INTERPRETATION OF IR SCANS
PARAFFINS (ALIPHATIC ALKANES)
The paraffins are saturated hydrocarbons and thus contain only C-C and C-H single bonds.
IR radiation induces stretching and bending vibrations of these units.
 The C-C bending occurs at very low frequency, ~ 500 cm-1, so it does not appear in the scan.
 The C-C stretching bands are weak and appear over a broad range, i.e., 1200 to 800 cm-1, and are
generally of low utility (of little value for identification).
 A variety of C-H stretching and bending bands appear and they are of high utility.
1. CH at 2960-2850 cm-1 (2 sets of split peaks, just below 3000 cm-1)
2. CH at 1460, 1380, and 720 cm-1
Group
Band (cm-1)
Intensity ()
Assignments
Remarks
2960
s
70
as CH3
2870
m
30
s CH3
1460
m
15
as CH3
1380
s
15
(s CH3) umbrella
2925
s
75
as CH2
doublet with 2960 as CH3 stretch
2850
s
45
s CH2
doublet with 2870 s CH3 stretch
1470
m
8
(s CH2) scissor
doublet with 1460 as CH3 bend
725-720
s
3
(CH2)n rock
1170
s
15
 C-C (skeletal)
1385& 1370
s
15
gem-dimethyl
1255
s
15
 C-C (skeletal)
t-butyl group
1210
s
15
 C-C (skeletal)
t-butyl group
CH3
1390& 1375
s
CH3
1195
s
1215
s
1380
s
-CH3
-CH2-
CH3
CH-
doublet in gem-dimethyls & t-butyls
present when n  4. Higher when n < 4,
i.e., propyl 740-730, ethyl 790-770
usually a doublet
doublet
CH3
CH3
CH3 - C -
R - C -R
CH3
t-butyl
doublet
15
skeletal
quaternary C
15
skeletal
quaternary C
15
(s CH3) umbrella
Cycloparaffins: absorption bands are much the same as acyclic paraffins provided they are
unstrained, e.g., cyclohexane and cyclopentane. Ring strain in cyclopropane raises CH stretching
bands from the 2960-2850 cm-1 region to the 3100-2900 cm-1 region.
Halogenated Paraffins: The C-H stretch is raised above 3000 cm-1 due to the presence of
electronegative halogens, e.g., CHCl3 absorbs at 3010 cm-1. The following C-X stretches are also
observed ...
 An aliphatic C-Cl band occurs between 850&550 cm-1. With several Cl substitutions, the
band occurs at the high frequency end of the range and its intensity also increases.
 A C-Br band appears @ 690-515 cm-1, C-I @ 600-500 cm-1, CH2X @ 1300-1150 cm-1
 Several strong C-F bands cover the wide range of 1400-730 cm-1
1
OLEFINS (ALKENES)
In addition to the paraffin bands, unsaturated organics show C-H stretching bands above 3000 cm-1.
They are rarely hidden by the strong main bands below 3000 cm-1. The C=C double bond absorbs
sharply in the 1650 cm-1 region & C-H out-of-plane bending bands appear between 1000-650 cm-1,
the exact position depending upon the substitution pattern on the double bond.
Band (cm-1)
Group
Intensity ()
Assignments
Remarks
3080
m
30
as CH2
CH > 3000 cm-1 indicates alkene or aromatic.
Seen in terminal alkenes
2975
m
-
s CH2
Overlaps with alkane absorption (not seen)
=CH-R
3020
m
-
 CH
Seen in some internal alkenes
terminal vinyl
1650
s
30
 C=C
vinylidene
1670
s
30
Shifts to 1670 cm-1 in cis, trans, tri- and tetrasubstituted alkenes. Trans is very weak.
C=CH2
990
s
50
910
s
110
890
s
125
 CH(out-of-plane
wag)
730-675
m
40
 CH(out-ofplane)
965
s
100
 CH(out-ofplane)
840-800
s
40
 CH(out-ofplane)
--
-
--
 CH(out-ofplane)
=CH2
R
H
R
R
terminal vinyl
C=CH2
terminal vinylidene
R
H
R
H
R
R
R
R
C=C
cis-
R
H
H
C=C
trans-
C=C
tri-
C=C
tetra-
R
R
H
R
R
 CH(out-of-plane
wag)
absent due to symmetry (IR inactive)
Conjugated Systems: Olefinic bond stretching vibrations in conjugated olefins, without a center of
symmetry, interact to produce two C=C stretching bands. For example...
 1,3-pentadiene (CH2=CH-CH=CH-CH3) shows C-C stretching at 1650 cm-1 and 1600 cm-1.
 Symmetrical 1,3-butadiene (CH2=CH-CH=CH2) shows only 1 band near 1600 cm-1.
2
AROMATICS
The presence of an aromatic group is shown by the 3030, 1600, and 1500 cm-1 bands as well as
aromatic overtones and combination band patterns between 2000 and 1700 cm-1. C-H out-of-plane
bending band(s) below 900 cm-1 indicates substitution patterns.
Group
Band (cm-1)
aromatic
Intensity ()
Assignments
3030,
several
m
<60
 CH and
combination
2000-1700
several
w
ca. 5
combinations and
 CH overtones
1600, (1580)
s
s
< 100
< 100
phenyl nucleus
m
>100
CH out-of-plane
bending
1500, (1450)
<900
several
Remarks
Sometimes appear as a weak shoulder on the
main  CH aliphatic band.
Group of 2 to 6 bands characteristic of the
substitution pattern
Variable intensity, 1500 usually stronger than
1600. 1580 band appears when phenyl is
conjugated with unsaturated groups or atoms
with lone electron pairs.
1450 peak overlaps with alkane CH band.
Identified by comparison with scans of knowns.
Vary with substitution patterns of aromatic.
Electron-withdrawing groups (e.g., nitro)
increase  about 30 cm-1.
Polynuclear Aromatics: have IR spectra, which are very similar to mononuclear aromatics.
XY GROUPS
-1
The 2200 cm region:
 Absorptions in this region originate from the XY type groups, i.e., acetylenic (-CC-) and nitrile
(-CN). They are weak but distinctive since no other bands appear in this region.
 CH stretching band frequencies are significantly raised in alkynes.
Group
Band (cm-1)
Intensity ()
Assignments
Remarks
acetylenic

-CC-H
~3300
s
100
 CH
terminal alkynes.
-CC-
~2120
s
5
 CC
terminal alkynes.
~2220
s
1
 CC
internal alkynes- but is completely absent when
symmetry is high
-CC-H
600-700
m
100
 CH
nitrile
-CN
~2240
s
10-150
 CN
aliphatic, e.g., acetonitrile (2250)
aromatic, e.g., benzonitrile (2230)
It is difficult to distinguish between internal alkynes and nitriles. Neither have  C-H at 3300 cm-1
and both have show absorption at ca. 2200 cm-1. However,  -CN at 2200 cm-1 is much more
intense than  -CC because the C-to-N bond is more polar than the C-to-C bond.
3
ALCOHOLS AND PHENOLS
The characteristic bands are the O-H stretch, C-O stretch, and O-H bend modes. They are all
sensitive to hydrogen bonding.
 Free O-H stretch (dilute solns. in nonpolar solvent) produce sharp, strong absorption at 3600 cm-1.
The band grows wide and moves to a lower frequency (3300 cm-1) with strong H-bonding, e.g.,
neat liquid samples.
 Strong C-O stretch occurs at 1260-1000 cm-1.
 O-H bending is seen at 1420-1050 cm-1 and below 800 cm-1.
Water of hydration or water contamination in samples produces a broad band in the same region as
H-bonded O-H stretch, i.e., ~3300 cm-1.
Phenols: will show the OH and CO bands as well as aromatic bands.
Group
Free O-H,
1
2º
3º
phenolic O-H
Band (cm-1)
Intensity ()
Assignments
Remarks
3640
3630
3620
3610
s
s
s
s
70
55
45
~50
3400-3200
w
~50
 OH
In neat liquids
-C-O-H
1260-1000
m
~ 60
 C-O
Variable position reduces utility of these strong
bands.
O-H
1420-1050
m
--
 OH in-plane
770-650
w
--
 OH out-of-plane
Variable position and possible overlap with
CH in alkanes reduces utility of these bands.
In liquid alcohols and phenols
all H-bonded
O-H
including
phenols
 OH
In dilute, non-polar solvents such as CCl4 and
CHCl3.
ETHERS
The mass of the oxygen atom and the strength of C-O bonds do not differ much from those of the C
atom and C-C bonds, and hence the positions of ether absorptions (1150-1085 cm-1) are not
characteristic - they lie in the same region as the C-C stretching bands (1200-800 cm-1). C-O
stretching bands are stronger because their dipole moment change is greater than for C-C bonds.
Identification of ether groups from IR spectra is not easy because organic compounds frequently
contain other C-O bonds, e.g., alcohols, esters, acids. In summary, a peak at ~ 1100 cm-1 or ~1200
cm-1 only infers the presence of a C-O bond or a =C-O bond, respectively. Band intensities are
generally large, i.e., > 200.
Group
Band (cm-1)
-C-O-C
1150-1085
m
---
as C-O-C
Aliphatic C-O-C
Common with other C-O bands.
=C-O-C
1275-1200
m
---
as =C-O-C
Aromatic and vinyl ethers stretch.
1075-1020
m
---
s =C-O-C
Weaker than the 1250 cm-1 band.
Intensity ()
Assignments
4
Remarks
THE CARBONYL GROUP COMPOUNDS
Ketones, aldehydes, carboxylic acids, esters, acid halides, acid anhydrides, amides and lactams all
show a strong C=O stretching absorption band in the 1870-1540 cm-1 region. Its relatively constant
position, high intensity ( = 300-1500), and freedom from interfering bands makes this one of the
easiest bands to recognize in the IR spectra.
Ketones: For neat saturated aliphatic ketones, the normal stretching vibration band occurs at 1715
cm-1. For example, acetone and cyclohexanone absorb at 1715 cm-1. Replacement of an alkyl group
at the -C of a saturated ketone by a heteroatom shifts the carbonyl absorption.
 Inductively withdrawing heteroatoms, i.e., the halogens, increase the C=O force constant and thus
increase the carbonyl absorption frequency.
 Resonance donating heteroatoms, i.e., -NH2 and -SR, have the opposite effect.
X
X
C
+
C
O
O
R
R
INDUCTIVE
ELECTRON
DENSITY
WITHDRAWAL
RESONANCE
ELECTRON
DENSITY
DONATION
Conjugation with a C=C bond results in delocalization of the  electrons of both unsaturated groups
and this reduces the double bond character causing a shift to a lower frequency.
O
C
1685-1665 cm-1
C
C
C
O
C
C
 C=O (cm-1)
Group
Inductive Withdrawal Groups
Br
1760
Cl
1815-1785
F
1870
OH
1760
OR
1735
Resonance Donating Group
NH2
1695-1650
SR
1720-1690
conjugate (, -) unsaturation
1685-1665
aromatic ring
1690
Acid Halides: The spectra of acid halides are very similar to ketones with the exception that the
carbonyl band of an acid halide occurs at a higher frequencies than that of a ketone. Conjugate
unsaturation with acid halides lowers the carbonyl band frequency. For example, benzoyl chloride
absorbs at 1770 cm-1 compared to 1800 cm-1 for most aliphatic acid chlorides.
5
Aldehydes: The C=O stretch of normal (aliphatic) aldehydes occurs at 1740-1720 cm-1. As in
ketones, electronegative substituents on the -C increase the frequency of carbonyl absorption. For
example, acetaldehyde absorbs at 1730 cm-1 and trichloroacetaldehyde at 1770 cm-1.
The distinguishing feature between an aldehyde and a ketone is that the C-H stretch of aldehydes
occurs at 2820-2695 cm-1. In most aldehydes, this C-H band is split. A sharp band of medium
intensity near 2720 cm-1 accompanied by a carbonyl absorption is good evidence for the presence of
an aldehyde group.
Carboxylic Acids: In liquid or solid state, and even in nonpolar solutions at concentrations above
0.01 M, carboxylic acids exist as dimers due to strong hydrogen bonding.
O
R
H
O
C
C
O
H
R
O
DIMERIZED CARBOXYLIC ACID
Dimerized carboxylic acids exhibit carbonyl bands at 1710 cm-1, very close to the normal ketone
band, but much more intense. The OH in carboxylic acids is easily recognized. It is very broad and
lowered from its alcoholic position to 3300-2500 cm-1.
As expected, carboxylic acids show both C-O stretching and in-plane O-H bending bands in the
1400-1200 cm-1 region & broad, medium to low intensity, out-of-plane O-H bending near 920 cm-1.
Esters: Esters show strong characteristic absorption bands for C=O and C-O stretches.
 C=O for aliphatic esters at 1750-1735 cm-1
 C=O for aromatic esters at 1730-1715 cm-1
 C-O for all esters between 1300-1000 cm-1
Esters are easily distinguished from carboxylic acids since they lack the broad  O-H band.
The IR spectra of esters look similar to those of ketones and acid halides since all have absorption
bands in the 1200 cm-1 region, however ester C-O stretches are larger than the C-C stretches of
ketones and acid halides. Esters are difficult to distinguish from ketones. Esters have two C-O
stretches (1300 - 1050 cm-1) in addition to the C-C stretch (1200 - 800 cm-1) of ketones and esters.
Acid Anhydrides: Acid anhydrides have 2 carbonyl groups joined by an O atom. Their IR spectra
are unique and easy to recognize as 2 carbonyl peaks are seen. They usually appear as one split band
with their tips about 60 cm-1 apart.
Saturated acyclic anhydrides absorb near 1820 and 1760 cm-1 (e.g., propionic anhydride)
Conjugated acyclic anhydrides absorb near 1775 and 1720 cm-1.
Saturated cyclic anhydrides with 5-membered rings absorb at 1865 and 1780 cm-1 (at higher
frequencies due to ring strain). (e.g., succinic anhydride)
Phthalic anhydride, a conjugated aromatic anhydride absorbs at 1785 and 1760 cm-1.
As expected for acid anhydrides, strong  C-O-C bands are seen in the 1300-1050 cm-1 region.
6
Amides:
 All amides show a carbonyl absorption band known as the amide I band. Its exact position varies
(~ 1670 cm-1) but as previously stated, is lower than the normal ketone carbonyl band at 1715
cm-1 due to resonance with the unpaired electrons on N.
 1º and 2º amides and some lactams, display another, weaker band in the 1650-1515 cm-1 region
due to  NH. This second band, called the amide II band, is usually under the envelope of the
amide I band and, on rare occasions, is completely overlapped by the amide I band and therefore
not distinguishable. 3º amides do not display this band since they have no N-H bonds.
 1º and 2º amides (and amines) display a broad, medium-to-weak, out-of-plane N-H rocking band
in the 800-670 cm-1 region, reminiscent of the O-H rocking band in alcohols at 770-650 cm-1.
 1º and 2º amides (and amines) show N-H stretching vibrations in the 3500-3300 cm-1 region, i.e.,
within the region of O-H stretching. N-H stretching bands however are narrower and less intense
than O-H bands so they are readily recognized.
1º amides (R-CONH2) and 1º amines usually show 2 bands here for as NH2 and s NH2.
2º amides (R-CONHR’) and 2º amines show only 1 band here, corresponding to  N-H.
3º amides (R-CONR’2) and 3º amines have no N-H stretching bands.
 All amides and amines also show medium-intensity C-N stretching bands over the broad range of
1400-1050 cm-1. These are difficult to distinguish but help to confirm the presence of N in a
compound when interpreted with other data.
Amines: The N-H stretching and bending bands and C-N stretching bands in amines are basically
the same as in amides. They are listed again along with details specific to amines.
In the 3500-3300 cm-1region, 1º amines (R-NH2) show 2  N-H bands, 2º amines (R2-NH)
show 1 band and 3º amines (R3-N) have no  N-H bands.
In plane  N-H (scissoring) occurs at 1650-1500 cm-1. (Equivalent to Amide II band of
1 and 2 amides). The band is strongest in 1º amines, small and difficult to locate in 2º
amines, and does not occur in 3º amines.
Out-of-plane  N-H (rocking) occurs at 910-670 cm-1. The band is strongest in 1º amines,
and does not occur in 3º amines.
Medium to weak  C-N occurs at 1250-1020 cm-1in aliphatic amines. Oddly, the frequencies
are increased to 1340-1265 cm-1 in aromatic amines.
Amine Salts:
Ammonium ion (NH4+ Cl - ) displays strong, broad absorption at 3300-3030 cm-1.
Salts of 1º amines (R-NH3+ Cl - ) display strong, broad absorption at 3000-2800 cm-1 with
multiple combination bands of medium intensity occurring at 2800-2200 cm-1.
Salts of 2º amines (R2-NH2+ Cl - ) display strong, broad absorption at 3000-2700 cm-1 with
multiple combination bands extending to 2270 cm-1.
Salts of 3º amines (R3-NH+ Cl - ) display strong, broad absorption at 2700-2250 cm-1.
Quaternary ammonium salts (R4-N+ Cl - ) can have no N-H stretching vibrations.
As expected, the 1600-1400 cm-1 region contains  N-H bands for salt of 1º, 2º, and 3º amines.
7
Group
Band (cm-1)
Intensity ()
m
Ketone
>300
Assignments
Remarks
 C=O
-CO-
1715
 C=O
aliphatic, e.g., acetone
, -unsatd.
1685
 C=O
e.g., 3-buten-2-one (methyl vinyl ketone)
Ar-CO-
1690
 C=O
e.g., acetophenone (methyl phenyl ketone)
7-memb. ring
1705
 C=O
e.g., cycloheptanone
6-memb. ring
1715
 C=O
e.g., cyclohexanone
5-memb. ring
1750
 C=O
e.g., cyclopentanone
4-memb. ring
1785
 C=O
e.g., cyclobutanone
-C-C-
1200-1000
m
~70
 C-CO
aliphatic, e.g., acetone
-C-C-
1300-1100
m
~70
 C-CO
aromatic, e.g., acetophenone
m
>300
 C=O
acid halide
R-CO-Cl
1810
 C=O
aliphatic acid chloride, e.g., acetyl chloride
Ar-CO-Cl
1770
 C=O
aromatic acid chloride, e.g., benzoyl chloride
R-CO-Br
1760
 C=O
aliphatic acid bromide, e.g., acetyl bromide
R-CO-I
1725
 C=O
aliphatic acid iodide, e.g., acetyl iodide
R-CO-F
1870
 C=O
aliphatic acid fluoride, e.g., acetyl fluoride
-C-C-
1300-1000
aldehyde
m
~70
 C-CO
m
~70
 C=O
R-CHO
1730
 C=O
aliphatic aldehyde, e.g., acetaldehyde
Ar-CHO
1705
 C=O
aromatic aldehyde, e.g., benzaldehyde
, -unsatd.
1705
 C=O
e.g., 2-propenal
-CHO
2820 (2720)
s
<50
 C-H
may be one or two bands
-C-CHO
1300-1000
m
~70
 C-CO
m
1000
 C=O
intense,  up to 1500
acid (dimers)
R-COOH
1710
 C=O
aliphatic, e.g., acetic acid (1760 in monomer)
, -unsatd.
1690
 C=O
conjugate unsatd. aliphatic, e.g., propenoic acid
Ar-COOH
1690
 C=O
aromatic acid, e.g., benzoic acid
-O-H
3300-2500
w
~70
 O-H
very characteristic
-CO-OH
1420
m
50
 C-O
-CO-O-H
1300-1200
m
50
 CO-OH
-O-H
920
m
<50
 O-H
out-of-plane bend
m
700
 C=O
intensity between ketone and acid (500-1000)
ester
in-plane bend
R-CO-O-R
1735
 C=O
e.g., ethyl acetate
Ar-CO-O-R
1720
 C=O
e.g., ethyl benzoate
Ar-CO-O-Ar
1735
 C=O
e.g., phenyl benzoate
R-CO-O-Ar
1775
 C=O
e.g., phenyl acetate & vinyl acetate (, -unsat.)
-C-O
1300& 1050
 C-O
occasionally appears as 1 split peak
m
>700
8
Band (cm-1)
Group
Intensity ()
m
acid anhydride
>300
Assignments
Remarks
 C=O
usually appears as a split band
-OC-O-CO-
1820& 1760
 C=O
aliphatic, e.g., acetic anhydride
, -unsatd.
1775& 1720
 C=O
e.g., 3-buten-2-one (methyl vinyl ketone)
5-memb. ring
1865& 1780
 C=O
e.g., succinic anhydride (ring strain)
-OC-O-CO-
1300-1050
 C-O
1 to 2 strong bands in all anhydrides
amide
amide I (II)
1º R-CO-NH2
1690 (1600)
1000 (300)
 C=O ( N-H)
free aliphatic, e.g., acetamide
"
1650 (1630)
1000 (300)
 C=O ( N-H)
associated aliphatic, e.g., acetamide
1º Ar-CO-NH2
1675 (1600)
1000 (300)
 C=O ( N-H)
free aromatic, e.g., benzamide
1º R-CO-NH2
3400& 3200
m
<30
as & s N-H
"
800-670
m
<30
 N-H (rocking)
single band
2º R-CO-NRH
1680 (1580)
1000 (300)
 C=O ( N-H)
free aliphatic, e.g., N-methylacetamide
"
1655 (1550)
1000 (300)
 C=O ( N-H)
associated aliphatic, e.g., N-methylacetamide
"
3350
m
<30
 N-H
"
800-670
m
<30
 N-H (rocking)
3º R-CO-NR2
1650 (none)
1000 (300)
 C=O
free aliphatic, e.g., N,N-dimethylacetamide
"
1650 (none)
1000 (300)
 C=O
assoc. aliphatic, e.g., N,N-dimethylacetamide
1º, 2º, 3º -CO-N
1400-1050
m
<50
1º R-NH2
3500
m
<30
as
N-H
"
3400
m
<30
s
N-H
"
1650-1580
m
<50
 N-H in-plane
"
910-670
m
50
 N-H
out-of-plane bend, stronger than in 2º amines
2º R2-NH
~3330
m
<30
 N-H
shoulder on low  side may look like 2rd band
"
1580-1500
m
<15
 N-H in-plane
"
910-670
m
50
 N-H
out-of-plane bend
1º, 2º, & 3º
1250-1020
m
<70
 C-N
increasing intensity from 1º 2º 3º
Cl-
3300-3030
w
>100
 N-H
R-NH3+ Cl-
3000-2800
w
>100
 N-H
also multiple combination bands at 2800-2200
R2-NH2+ Cl-
3000-2700
w
>100
 N-H
also multiple combination bands down to 2270
2700-2250
w
>100
 N-H
m
>300
 C-N (amide III)
usually 2 narrow bands & possible shoulder
1 band and possible shoulder to the right
single band
increasing intensity from 1º 2º 3º
amine
shoulder on low  side may look like 3rd band
(scissoring) stronger than in 2º amines
weak in 2º amines & buried in aromatic amines
amine salts
NH4+
+
-
R3-NH Cl
+
-
R4-N Cl
none
salts of 1º, 2º, 3º
1600-1400
no N-H bonds
m
<100
 N-H
9
NITRO GROUP
Nitro groups produce two N-O stretches, a C-N stretch and a C-NO2 bend. The CNO bend occurs at
610 cm-1 and so is not seen on the usual IR scan but shifts aromatic  CH oop higher and destroys the
reliability of the  CH oop for identifying substitution patterns of aromatics.
Group
Band (cm-1)
Intensity ()
N-O
1550& 1370
m
as N-O& s N-O
aliphatic
1530& 1350
m
as N-O& s N-O
conjugated
C-N
870
w
 C-N
C-NO2
610
Assignments
 C-NO2
Remarks
not seen but shifts CH oop higher
Nitro groups are difficult to identify especially in aromatic compounds where phenyl nucleus
stretches occur at 1600 cm-1 and -CH3 groups absorb at 1380 cm-1. The two N-O stretches are of
similar intensity and broader than the phenyl and methyl group bands just mentioned.
10
Interpretation of IR Spectra
For relatively simple molecules much information can be gained about its functional groups from
spectral correlations. To analyze an IR spectra, 1) pay most attention to the strongest absorptions, 2)
pay more attention to peaks to the left (shorter ) of 1250 cm-1, and 3) pay as much attention to the
absence of certain peaks as to the presence of others. The absence of characteristic peaks will
definitely exclude certain functional groups. Be wary of O-H peaks because water is a common
contaminant in many samples, especially when scanned in KBr disks.
A stepwise analysis:
1. Is there a strong C=O stretching peak between 1820 to 1625 cm-1? If not, carbonyl groups are
absent, so go to step 2. If yes, continue with a), b), c), etc.
a)
Is there also a strong, broad O-H peak between 3500 and 2500 cm-1? If so, the compound
is a carboxylic acid. If not then:
b)
Is there a medium-to-weak N-H stretching band between 3520 to 3070 cm-1? If there are
2 peaks, then it is a primary amide. If there is only 1 peak, then it is a secondary amide.
Tertiary amides do not absorb in this region since they do not have N-bonded hydrogens.
Thus, if there is no absorption band, then 1 and 2 amides are eliminated and then:
c)
Are there 2 strong peaks, 1 in the region 1870 to 1800 cm-1 and the other in the region
1800 to 1740 cm-1 ? If so, an acid anhydride is present. If not:
d)
Is there a weak-to-medium, narrow peak in the region of 2720 cm-1 ? If so, is the carbonyl
peak in the region 1715 to 1680 cm-1 ? If so, it is a conjugated aldehyde; if not, it is an
isolated aldehyde. However, if there is not a peak near 2720 cm-1:
e)
Does the strong carbonyl peak fall in the range 1815 to 1770 cm-1 and the molecule give a
positive Beilstein test? If so, it is an acid halide. If not:
f)
Does the strong carbonyl peak fall in the range 1690 to 1675 cm-1? If so, if so it is a
conjugated ketone. If not:
g)
Does the strong carbonyl peak fall in the range 1670 to 1630 cm-1 ? If so, it is a tertiary
amide. If not:
h)
Does the molecule have a strong, wide peak in the range 1310 to 1100 cm-1 ? If so, does
the carbonyl peak fall in the range 1730 to 1715 cm-1 ? If so, it is a conjugated ester; if
not, the ester is not conjugated. If there is not a strong, wide peak in the range 1310 to
1100 cm-1, then:
i)
The molecule is an ordinary, nonconjugated ketone.
11
2. If the molecule lacks a carbonyl peak in the range 1820 to 1625 cm-1, does it have a broad O-H
band in the region 3650 to 3200 cm-1 ? If so, does it also have a secondary alcohol peak about
1200 cm-1, a C-H stretching peak to the left of 3000 cm-1, and a phenyl group peak in the region
1600 to 1470 cm-1 ?. If so, it is a phenol. If it does not meet these latter 3 criteria, it is an
alcohol. However, if there is no broad band in the region 3650 to 3200 cm-1, then it is not an
alcohol, so:
a) Is there a moderately broad band in the region 3500 to 3300 cm-1, does the molecule smell like an
amine, or does it contain nitrogen? If so, are there 2 peaks in this region? If so, it is a primary
amine: if not, it is a secondary amine. However, if there is no broad band in the region 3500 to
3300 cm-1, then:
b) Is there a sharp peak of medium-to-weak intensity at 2260 to 2100 cm-1 ? If so, is there also a
CC-H stretching peak at 3320 to 3310 cm-1 ? If so, the molecule is a terminal acetylene. If not,
then the molecule is most likely a nitrile, although it may be a substituted acetylene. If there is no
sharp peak of medium-to-weak intensity at 2260 to 2100 cm-1 , then:
c) Are there strong peaks in the region 1600 to 1540 cm-1 and 1380 to 1300 cm-1?
If so, then the molecule contains a nitro group. If not:
d) Is there a strong peak in the region 1270 to 1060 cm-1 ? If so, the molecule is an ether. If not:
e) The molecule is either a tertiary amine (check odor?), a halogenated hydrocarbon (Beilstein test?),
or just an ordinary hydrocarbon.
Some Additional Clues:
 Dilute solutions of alcohols will show a sharp peak at about 3600 cm-1 for non-hydrogen
bonded O-H in addition to the usual broad hydrogen-bonded O-H peak.
 Aromatic hydrogens give C-H stretching peaks just to the left of 3000 cm-1, while aliphatic
hydrogens appear just to the right of 3000 cm-1.
 The carbonyl wavenumbers listed above refer to the open chain or unstrained functional
group in a nonconjugated system. If the carbonyl group is conjugated with a double bond
or an aromatic ring, the peak will be displaced to the right by 30 cm-1. When the carbonyl
is in a ring smaller than six members or if there is oxygen substitution on the carbon
adjacent to an aldehyde or ketone carbonyl, the peak will be moved to the left.
 Methyl groups often give a weak-to-medium peak near 1375 cm-1
 Phenyl groups give strong, sharp C=C peaks at about 1600 cm-1 and 1500 cm-1
 Alicyclic alkenes also show medium, sharp C=C-H stretches at about 1650 cm-1.
12
INTERPRETATION OF IR SCANS – HOMEWORK ASSSIGNMENT
For each of the attached IR scans, complete the following …
1. Write assignments for all important absorption bands. Do as many assignments as possible
for each scan. Report the frequency (cm-1), absorbing group (e.g., OH), and cause (e.g., ).
For example, on the first scan we see a strong OH band at 3300 cm-1. Writing the
frequencies helps you remember them.
2. State whether the compound is aliphatic or aromatic. If it is aromatic give its substitution
pattern, i.e., o-, m-, p-, etc.
3. Give the functional group(s) present, e.g., a compound containing carbonyl and hydroxyl
bands is identified as a carboxylic acid.
4. List all types of atoms present, e.g., contains C, H, N, O, Cl, etc.
5. Use the molecular weight along with the other information you have gathered to calculate
the empirical formula, e.g., compound # 1 has empirical formula C3H6O. Be sure that the
degree of unsaturation (D.U.) of the proposed empirical formula is consistent with the
functional groups present. For example, one aromatic ring requires a D.U. = 4
6. Draw a possible structure for the compound and give its name.
Example: Determination for IR scan # 1:
 strong  OH at 3300 cm-1 (but no carbonyl band near 1700 cm-1 so it must be an alcohol)
 looks like strong  C-O and  O-H at 1050 and 1000 cm-1 (consistent with presence of hydroxyl
group). The band at 950 cm-1 cannot be identified with any certainty, however, note that terminal
vinyl groups absorb at both 910 & 990 cm-1. This may account for the band at 1000 cm-1.
 small, sharp  =C-H at 3050 cm-1 and sharp  C=C at 1650 cm-1is definite evidence of C=C group
but absence of phenyl bands at 1600 and 1580 cm-1 as well as lack of a distinct overtone pattern at
2000-1700 cm-1 rules out aromatic. So it is an aliphatic alkenol (ene + ol).
 The molecular weight (58.08 g/mol) rules out the presence of odd numbers of Cl (atomic mass =
35.5) and odd numbers of N atoms (Odd numbers of N (& P) give odd-number molecular weights)
 The atoms likely present are C, H, and O.
 Using this data and a molecular weight of 58, the only possible empirical formula is C3H6O and
this has a D.U. = 1, which is consistent with an aliphatic alkenol.
 Three 3-C aliphatic enol structures can be drawn, i.e., 1-propen-1-ol, 1-propen-2-ol and
2-propen-1-ol. Draw these structures. The first two structures are incorrect for two reasons.
Firstly, both contain a –CH3 group but the scan shows no CH3 umbrella at 1380 cm-1. Secondly,
these 2 structures are unstable and tautomerize to propanal and propanone, respectively. The third
structure, CH2=CH-CH2OH, i.e., 2-propen-1-ol is the only possible answer.
 In the final semester, we will study several other analytical techniques which will enable us
identify unknowns with greater specificity. Mass Spectrometry will give us molecular weights and
identify the presence of specific atoms and groups of atoms. Nuclear Magnetic Resonance will
give more information on assembly and structure of molecules.
13