nitrogen containing compounds including amides and amines. Amides contain both nitrogen and a C=O group and are found in proteins and polymers. Amines contain carbon, nitrogen, and hydrogen, and are ubiquitous in medicines. As always, concepts will be illustrated with reference spectra.

Since the inception of this column series over ten years ago (!) we have covered in detail the theory of infrared spectral interpretation and the spectra of C-H bonds, C-O bonds, C=O bonds, and organic nitrogen compounds. The last several columns have involved a review of these topics which I have dubbed The Grand Review (1–7). In past Grand Review columns, we discussed all the major topics of this series except organic nitrogen compounds, which we will discuss here.

So far in our review we have restricted our discussion to organic functional groups that contain carbon, hydrogen, and oxygen. Traditionally, the chemical element nitrogen is considered part of organic chemistry, hence the term organic nitrogen compounds. Thus here we will discuss organic functional groups that contain carbon, hydrogen, and nitrogen, and organic functional groups that contain carbon, hydrogen, nitrogen, and oxygen.

The nitrogen atom has an atomic number of seven and five outer shell electrons. The drive of chemical elements is to have their outer shells filled with electrons. In most organic compounds nitrogen forms three chemical bonds to other atoms. When it comes to carbon-nitrogen bonds single, double and triple bonds can form as seen in Figure 1.

Note that for the C-N bond seen in Figure 1a the bond angles are approximately 120° and that there are three atoms bonded to the nitrogen. For the C=N bond seen in Figure 1b the bond angles are approximately 90° and there are two atoms attached to the nitrogen. Lastly, for the carbon-nitrogen triple bond seen in Figure 1c the bond angles are approximately 180° and there is one atom attached to the nitrogen. So the question is “Are can any of the peaks from the three types of carbon-nitrogen bond serve as a marker for the presence of nitrogen in a compound?” The short answer is no for reasons discussed in detail in the original article on this topic (8). Briefly, the peaks from C-N bonds are too weak to be a useful spectral identification band; C=N bonds are unstable, and carbon-nitrogen triple bonds have a lovely spectral band at around 2200 cm^-1. (Going forward all peak positions will be in wavenumber units even if not noted.) As it turns out the best infrared peaks for disclosing the presence of nitrogen in a compound are N-H stretching peaks. These bands show up in the 3500-3300, range right where O-H stretches fall (9). However, the width of O-H and NH stretches are significantly different as seen in Figure 2.

The left hand spectrum in Figure 2 shows the N-H stretching peaks of propylamine, and the peak envelope width as measured at its base is approximately 400 cm^-1. The right hand spectrum is of the O-H stretch of ethyl alcohol, and note its peak width measured at its base is over 1000 cm^-1. The N-H stretching peaks are significantly narrower than the O-H stretching peaks, and this peak width difference can be used to distinguish the OH and NH functional groups from each other even though their peaks show up in the same spectral region. The difference in peak widths seen in Figure 2 is due to the difference in hydrogen bond strength between N-H and O-H containing functional groups as discussed previously (8).

Amides

The original articles on the spectroscopy of amides are found in references (10,11). The generic structural framework of amides is seen in Figure 3.

The amide functional group consists of a central carbonyl carbon with a single nitrogen atom bonded to it. This nitrogen is called the amide nitrogen, and can have carbons or nitrogens attached to it. The carbon attached to the carbonyl carbon is called the alpha carbon.

Figure 4 shows the structures of primary, secondary, and tertiary amides. The difference between them is the number of carbons attached to the nitrogen, that is the number of C-N bonds.

Thus, a primary amide has one C-N bond and two N-H’s, a secondary amide has two C-N bonds and one N-H, and a primary amide has three C-N bonds and no N-H bonds.

Amides engage in conjugation (12) as seen in Figure 5.

The nitrogen atom in amides contains a p-orbital with a lone pair of non-bonding electrons in it. This orbital points in space towards the pi-electron cloud of the carbonyl group. There is some orbital overlap as illustrated in Figure 5 which leads to conjugation. Like with aromatic carbonyl groups, because of conjugation some of the electron density is withdrawn from the carbonyl bond, weakening it and lowering its force constant. This causes the amide C=O stretch to be on the low side compared to other carbonyl containing functional groups, typically from 1680 to 1630 cm^-1. Note however that we do not speak of “saturated” or “aromatic” amides as conjugation takes place in all amides, so all amides have the same carbonyl stretching peak range.

Primary Amides

The infrared spectrum of a primary amide, benzamide, is seen in Figure 6.

Since primary amides have a NH₂ group they have two N-H stretching vibrations, whose peaks are seen labeled A at 3366 and 3710 in Figure 6. Both peaks must be present for a molecule to be a primary amide. The C=O stretch in Figure 6 is seen at 1656 and is labeled B, well within the range expected. Benzamide also has a primary amide scissoring peak labeled C in the figure and seen at 1622. In general this peak falls from 1650 to 1620. The NH₂ wag is a broadened envelope due to hydrogen bonding, and is seen at ~700 in the figure and is labeled E. This envelope normally tops out between 750 and 600. The group wavenumbers for primary amides are summarized in Table 1.

The diagnostic pattern for primary amides then is a pair of N-H stretches in combination with a C=O stretch.

Secondary Amides

The secondary amide linkage is important because it is found in polymers and proteins. The infrared spectrum of a polymeric secondary amide, nylon-6,6, is seen in Figure 7.

Because secondary amides contain one N-H bond there is only one N-H stretch, labeled A at 3301 as seen in Figure 7. Generally for secondary amides this peak falls from 3370 to 3170. The C=O stretch of our secondary amide is seen at 1641 and is labeled C, in the 1680 to 1630 range typical of all amides. Note however the carbonyl stretch has a companion peak at 1542 labeled D. This is the in-plane N-H bend of the secondary amide group and is normally found from 1570 to 1515. This peak’s position is unusual in that there are very few group wavenumber peaks that fall around 1550, and it is unusually intense for an in-plane bend. The N-H wag of secondary amides forms a broadened envelope from 750 to 680 and is seen in Figure 7 at 691 labeled F. The summary of secondary amide group wavenumbers is seen in Table 2.

The diagnostic pattern of peaks for secondary amides is one N-H stretch, a carbonyl stretch, and an unusually intense in-plane bend.

Tertiary amides are difficult to detect by infrared spectroscopy because they lack NH bonds and the N-H stretching and bending peaks that are the useful group wavenumbers of primary and secondary amides do not exist.

Amines

The amine functional group contains carbon, hydrogen, and nitrogen and consists of C-H and C-N bonds. Amines are divided into three categories called primary, secondary, and tertiary. Their molecular structures are seen in Figure 8.

Amines consist of a central nitrogen atom, called the amine nitrogen, which is single-bonded to various numbers of carbons and hydrogens. Any carbon attached directly to the nitrogen is called an alpha carbon. The amine group structural names count the number of carbons attached to the amine nitrogen. Thus, a primary amine contains one C-N bond, a secondary amine contains two C-N bonds, and a tertiary amine contains three C-N bonds. Also note that as the number of C-N bonds goes up, the number of N-H bonds goes down. Thus, a primary amine contains two N-H bonds, a secondary amine contains one N-H bond, and a tertiary amine contains no N-H bonds.

The nature of the alpha carbons in amines also plays a role in determining the type of amine. If all the alpha carbons are saturated the functional group is called a saturated amine. If all the alpha carbons are aromatic, then the moiety is called an aromatic amine. But what happens when one alpha carbon is saturated and the other is aromatic as is the case for the molecule N-methylaniline seen in Figure 9?

The answer is that aromaticity wins. If any of the alpha carbons in an amine are aromatic, the amine is aromatic. Thus, N-methylaniline is an aromatic amine. Ultimately then there are 6 different kinds of amine: primary saturated, primary aromatic, secondary saturated, secondary aromatic, tertiary saturated, and tertiary aromatic. Note it takes two adjectives to properly describe an amine. All these types of amine may be bewildering, but as we will see in the following text that infrared spectroscopy distinguishes all the different types of amines from each other.

Primary Amines

Primary amines have N-H stretching and bending vibrations as seen in Figure 10.

The peak ranges for these vibrations are given in Table 3.

Note that there are two N-H stretches for primary amines, and that both peaks are sensitive as to whether the amine is saturated or aromatic. The spectrum of a primary amine, propylamine, is seen in Figure 11.

The N-H stretches of propylamine fall at 3369, which is assigned as the asymmetric stretch, and 3298 cm^-1 which is the symmetric stretch. They are labeled A and B in Figure 11. The positions of both these peaks indicate the amine is saturated. The primary amine group also has a scissoring vibration as seen in Figure 11 and is labeled C at 1606. This peak generally falls from 1650 to 1580 and note it is somewhat broadened due to hydrogen bonding. The last useful group wavenumber for primary amines is the NH₂ wagging peak. It falls from 850-750 and in the spectrum of propylamine in Figure 11 it is found at 812 and is labeled E. Note that it is somewhat broadened like the primary amine N-H stretching and scissoring peaks, again due to hydrogen bonding.

Secondary Amines

The original article on secondary amines is here (13). The spectrum of a secondary amine, N-methylcyclohexylamine, is seen in Figure 12.

The peak labeled A at 3284 is the single N-H stretching peak of this molecule. Note that secondary amines have only one N-H stretching peak, whereas primary amines have two N-H stretching peaks. This is because primary amines have two NH bonds whereas secondary amines have only one. Like primary amines, the secondary amine N-H stretch also discloses whether the amine is saturated or aromatic. For saturated secondary amines the N-H stretching peak falls from 3320 to 3280 while for aromatic secondary amines it is found near 3400. Peak A in Figure 12 confirms that N-methylcyclohexylamine is a saturated amine.

The only other useful group wavenumber for secondary amines is the N-H out-of-plane bend, or more simply called the N-H wag. Imagine the N-H bond bending above and below the plane of this page. It is labeled D in Figure 12 at 736. In general, this peak falls from 750 to 700. The group wavenumber table for secondary amines is seen in Table 4.

Like tertiary amides, tertiary amines contain no N-H bonds and hence do not have any useful group wavenumbers.

Conclusions

The best infrared spectroscopic indicator of the presence of nitrogen in a compound are the N-H stretches. Amides contain C, H, N, and O atoms, while amines contain carbon, hydrogen, and nitrogen only. For both functional groups they come in primary, secondary, and tertiary versions which contain one, two, and three C-N bonds, respectively.

The diagnostic peak pattern for primary amides is two N-H stretching peaks and a carbonyl stretch, while for secondary amides it is one N-H stretch and a carbonyl stretch. The pattern for secondary amides is one N-H stretch, a carbonyl stretch, and an unusually strong N-H in-plane bend.

Amines come in six types as discussed above. The number and position of the N-H stretches can be used to distinguish primary from secondary amines, and saturated from aromatic amines.

References

1. 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
2. Smith, B. C. The Big Review II: The Physical Mechanism of Infrared Absorbance and Peak Types. Spectroscopy 2024, 39 (7), 28-33. DOI: 10.56530/spectroscopy.kj1475f8
3. Smith, B. C. The Big Review III: Molecular Vibration Theory. Spectroscopy 2024,39 (8), 31-34. DOI: 10.56530/spectroscopy.gn8689j7
4. Smith, B. C. The Big Review IV: Hydrocarbons. Spectroscopy 2025, 40 (1), 16-19. DOI: 10.56530/spectroscopy.vt7783b7
5. Smith, B. C. The Big Review V: The C-O Bond. Spectroscopy 2025, 40 (3), 10-13. DOI: 10.56530/spectroscopy.mq4273p5
6. Smith, B. C. The Big Review VI: Carbonyl Compounds. Spectroscopy 2025, 40 (4), 12-18. DOI: 10.56530/spectroscopy.zd9677w3
7. Smith, B. C. The Big Review VII: More Carbonyl Compounds. Spectroscopy 2025, 40 (8), 27-30. DOI: 10.56530/spectroscopy.dg4483d4
8. Smith, B. C. Organic Nitrogen Compounds, Part I: Introduction. Spectroscopy 2019, 34 (1), 10-15.
9. Smith, B. C. The C-O Bond, Part I: Introduction and the Infrared Spectroscopy of Alcohols. Spectroscopy 2017, 32 (1), 14-21.
10. Smith, B. C. Organic Nitrogen Compounds VI: Introduction to Amides Spectroscopy 2019, 34 (11), 32-35.
11. Smith, B. C. Organic Nitrogen Compounds, VII: Amides—The Rest of the Story. Spectroscopy 2020, 35 (1), 10-15.
12. Smith, B. C. The Carbonyl Group, Part I: Introduction. Spectroscopy 2017, 32 (9), 31-36.
13. Smith, B. C. Organic Nitrogen Compounds III: Secondary and Tertiary Amines. Spectroscopy 2019, 34 (5), 22-28.