Che 440/540 Infrared (IR) Spectroscopy
Some Major IR Absorption Bands
3500-3300 cm-1
N–H stretch
Amines
3500-3200 cm-1
O–H stretch
alcohols, a broad,
strong band
3100-3000 cm-1
C–H stretch
Alkenes
3000-2850 cm-1
C–H stretch
Alkanes
1760-1665 cm-1
C=O stretch
ketones, aldehydes,
esters
1680-1640 cm-1
C=C stretch
alkenes
How to Analyze an IR Spectrum
1.
Look first for the carbonyl C=O band. Look for a strong band at 1820-1660 cm-1.
This band is usually the most intense absorption band in a spectrum. It will have a medium width. If
you see the carbonyl band, look for other bands associated with functional groups that contain the
carbonyl by going to step 2. If no C=O band is present, go to step 3 and check for alcohols.
2. If a C=O is present determine if it is part of an acid, an ester, aldehyde, ketone or anhydride.
anhydrides show two C=O absorptions near 1810 and 1760 cm-1
ACID
Look for indications that an O-H is also present. It has a broad absorption near
3300-2500 cm-1. This actually will overlap the C-H stretch. There will also be a C-O
single bond band near 1100-1300 cm-1. Look for the carbonyl band near 17251700 cm-1.
ESTER
Look for C-O absorption of medium intensity near 1300-1000 cm-1. There will be no
O-H band.
ALDEHYDE
Look for aldehyde type C-H absorption bands. These are two weak absorptions to
the right of the C-H stretch near 2850 cm-1 and 2750 cm-1 and are caused by the
C-H bond that is part of the CHO aldehyde functional group. Look for the carbonyl
band around 1740-1720 cm-1.
KETONE
The weak aldehyde CH absorption bands will be absent. Look for the carbonyl
C=O band around 1725-1705 cm-1
AMIDE
Carbonyl C=O band from 1680-1630. In addition, NH stretch(es) will be present
from 3475-3150 cm-1; primary amides show two bands, secondary amides one
band, and tertiary amides no bands in this region
3. If the C=O band is absent, look for the following:
ALCOHOLS, PHENOLS
Broad absorption near 3400-3300 cm-1.
Confirm by finding C-O stretch near 1300-1000 cm-1
AMINES
Check for N-H stretch. Medium absorption(s)
Near 3400 cm-1.
ETHERS
Check for a C-O stretch near 1300-1000 cm-1 (and an
absence of O-H near 3400 cm-1).
4. If no functionalities in step 3 are present, check for double bonds and/or aromatic
rings.
C=C is a weak absorption near 1650 cm-1.
Aromatic rings show medium to strong absorptions
from 1600-1450 cm-1.
Consult the C-H region to confirm. Aromatic and vinyl
C-H absorptions occur to the left of 3000 cm-1.
Aliphatic C-H occurs to the right of 300 cm-1.
5. Triple bonds
CN shows a medium, sharp absorption near 2250 cm-1.
CC shows a weak, sharp absorption near 2150 cm-1.
6. Nitro groups
Shows two strong absorptions at 1600-1530 cm-1 and
1390-1300 cm-1.
7. Hydrocarbons
This may be indicated if none of the other functional groups
are present
Major absorptions are found to the right of 3000 cm-1
to C-H.
Hydrocarbons show very simple spectra. The only other
absorptions appear near 1460 and 1375 cm-1.
C–H stretch from 3000–2850 cm-1
C–H bend or scissoring from 1470-1450 cm-1
C–H rock, methyl from 1370-1350 cm-1
C–H rock, methyl, seen only in long chain alkanes, from 725-720 cm-1
C=C stretch from 1680-1640 cm-1
=C–H stretch from 3100-3000 cm-1
=C–H bend from 1000-650 cm-1
–C≡C– stretch from 2260-2100 cm-1
–C≡C–H: C–H stretch from 3330-3270 cm-1
–C≡C–H: C–H bend from 700-610 cm-1
C–H wag (-CH2X) from 1300-1150 cm-1
C–X stretches (general) from 850-515 cm-1
C–Cl stretch 850-550 cm-1
C–Br stretch 690-515 cm-1
C–H stretch from 3100-3000 cm-1
overtones, weak, from 2000-1665 cm-1
C–C stretch (in-ring) from 1600-1585 cm-1
C–C stretch (in-ring) from 1500-1400 cm-1
C–H "oop" from 900-675 cm-1
O–H stretch, hydrogen bonded 3500-3200 cm-1
C–O stretch 1260-1050 cm-1 (s)
C=O stretch:
aliphatic ketones 1715 cm-1
α, β-unsaturated ketones 1685-1666 cm-1
H–C=O stretch 2830-2695 cm-1
C=O stretch:
aliphatic aldehydes 1740-1720 cm-1
alpha, beta-unsaturated aldehydes 1710-1685 cm-1
(aromatic)
O
H3C
O
O
CH3
Asymmetric C=O
stretch
Symmetric C=O
stretch
O–H stretch from 3300-2500 cm-1
C=O stretch from 1760-1690 cm-1
C–O stretch from 1320-1210 cm-1
O–H bend from 1440-1395 and 950-910 cm-1
C=O stretch
aliphatic from 1750-1735 cm-1
α, β-unsaturated from 1730-1715 cm-1
C–O stretch from 1300-1000 cm-1
N–H stretch 3400-3250 cm-1
1° amine: two bands from 3400-3300 and 3330-3250 cm-1
2° amine: one band from 3350-3310 cm-1
3° amine: no bands in this region
N–H bend (primary amines only) from 1650-1580 cm-1
C–N stretch (aromatic amines) from 1335-1250 cm-1
C–N stretch (aliphatic amines) from 1250–1020 cm-1
N–H wag (primary and secondary amines only) from 910-665 cm-1
N–O asymmetric stretch from 1550-1475 cm-1
N–O symmetric stretch from 1360-1290 cm-1
Effect of Conjugation on C=O absorptions
Conjugation moves the C=O absorption to a smaller wave number.
Non-conjugated
Compound
Frequency
cm-1
Conjugated
Compound
Frequency
cm-1
butanal
1725
2-butenal
1691
benzaldehyde
1702
methyl vinyl ketone
1700 (s-cis)
1681
acetophenone
1685
2-butanone
1717
Frequency
cm-1
(s-trans)
propanoic acid
1715
propenoic acid
1702
benzoic acid
1688
ethyl propionate
1740
ethyl acrylate
1727
ethyl benzoate
1718
butanoic anhydride
1819,
1750
2-butenoic
anhydride
1782,
1722
benzoic
anhydride
1786,
1726
Match the following spectra with the structures shown below.
Support your answers with specific approximate frequencies.
OH
O
O
O
OH
H
OH
O
O
O
OH
H
OH
O
O
O
OH
H
OH
O
O
O
OH
H
OH
O
O
O
OH
H
aliphatic C-H
stretch
arom C-H
stretch
arom C=C
O-H stretch
C-O
stretch
Propose a structure for a compound with molecular formula C7H12O,
whose IR spectrum appears below.
Propose a structure for a compound with molecular formula C6H12O,
with the following IR spectrum:
Propose a structure for a compound with molecular formula C6H13N,
whose IR spectrum appears below.
Alkenes (out-of-plane C-H bending)
Alkene
Frequency (cm-1)
RCH=CH2
(mono-substituted)
R2C=CH2
(gem-di-substituted)
985-1000
905-920
880-900
RCH=CHR
(trans-di-substituted)
RCH=CHR
(cis-di-substituted)
960-975
675-730
993 cm-1
912 cm-1
890 cm-1
965 cm-1
(trans)
698 cm-1
(cis)
Limitations of IR in Identifying Alkenes
What would be a better
way to identify these?
Combination and
overtone bands
Benzenoid Aromatics
C-H out-of-plane
bending
C=C out-of-plane
ring bending
monosubstituted: strong absorption near 690 cm-1
ortho-disubstituted: strong absorption near 750 cm-1
meta-disubstituted: the 690 cm-1 band, one near 780 cm-1, and often one near 880 cm-1
para-disubstituted: strong absorption in the region 800-850 cm-1
CH2CH3
monosubstitution
oop
aromatic
C=C stretch
CH3
CH3
ortho-disubstitution
aromatic
C=C stretch
oop
CH3
CH3
meta-disubstitution
aromatic
C=C stretch
oop
CH3
CH3
para-disubstitution
aromatic
C=C stretch
oop
Propose a structure for an aromatic compound with the molecular
formula C9H12 which gives the following IR spectrum. Justify your answer.
Propose a structure for an aromatic compound with the molecular
formula C10H14 which gives the following IR spectrum. Justify your answer.
O
C-O stretch
O
NH2
overlap
N-H bend
C=O stretch
NH2 stretch
(primary amide)
C-N stretch
O
CH3
N
H
NH stretch
(secondary
amide)
Overtone of 1540 band
(3080 cm-1)
C=O
stretch
N-H
Bend
(1540 cm-1)
O
N
CH3
CH3
Notice that there
is no N-H stretch
in the 3350-3180 cm-1
region in a tertiary amide.
C-N
stretch
C=O
stretch
Propose a structure for a compound with the molecular formula C8H14O2 which gives
the following IR spectrum. Justify your answer.
Propose a structure for a compound with the molecular formula C8H14O2 whose IR
spectrum appears below. Justify your answer.
Propose a structure for a compound with the molecular formula C8H9NO which
gives the following IR spectrum. Justify your answer.
Propose a structure for a compound with the molecular formula C10H12O2 whose
IR spectrum appears below. Justify your answer.
An unknown compound has the mass spectrum shown below. The IR spectrum
shows peaks in the 3100-3030 and the 2979-2879 cm-1 ranges and a strong
absorption at 1688 cm-1. Suggest a structure consistent with this data.
An unknown compound has the mass spectrum shown below. The IR
spectrum shows peaks in the 2963-2861 cm-1 range and a strong
absorption at 1718 cm-1. Suggest a structure consistent with this data.