Appendix 4: IR ABSORPTION FREQUENCIES
TABLE 1: GENERAL ABSORPTION RANGES
Abbreviations: (s) = strong, (m) = medium, (v) = variable, (w) = weak
BOND
TYPE
C−H
COMPOUND TYPE
Alkanes (aliphatic sp3 C−H stretch)
(C−H bend)
Alkenes (olefinic sp C−H stretch)
2
(C−H bend)
Alkynes (acetylenic sp C−H stretch)
(C−H bend)
Aromatic rings (aryl C−H stretch)
(C−H bend)
C−C
C=C
C≡C
…
C−−C
C−O
C=O
O−H
N−H
C−N
C≡N
N−O
Alkanes (C−C stretch)
Alkenes
Alkynes
Aromatic rings (in-plane vibrations)
Alcohols, ethers, carboxylic acids,
esters
Aldehydes, ketones, carboxylic
acids, esters, amides
Monomeric alcohols, phenols
H-bonded alcohols, phenols
carboxylic acids,
water contaminant
Amines, Amides
Amines
Nitriles
Nitro compounds (N−O stretch)
as in C−NO2
FREQUENCY RANGE
(cm−1)
2960-2850 (m-s)
1470-1350 (v)
3080-3020 (m)
1000-675 (v)
(See details in Table 2)
~ 3300 (s)
~ 630 (s)
3000-3100 (v)
(often weak)
675-870
(See details in Table 3)
1300-1100 (v)
(not useful)
1680-1640 (v)
2260-2100 (v)
1500, 1600 (v) (usually 4 peaks, see example p.A-12)
1300-1080 (s)
1800−1650 (s)
(See details given in Table 5.)
3640-3610 (v)
3600-3200 (broad)
3000-2500 (broad)
3710 & 1630
3500-3300 (m)
1360-1180 (v)
2260-2210 (v)
1600-1500 (s)
1400-1300 (s)
TABLE 2: C−H BENDING OF ALKENES (in cm-1)
920-910 (s)
RCH=CH2
cis-RCH=CHR
& 1000-990 (s)
R2C=CH2
900-880 (s)
730-675 (v)
trans-RCH=CHR 975-965 (s)
Characteristic Absorptions of Aromatic Compounds
There are four regions of absorptions for aromatic compounds. You will be mainly concerned with
only Region I this semester.
Region I:
C−H stretch slightly above 3000 cm-1 (Indicative of presence of benzene ring)
Region II: Overtones & combination bands at 2000-1670 cm-1 (See Table 4)
Region III: C=C in-plane vibrations at 1667-1429 cm-1: four bands at ~1600 (s), 1580 (w, as
shoulder), 1500 (s) and 1460 (usually obscured by others). Absence of these bands is fair
assurance that there is no aromatic ring in the compound.
Region IV: C−H bend at 1000-670 cm-1 (See Table 3)
A-11
A-12
APPENDIX 4: IR ABSORPTION FREQUENCIES
TABLE 3: For aromatic rings, out-of-plane C−H bending (in cm-1)
monosubstituted
m-disubstituted
~750 (s) (range 770-730)
R
R
& ~700 (s) (range 710-690)
~780 (v) (range 810-750)
R
p-disubstituted
o-disubstituted
R
~750 (s) (range 770-735)
R
R
~830 (v) (range 840-810)
R
IR Spectrum of Propylbenzene
(Typical Absorptions of Aromatic Compounds)
Region IV
698 cm-1
743 cm-1
monosubstituted C−H bend
Region III
1496 & 1453 cm-1
←1538 cm-1
←1584 cm-1
Region II
aromatic overtones
C=C in-plane vibration
Table 4: IR Absorption Overtones
of Substituted Benzenes
These are weak absorptions that may not be
observed if the sample is not concentrated
enough. They are helpful in determining the
type of substitution if no interfering absorption
is present, such as that of C=O.
1941, 1870, 1801, 1604 cm-1
(monosubstituted shown here)
Region I
aryl C−H stretch 3100-3050 cm-1
APPENDIX 4: IR ABSORPTION FREQUENCIES
A-13
Characteristic IR Absorptions of Carboxylic Acid Derivatives
TABLE 5: IR Absorptions of Carboxylic Acid Derivatives
Functional Group
ketone
Frequency
C=O 1710 cm−1
acid
C=O 1710 cm−1
O−H 2500−3500 cm−1
C=O 1735 cm−1
ester
amide
acid chloride
acid anhydride
nitrile
C=O 1640−1680 cm−1
N−H 3200−3500 cm−1
C=O 1800 cm−1
C=O 1800 and 1750 cm−1
C≡N 2200 cm−1
Comments
lower if conjugated,
higher if strained
lower if conjugated
broad, on top of C−H stretch
lower if conjugated,
higher if strained
two peaks for R−CO−NH2
one peak for R−CO−NHR'
very high frequency
two peaks
just above 2200 cm−1
(continued on the next page)