Area D (2000 - 1630 cm-1)

Area D

This Area is where the characteristic carbonyl (-C=O) stretch absorption appears. The exact frequency of this very strong absorption depends on the type of carbonyl (aldehyde, amide, carboxylic acid, ester, ketone, ...) and on whether or not the carbonyl is conjugated (next to an alkene, alkyne or aromatic ring). Conjugation lowers the frequency somewhat due to the partial double bond character of the carbonyl.

As a very general rule-of-thumb, the order of the -C=O stretching frequencies is:

esters > aldehydes > carboxylic acids > ketones > amides

But there is considerable overlap between the regions where the particular absorptions are found:

  • Esters: 1765 cm-1 – 1720 cm-1
  • Aldehydes: 1725 cm-1 – 1695 cm-1.
  • Acids: 1710 cm-1 – 1680 cm-1 for the dimer (the normal situation for a neat liquid or a solid), upon dilution it shifts to 1800 cm-1 – 1740 cm-1.
  • Ketones: 1725 cm-1 – 1650 cm-1.
  • Amides 1680 cm-1 – 1640 cm-1.

Because of the overlap, it is necessary to look for corroborative evidence in other Areas to positively identify the presence of one of these groups.