Practical infrared
The graph shows the regions of the spectrum where the following types of bonds
normally absorb.
Alkanes
Alkanes are build of -CH3 and -CH2- groups. Their IR spectrum displays only
C-C and C-H bond vibrations. The most useful are the C-H bands. The C-H
stretching vibrations originating from CH3 and -CH2- groups are split into four
peaks and appear around 2800 - 3000 cm-1. The order of wavenumber in this
broad band is as follows
methyl(asymm. str.), methyl(symm. str.), methylene (asymm. str.),
methylene(symm. str.).
Asymmetric vibrations appear always at higher energy than its symmetric
counterpart. Since most organic molecules have C-H bonds, most organic
molecules will display those bands in their spectrum.
1
Alkenes
Besides the presence of C-H bonds, alkenes also show sharp, medium bands corresponding to
the C=C bond stretching vibration at about 1600-1700 cm-1.
Some alkenes might also show a band for the =C-H bond stretch, appearing around 3080 cm-1
as shown below. However, this band could be obscured by the broader bands appearing
around 3000 cm-1
Alkines
The most prominent band in alkynes corresponds to the carbon-carbon triple
bond. It shows as a sharp, weak band at about 2100 cm-1. The reason it’s weak is
because the triple bond is not very polar.
2
Triple bond and symmetry
Let’s compare spectra 1-octyne and 4-octyne. At 2119 cm-1 the peak of C≡C
bond disappears in the case of 4-octyne, because of symmetric position of the
group.
Analysing the frequency dependence on force constant

1
k

2 
when μ can be regarded as constant as it is in the C-C, C=C, C≡C series.
Bond, type of vibration
C-C
C=C
C≡C
Range of absorption / cm-1
1200 - 1000
In molecule
Peak at / cm-1
1600-1700
1-hexene
1-octine
1642
2119
2100 - 2250
Nitriles
In a manner very similar to alkynes, nitriles show a prominent band around 2250
cm-1 caused by the CN triple bond. This band has a sharp, pointed shape just like
the alkyne C-C triple bond, but because the CN triple bond is more polar, this
band is stronger than in alkynes.
3
Aldehydes and ketones, C=O functional group
Aldehydes and ketones show a strong, stake-shaped band around
1710 - 1720 cm-1. This band is due to the highly polar C=O bond. Because
aldehydes also contain a C-H bond to the sp2 carbon of the C=O bond, they also
show a pair of medium strength bands positioned about 2700 and 2800 cm-1.
These bands are missing in the spectrum of a ketone because the sp2carbon of
the ketone lacks the C-H bond.
Carboxylic acid
A carboxylic acid functional group combines the features of alcohols and
ketones because it has both the O-H bond and the C=O bond. Therefore
carboxylic acids show a very strong and broad band covering a wide range
between 2800 and 3500 cm-1 for the O-H stretch. At the same time they also
show the stake-shaped band in the middle of the spectrum around 1710 cm-1
corresponding to the C=O stretch.
4
Amines
The most characteristic band in amines is due to the N-H bond stretch, and it
appears as a
weak to medium, somewhat broad band (but not as broad as the O-H band of
alcohols). This
band is positioned at the left end of the spectrum, in the range of about 3200 3600 cm-1.
Primary amines have two N-H bonds, therefore they typically show two spikes
that make this band. Secondary amines have only one N-H bond, which makes
them show only one spike. Finally, tertiary amines have no N-H bonds, and
therefore this band is absent from the IR spectrum altogether.
Amide functional group
The amide functional group combines the features of amines and ketones
because it has both the N-H bond and the C=O bond. Therefore amides show a
very strong, somewhat broad band at the left end of the spectrum, in the range
between 3100 and 3500 cm-1 for the N-H stretch. At the same time they also
show the stake-shaped band in the middle of the spectrum around 1710 cm-1 for
the C=O stretch. As with amines, primary amides show two spikes, whereas
secondary amides show only one spike.
5
6
Store
Transfer from Lecture: Vibrational transitions
Fig. 6. The IR spectrum of laboratory air. Double-beam mode produces a
straight line, while the single beam is referenced by N2.
Hexane
C-H bend
CH3 CH2 CH2 CH2 CH2 CH3
C-H stretch
WWU Chemistry
Fig. 7. Vapour phase spectrum of hexan.
7
The number of normal modes in hexan, using 3N-6 rule, is 20. A simpler
explanation for bands appear is the so called group frequencies. The molecule
consists of CH2 and CH3 groups.
-1
Band position / cm
2962
2929
2864
1462
1381
Group frequencies of hexan.
Chemical group
Vibrational mode
CH3
asym. str.
CH2
asym. str.
CH3, CH2
sym. str.
CH3, CH2
asym. bend.
CH3, CH2
sym. bend.
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Cyclohexane
C-H
bend
C-H stretch
WWU Chemistry
Fig. 8. Vapour phase spectrum of cyclohexan.
The molecule consists of six CH2 groups.
Group frequencies of cyclohexan.
Band position / cm
Chemical group
Vibrational mode
2930
CH2
asym. str.
2855
CH2
sym. str.
1452
CH2
asym. bend.
-1
9
1-Hexene
C-H “oop”
bending
C=C stretch
=C-H stretch
C-H
bend
CH2 CH CH2 CH2 CH2 CH3
C-H stretch
WWU Chemistry
Fig. 9. Vapour phase spectrum of 1-hexene.
-1
Band position / cm
3082
2963
2931
2865
1644
1464
Group frequencies of 1-hexene.
Chemical group
Vibrational mode
=C-H
asym. str.
CH3
asym. str.
CH2
asym. str.
CH3, CH2
sym. str.
C=C
str.
C-H
bend
10