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Organic Chemistry Text Book (CHEM 3401 and 3402)

14.2.3 Group Frequencies

Detailed information about the infrared absorptions observed for various bonded atoms and groups is usually presented in tabular form. The following table provides a collection of such data for the most common functional groups. Following the color scheme of the chart, stretching absorptions are listed in the blue-shaded section and bending absorptions in the green shaded part. More detailed descriptions for certain groups (e.g. alkenes, arenes, alcohols, amines & carbonyl compounds) may be viewed by clicking on the functional class name. Since most organic compounds have C-H bonds, a useful rule is that absorption in the 2850 to 3000 cm-1 is due to sp3 C-H stretching; whereas, absorption above 3000 cm-1 is from sp2 C-H stretching or sp C-H stretching if it is near 3300 cm-1.

 

            Typical Infrared Absorption Frequencies
 

Stretching Vibrations

Bending Vibrations

Functional Class

Range (cm-1)

Intensity

Assignment

Range (cm-1)

Intensity

Assignment

Alkanes

 2850-3000 str CH3, CH2 & CH
2 or 3 bands		 1350-1470
1370-1390
720-725		 med
med
wk		 CH2 & CH3 deformation
CH3 deformation
CH2 rocking

Alkenes

 3020-3100
1630-1680

1900-2000		 med
var

str		 =C-H & =CH2 (usually sharp) 
C=C (symmetry reduces intensity)

C=C asymmetric stretch		 880-995
780-850
675-730		 str
med
med		 =C-H & =CH2
(out-of-plane bending)
cis-RCH=CHR

Alkynes

 3300
2100-2250		 str
var		 C-H (usually sharp)
C≡C (symmetry reduces intensity)		 600-700 str C-H deformation

Arenes

 3030
1600 & 1500		 var
med-wk		 C-H (may be several bands)
C=C (in ring) (2 bands)
(3 if conjugated)		 690-900 str-med C-H bending &
ring puckering

Alcohols & Phenols

 3580-3650
3200-3550
970-1250		 var
str
str		 O-H (free), usually sharp
O-H (H-bonded), usually broad
C-O		 1330-1430
650-770		 med
var-wk		 O-H bending (in-plane)
O-H bend (out-of-plane)

Amines

 3400-3500 (dil. soln.)
3300-3400 (dil. soln.)
1000-1250		 wk
wk
med		 N-H (1°-amines), 2 bands
N-H (2°-amines)
C-N		 1550-1650
660-900		 med-str
var		 NH2 scissoring (1°-amines)
NH2 & N-H wagging
(shifts on H-bonding)

Aldehydes & Ketones

 2690-2840(2 bands)
1720-1740
1710-1720

1690167517451780		 med
str
str

str
str
str
str		 C-H (aldehyde C-H)
C=O (saturated aldehyde) 
C=O (saturated ketone)

aryl ketone
α, β-unsaturation
cyclopentanone
cyclobutanone
1350-1360
1400-1450 
1100
str
str
med
α-CH3 bending
α-CH2 bending
 C-C-C bending

Carboxylic Acids & Derivatives

 2500-3300 (acids) overlap C-H
1705-1720 (acids)
1210-1320 (acids)

1785-1815 ( acyl halides)1750 & 1820 (anhydrides)    1040-11001735-1750 (esters)    1000-1300 1630-1695(amides)		 str
str
med-str

str
str
str
str
str
str		 O-H (very broad)
C=O (H-bonded) 
O-C (sometimes 2-peaks)

C=O
C=O (2-bands)
    O-C
C=O
    O-C (2-bands)
C=O (amide I band)		 1395-1440







1590-1650 
1500-1560 		 med







med
med		 C-O-H bending







N-H (1¡-amide) II band
N-H (2¡-amide) II band

Nitriles

Isocyanates,Isothiocyanates,
Diimides, Azides & Ketenes

 2240-2260

2100-2270		 med

med		 C≡N (sharp)

-N=C=O, -N=C=S
-N=C=N-, -N3, C=C=O		  

 

To illustrate the usefulness of infrared absorption spectra, examples for five C4H8O isomers are presented below their corresponding structural formulas. The five spectra may be examined in turn by clicking the "Toggle Spectra" button. Try to associate each spectrum (A - E) with one of the isomers in the row above it. When you have made assignments check your answers by clicking on the structure or name of each isomer.