Instant Notes: Analytical Chemistry

shows the IR spectrum of a triglyceride fat, with the ester group R–O–CO–R¢.

The long-chain of the carboxylic acid gives –CH stretching vibrations just below

3000 cm-^1 and chain rocking at 720 cm-^1. The prominent peak near 1750 cm-^1

clearly shows the presence of the carbonyl group.

The changes that may occur during a reaction are readily studied. For

example, the dehydration of an alcohol to an alkene is observed by the disap-

pearance of the strong, broad –OH bands at 3300 and 1350 cm-^1 and the –C–O–

stretch near 1100 cm-^1 , arrowed in the spectrum of cyclohexanol in Figure 4aand

the appearance of the –C=C–bands, especially around 1600 cm-^1 as shown in

the spectrum of cyclohexene shown in Figure 4b.

Acidsare rather exceptional, as in the solid or liquid state the IR bands are

broadened and weakened by hydrogen bonding. As seen in the spectrum of

acetyl salicylic acid (aspirin) of Figure 5, the –OH peak is now so broad that

it overlaps with the –CH peaks at 3000 cm-^1. The acid carbonyl peak is also

broadened.

Raman spectra are equally useful for the interpretation of group frequencies.

Figure6 shows the Raman spectrum of methyl benzoate. Although there may

244 Section E – Spectrometric techniques

4000 3000 2000

cm–1

1500 1000 500

% T

(a)

(b)

Fig. 4. (a) Cyclohexanol (liquid film); (b) cyclohexene (liquid film).

4000 3000 2000

cm–1

1500 1000 500

% T

Fig. 5. Acetylsalicylic acid (KBr disk).