Infrared spectroscopy

Infrared spectroscopy is a powerful analytical technique used in the identification of organic molecules. It is based on the principle that molecules absorb infrared radiation at specific frequencies that correspond to the vibrations of their constituent atoms. Infrared spectra are used to identify functional groups in molecules, which can provide insight into their chemical properties and reactivity.

Functional groups are specific groups of atoms that are responsible for the chemical behaviour of a molecule. They determine the molecule's reactivity, solubility, and physical properties. By analysing the infrared spectra of a molecule, it is possible to identify the functional groups present in the molecule.

Infrared spectra are typically displayed as a plot of the intensity of the infrared radiation absorbed by the molecule versus the frequency of the radiation. The frequency is measured in units of wavenumbers, which are inversely proportional to the wavelength of the radiation. The wavenumber scale ranges from 4000 cm-1 to 400 cm-1, and the absorption bands are typically broad and intense.

The interpretation of infrared spectra requires an understanding of the characteristic absorption frequencies associated with different functional groups. The most important functional groups and their characteristic absorption frequencies are listed below:

1. Alkanes:

C-H stretching vibrations occur in the range of 2850-2960 cm-1, while C-H bending vibrations occur in the range of 1350-1470 cm-1.

2. Alkenes:

The C-H stretching frequency is like that of alkanes, but the C=C stretching vibration occurs in the range of 1620-1680 cm-1.

3. Alkynes:

The C-H stretching frequency is like that of alkanes, but the C≡C stretching vibration occurs in the range of 2100-2260 cm-1.

4. Aromatic compounds:

The C-H stretching frequency is typically found in the range of 3000-3100 cm-1, while the C=C stretching vibration occurs in the range of 1450-1600 cm-1.

5. Alcohols:

The O-H stretching vibration occurs in the range of 3200-3600 cm-1, and the C-O stretching vibration occurs in the range of 1050-1150 cm-1.

6. Amines:

The N-H stretching vibration occurs in the range of 3300-3500 cm-1, while the C-N stretching vibration occurs in the range of 1000-1300 cm-1.

7. Carboxylic acids:

The O-H stretching vibration occurs in the range of 2500-3500 cm-1, and the C=O stretching vibration occurs in the range of 1680-1750 cm-1.

8. Esters:

The C=O stretching vibration occurs in the range of 1730-1750 cm-1, and the C-O stretching vibration occurs in the range of 1250-1300 cm-1.

9. Amides:

The C=O stretching vibration occurs in the range of 1650-1690 cm-1, while the N-H bending vibration occurs in the range of 1600-1650 cm-1.

10. Ethers:

The C-O stretching vibration occurs in the range of 1000-1300 cm-1, while the C-O-C bending vibration occurs in the range of 1200-1300 cm-1.

In addition to these functional groups, there are many other absorption frequencies that can be used to identify specific functional groups or chemical bonds in a molecule. The interpretation of infrared spectra is a complex process that requires careful analysis and comparison to reference spectra.

Luckily for us, exam questions usually involve comparing spectra and are limited to only a few well known functional groups. Another thing to remember - don't try to memorise the regions since all informatio is given in the supplied data booklet. The fingerprint region (below 1500 cm-1) can usually be ignored for our purposes - all the action happens above this wavenumber.

Example One

This is the spectrum for butanoic acid. The broad stretch centred around 3000 cm-1 is the O-H stretch in an acid. The strong 'peak' at 1700 cm-1 is due to a carbonyl (C=O).

The C=O stretch varies in position:

Aldehydes (1740-1690)

Ketones (1750-1680)

Carboxylic acids (1780-1710)

You can see it can be difficult to differentiate between these carbonyl-containing groups. The combination of broad O-H and C=O could only make this a carboxylic acid

Example Two

This is the sprctrum for ethanol - notice the wide stretch of O-H and absencde of C=O peak. The C-O stretch in the fingerprint region is also a good indicator.

Example Three

This is the spectrum for a ketone - there's the unmistakeable C=O stretch. The absence of an O-H stretch rules out an alcohol and carboxylic acid. It could be an aldehyde but, for an aldehyde, a peak usually appears around 2720 cm-1, appearing as a "shoulder-type" peak to the right of the C–H stretches. (You would require a data sheet to rule out an ester, for example - the data sheet should lead you to a definitive answer.