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Carbon Nitrogen Bonds

The IR spectra of nitrogen-containing compounds can be messier than the ones you have seen so far. N-H bends and C-N stretches tend to be broader and weaker than peaks involving oxygen atoms. However, some peaks in nitrogen compounds are useful. The problems in this section will guide you through some of these features.

Problem IR.11.

Amines, such as butylamine, have a very diagnostic N-H peak, although the peak is sometimes weak.

Figure IR15. IR spectrum of butylamine.

Source: SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology of Japan, 14 July 2008)

Butylamine is a primary amine, meaning the nitrogen is attached to one carbon group and two hydrogens. An N-H bond is almost as strong as an O-H bond. Can you identify the N-H peak in this spectrum?

Problem IR.12.

Dibutylamine is another example of an amine. It is a secondary amine, meaning the nitrogen is attached to one hydrogen and two other groups.

Figure IR16. IR spectrum of dibutylamine.

Source: SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology of Japan, 14 July 2008)

Compare its N-H peak to that of butylamine. What do you notice about the number of N-H peaks in each spectrum?

Problem IR.13.

What peaks would you expect to see in the IR spectrum of triisobutylamine?

Problem IR.14.

Benzonitrile has no N-H bonds but it does have a carbon-nitrogen triple bond that shows up in the IR spectrum. Identify the corresponding peak on the spectrum, plus two other peaks.

Figure IR17. IR spectrum of benzonitrile.

Source: SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology of Japan, 14 July 2008)