Figure 9.18
Infrared spectrum of 1,2-dimethylbenzene.stretching band at ~ 3350 cm–^1 is broadened by hydrogen bonding whilst the strong band at ~ 1070 cm–^1
is due to a complex C—O stretching vibration. In the aldehyde a strong band assigned to stretching of
the carbonyl group appears at ~ 1730 cm–^1 and a sharp peak due to stretching of the aldehydic C—H (~
2720 cm–^1 ) serves to differentiate it from other types of carbonyl compound.
The spectrum of a carboxylic acid can be compared with that of an ester in Figures 9.22 and 9.23. Such
acids exist primarily as dimers, and the strongly hydrogen bonded structures give rise to a very broad
O—H stretching band between 2500 and 3500 cm–^1 and another at around 940 cm–^1 due to O—H out-
of-plane bending. An intense carbonyl stretching band occurs near 1710 cm–^1 , similar to that of an
aliphatic ketone because of hydrogen bonding, and the C—O stretch appears as a broad absorption
between 1200 and 1300 cm–^1. The ester can be distinguished by the absence of O—H bands around
3000 and 940 cm–^1 and the shift of the carbonyl band to ~ 1740 cm–^1 caused by replacement of the
acidic proton with an alkyl group. (N.B. carbonyl absorption in a monomer acid occurs at ~ 1760 cm–^1 .)
Furthermore the C—O stretching band
Figure 9.19
Infrared spectrum of 1-octene.