This document provides information about infrared (IR) spectroscopy and analyzing IR spectra of different functional groups. It discusses: 1. The conditions required for IR absorption and the division of the IR spectrum into the functional group and fingerprint regions. 2. The characteristic IR absorptions of common functional groups like alkanes, alkenes, alkynes, alcohols, phenols, ethers, aldehydes, ketones, carboxylic acids, esters, amides, amines, and aromatics. Specific examples and their spectra are provided. 3. Factors that affect IR frequencies, such as bond strength, mass of atoms, resonance, conjugation, and hydrogen bonding.

1. Infrared spectrum/Infrared frequency
Ravish Yadav
Hydrocarbons

2. CONDITION FOR
ABSORPTION OF IR by
BONDS
Molecular vibrations
must produce a
change in dipole
moment of bond
The frequency of IR
radiation = Frequency
of bond vibration
CONDTION FOR A MOLECULAR VIBRATION (BOND) TO
ABSORB IR RADIATION

3. REGION OF AN IR SPECTRUM
The IR spectrum is divided into 2 regions
1) Functional group region
2) Finger print region

4. HYDROGEN REGION
1. O-H
2. N-H
i) Primary
ii) Secondary
3. C-H
i) Sp
ii) Sp2
iii) sp3
TRIPLE
BOND
REGION
1. C≡C
2. C≡N
DOUBLE BOND
REGION
1. -C=C
2. C=O
3. C=N
SINGLE BOND
REGION,
BENDING
1. C-C
2. C-O
3. C-N
4. C-X
FUNCTIONAL GROUP REGION
FINGERPRINT
REGION
3650c
m
2500
cm
2300
cm
2100
cm
1900
cm
1650c
m
1500
cm
667
cm

5. Factors affecting IR frequency
Probable frequency/ wave no. at which particular
bond absorbs can be calculated with hook’s law:
frequency is directly proportional to bond strength and
inversely proportional to masses of atoms

6. Other factors
1) Fermi resonance
2) Electronic effects (related to chemical arrangement of atoms in
molecule )
i. Positive inductive effect- vibrational frequency decreases
ii. Negative inductive effect- vibrational frequency increases
iii. Resonance and conjugation- vibrational frequency decreases
iv. Field effect- as field increases vibrational frequency decreases
v. Bond angle- as bond angle increases vibrational frequency decreases
vi. Hydrogen bonding – as strength of OH decreases, vibrational
frequency decreases.

8. HYDROCARBONS
1.ALKANES:
•The spectra of simple alkanes are characterized by
absorptions due to C–H stretching and bending
•Note the strong bands in the 3000-2850 cm-1 region due to C-
H stretch.
•Example : octane
8

10. 2.ALKENE :
•Alkenes are compounds that have a carbon-carbon double
bond, –C=C–. The stretching vibration of the C=C bond usually
gives rise to a moderate band in the region 1680-1640 cm-1.
•The IR spectrum of 1-octene is shown below. Note the band
greater than 3000 cm-1 for the =C–H stretch and the several
bands lower than 3000 cm-1 for –C–H stretch (alkanes). The C=C
stretch band is at 1644 cm-1.
•Example: 1- octene
10

12. 3.ALKYNES:
•Alkynes are compounds that have a carbon-carbon triple bond
(–C≡C–). The –C≡C– stretch appears as a weak band from 2260-
2100 cm-1.
•The spectrum of 1-hexyne, a terminal alkyne, is shown below.
Note the C–H stretch of the C–H bond adjacent to the carbon-
carbon triple bond (3324), the carbon-carbon triple bond stretch
(2126).
•Example : 1- hexyne
12

14. AROMATIC HYDROCARBON
• The =C–H stretch in aromatics is observed at 3100-3000 cm-1.
• Aromatic hydrocarbons show absorptions in the regions 1600-
1585 cm-1 and 1500-1400 cm-1 due to carbon-carbon stretching
vibrations in the aromatic ring.
• The pattern of overtone bands in the region 2000-1665 cm-
1 reflect the substitution pattern on the ring.

16. CARBONYL CARBON COMPOUNDS
1)Carboxylic acid:
Carboxylic acid can be easily detected by IR spectroscopy .It
mainly considered two units like C=O and O-H.
• C=O shows very strong bands that appears in the range of
1730-1700cm-1 for simple aliphatic carboxylic acids.
 O-H stretching occurs at broad band in the range of 3000-
2500cm-1
Example : hexanoic acid , benzoic acid
16

17. Hexanoic acid
O-H 3300-2500cm-1
C-H 2950-2850cm-1
C=O 1721cm-1

18. Benzoic acid
O-H occurs at 3400-2400cm-1( broad peak)
C=O Occurs at 1730-1700 cm-1

19. 2) ESTER
Esters show a very strong band for the C=O group that
appears in the range of 1750-1735cm-1 for simple aliphatic
esters. The C=O band is shifted to lower frequency when it is
conjugated to C=C or phenyl group.
Ex:, ethyl benzoate, ethyl acetate.
19

21. C-H 2980-3000 cm-1
C=O 1752 cm-1
C-O 1250 cm-1

22. 3)ANHYDRIDES
EX: Propionic anhydride.
22
2980cm-1
1820cm-1
1748cm-1
1035cm-1

23. 4) ALDEHYDES
The carbonyl stretch C=O of saturated aliphatic aldehydes appears
from 1740-1720 cm-1.
If the carbons adjacent to the aldehyde group are unsaturated, this
vibration is shifted to lower wavenumbers, 1710-1685 cm-1
H–C=O stretch 2830-2695 cm-1
The spectra of benzaldehyde and butyraldehyde are shown. Note that
the O=C stretch of the alpha, beta-unsaturated compound --
benzaldehyde -- is at a lower wavenumber than that of the saturated
butyraldehyde.
Note the O=C–H stretches in both aldehydes in the region 2830-2695
cm-1, especially the shoulder peak at 2725 cm-1 in butyraldehyde and
2745 cm-1 in benzaldehyde.

26. 5) KETONES
The carbonyl stretching vibration band C=O of saturated
aliphatic ketones appears at 1715 cm-1. Conjugation of
the carbonyl group with carbon-carbon double bonds or
phenyl groups, shifts this band to lower wavenumbers,
1685-1666 cm-1.
The spectrum of 2-butanone is shown below. This is a
saturated ketone, and the C=O band appears at 1715.
Note the C–H stretches (around 2991) of alkyl groups.

29. 7) AMIDES
The carbonyl stretching vibration band C=O of saturated aliphatic
amides appear in between 1650 to 1690 cm-1.
EXAMPLE- PROPANAMIDE

31. 5.ALCOHOL AND PHENOL:
Alcohol and phenol show strong and broad hydrogen bonded O-
H stretching bands centering between 3400cm-1 and 3300cm-1.
In solution it will also be possible to observe a free O-H
stretching band at 3600cm-1.
Example: ethanol , p-cresol
3

33. O-H =3500-3300 cm-1
C-O = 1200 cm-1

36. 6.ETHERS:
Ether is derivative of alcohol and show characteristics c-o-c bonds .
Ether show prominent C-O stretching band at 1300 to 1000cm-1.
Absence of C=O and O-H is required to ensure that C-O stretch is
not due to an ester and phenol .phenyl alkyl ether gives two strong
bands at 1250cm-1 and 1040cm-1.
Example: di butylether, anisole
3

37. 2960
1462
1372 1124

38. AMINES
Amines show characteristic
N-H stretching in the hydrogen region.
Occurring between 3300-3500 cm-1.
C-N stretching in the single bond region.
Occurring between 1350-1000cm-1

39. AMINES: N-H stretch: 3300- 3500cm-1
occurs to the left of C-H absorptions.
Primary amines:3500- 3400cm-1
Less intensity of absorption
Primary amines display two small peaks.
Bands represent the asymmetric and symmetric
vibrations.
Secondary amines:3350-3310cm-1
Very less intensity
Consist of only a single peak small peak
Tertiary amine:
Peaks are non existent.

40. Aromatic amines:
High intensity of absorption.
Single & long peak.
C-N stretch:
Occurs at 1350-1000cm-1, varying intensity.
To the right of C-H bonds.
1250-1000cm-1 for aliphatic,
Primary amine: 2peaks with very less intensity.
Secondary amine: single, peak with high intensity.

41. Tertiary amine: single peak with comparatively less
intensity.
1350-1250cm-1 for aromatics.
Single , sharp peak with higher intensity
The higher frequency in aromatics is from resonance
increasing double-bond character between the ring and
attached nitrogen.

42. AMINES: primary amine e.g. Butyl amine

43. Secondary amine : egg: dibutyl amine

44. Tertiary amine : egg: Tributylamine

45. Aromatic amine: egg: N-methyl aniline