Chemistry, Third edition

20 · LIGHT AND SPECTROSCOPY

irrespective of the molecule in which they are found. Typical of such groups are C=O,

C–H, –NH 2 and –OH. For example, the higher amplitude vibration of the carbonyl

group (C=O), due to the transition v 0 to v 1 , is activated by infrared radiation between

1700 and 1800 cm–1, the exact wavenumber varying with the molecule in question.

This transition gives rise to a ‘carbonyl peak’ at 1715 cm–1in propanone

(CH 3 COCH 3 , Fig. 20.17), 1725 cm–1in ethanal (CH 3 CHO, Fig. 20.18), 1742 cm–1in

the ester ethyl ethanoate (CH 3 COOC 2 H 5 , Fig. 20.19) and 1720 cm–1in ethanoic acid

(CH 3 COOH, Fig. 20.20). The high intensity (peak height) and relatively narrow

wavenumber range over which the C=O peak is observed in compounds makes it easy

to recognise in the infrared spectrum of an unknown compound. This gives us a very

useful way of identifying compounds that contain carbonyl groups. For example,

photographic film consists of the ester cellulose triacetate and a spectrum of film

shows an intense absorption at about 1745 cm–1.

Other groups of atoms also give identifiable absorptions:

●C–H vibrations in molecules where the carbon atom of the C–H group is linked

to other carbon atoms using single C–C bonds, as in

H

||

⎯C⎯C⎯

||

●are observed as sharp peaks in a relatively narrow wavenumber band 3000–2850

cm–1(see hexane, Fig. 20.21). The exceptions are C–H vibrations within the CHO

group of aldehydes, which usually give two peaks between 2850 and 2750 cm–1

(Fig. 20.18). These are useful to distinguish aldehydes from ketones.

●C–H vibrations involving carbon atoms bonded to other carbon atoms by double

or triple bonds, such as

H

\⁄

C==C and H⎯CC⎯

⁄\

●give rise to peaks above 3000 cm–1. C–H peaks above 3000 cm–1are also observed

where the vibrating C–H group is part of a benzene ring (see benzene, Fig. 20.22).

●The hydrogen-bonded –OH group of alcohols and phenols gives rise to a charac-

teristic and very broad absorption in the range 3500–3200 cm–1(see ethanol, Fig.

384

Fig. 20.25Infrared spectrum of ethanenitrile CH 3 CN.