The C=O stretching frequency aldehydes and ketones

A summary of the principle infrared bands and their assignments A summary of the principle infrared bands and their assignments. R is an aliphatic group.

Aliphatic Aldehyde Figure IR-14. Butanal, neat liquid, thin film: CH3CH2CH2CHO

Aliphatic Ketone Figure IR-35. 5-Hexene-2-one, neat liquid: CH2=CH2CH2CH2COCH3

The C=O stretching frequency esters

A summary of the principle infrared bands and their assignments A summary of the principle infrared bands and their assignments. R is an aliphatic group.

Aliphatic Ester Figure IR-33. Methyl acetate in 0.1 mm NaCl cells: CH3CO2CH3

neat CCl4

Aliphatic Ester Figure IR-34. Vapor and liquid spectra of ethyl n-hexanoate: CH3CH2CH2CH2CH2CO2CH2CH3

The C=O stretching frequency aliphatic acids

A summary of the principle infrared bands and their assignments A summary of the principle infrared bands and their assignments. R is an aliphatic group.

Aliphatic Acid Figure IR-26. The liquid, vapor and CCl4 soln spectra of hexanoic acid: CH3(CH2)4 CO2H

The C=O stretching frequency aliphatic amides

A summary of the principle infrared bands and their assignments A summary of the principle infrared bands and their assignments. R is an aliphatic group.

KBr pellet Amide I  1662 cm-1 Amide II; 1634 cm-1

Secondary Amide Figure IR-21. N-Methyl acetamide, neat liquid; thin film: CH3CONHCH3

Tertiary Amide Figure IR-23. N,N-Dimethylacetamide, neat liquid; thin film: CH3CON(CH3)2

The C=O stretching frequency anhydrides

A summary of the principle infrared bands and their assignments A summary of the principle infrared bands and their assignments. R is an aliphatic group.

Figure IR-36. Propionic anhydride, neat liquid: CH3CH2CO2COCH2CH3 Anhydrides Figure IR-36. Propionic anhydride, neat liquid: CH3CH2CO2COCH2CH3

The C=O stretching frequency carboxylates

A summary of the principle infrared bands and their assignments A summary of the principle infrared bands and their assignments. R is an aliphatic group.

C=O in an acid 1700 cm-1; C-O-C stretch 1300 to 1000 cm-1, say in ethers  1150 cm-1. Averaging a C=O with a C-O-C results in: (1700 + 1150)/2 = 1425 cm-1. Naively, we would estimate a bond and a half would have a frequency of 1425 cm-1

1410 cm-1 1560 cm-1 (1560+ 1410)/2 = 1485 cm-1

The C=C bond stretch

A summary of the principle infrared bands and their assignments A summary of the principle infrared bands and their assignments. R is an aliphatic group.

Figure IR-9. 1-Heptene; neat sample, thin film: H2C=CH-CH2CH2CH2CH2CH3

The C=C bond stretch when next to a heteroatom

A summary of the principle infrared bands and their assignments A summary of the principle infrared bands and their assignments. R is an aliphatic group.

Figure IR-40. 2,5-Dimethoxy2,5-dihydrofuran, neat liquid:

Figure IR-41. 2,3-Dihydrofuran, neat liquid:

The cumulative C=C bond stretch

The NO2 stretching frequency

A summary of the principle infrared bands and their assignments A summary of the principle infrared bands and their assignments. R is an aliphatic group.

Aromatic Rings Ring breathing motions 1600-1585 1500-1400 Both often appear as doublets

Benzene, neat, thin film:

4-Chloro-2-nitrophenol, KBr pellet: