Ch13 - Sample IR spectra

Chapter 13: Spectroscopy

Sample IR Spectra : heteroatom functional groups
By looking at IR spectra that contain known functional groups and comparing and contrasting them with other IR spectra, one can develop the skills required to be able to "interpret" an "unknown" IR spectra. Remember that for an organic chemist, the primary role of IR is to identify the functional groups that are present. A few examples reflecting some of the more important functional groups are provided below.
Compare them to try to appreciate the subtle differences, comparing frequency, intensity and shape.

1. An alcohol. The characteristic absorption of an alcohol, such as 2-propanol, is the broad band due to the hydrogen bonded -OH group around 3200-3400 cm^-1.

IR spectra of isopropanol

2a. A primary amine: Here we have the IR of propyl amine, CH₃CH₂CH₂NH₂. The key absorptions are the NH which shows a symmetric and asymmetric band in the range of 3500 cm^-1.

IR of propylamine

2b. A secondary amine: Here we have the IR of dipropyl amine, (CH₃CH₂CH₂)₂NH. The key absorption is the single (broad) NH band near 3500 cm^-1.

IR of dipropylamine

2c. A tertiary amine:

3a. A ketone, acetone (2-propanone) is the "classic" carbonyl containing compound with the obvious C=O stretch in the middle of the spectra at 1715 cm^-1. Note that the peak is a very strong absorption due to the polarity of the bond. Make sure to compare it with the C=C alkenes which are weaker and sharper.

3b. An aldehyde, butanal, CH₃CH₂CH₂CH(=O), the a carbonyl containing compound with the obvious C=O stretch in the middle of the spectra at 1731 cm^-1 (a little higher than the simple ketone shown above).

IR of butanal

3c. Carboxylic acids, such as butanoic acid, CH₃CH₂CH₂CO₂H, contain both C=O (1712 cm^-1 ) and OH groups (near 3000 cm^-1). Note the broadness of both absorptions due to the hydrogen bonding and that the C=O is typically at slightly lower frequency than that of a ketone.

3d. An ester has the following key absorptions, the C=O (here 1746 cm^-1) and typically two bands for the C-O (not always easy to identify, here at about 1250 and 1050 cm^-1) since there are sp³ C-O and sp² C-O bonds.

IR spectrum of methyl ethanoate

3e. An amide, here propanamide, CH₃CH₂C(=O)NH₂, is characterised by a strong C=O bands at 1650 cm^-1 (note that these are well below the normal value for a ketone carbonyl). In addition, we can also see absorptions for the NH (symmetric and asymmetric) band in the range of 3200-3400 cm^-1. The broadness of the bands is likely due to hydrogen bonding.

IR of propanamide

3f. An acid halide (or acyl halide), here CH₃CH₂C(=O)Cl, is characterised by a strong C=O bands at 1790 cm^-1 (note that these are well above the normal value for a ketone carbonyl).

IR of propanoyl chloride

3g. An acid anyhydride, propanoic anhydride, (CH₃CH₂C(=O))₂O is characterised by 2 C=O bands (symmetric and asymmetric stretches) at 1810 and 1760 cm^-1 (note that these are well above the normal value for a ketone carbonyl).

IR of propanoic anhydride

4. Nitrile. This example is propionitrile, CH₃CH₂C≡N. This is is characterised by the strong and sharp C≡N at 2250 cm^-1 (Note that this is in the same region as the alkyne C≡C).

IR of propionitrile