This presentation provides information regarding the identification of organic compounds by IR spectroscopy

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2. IMPORTANCE OF
IR SPECTROSCOPY IN
STRUCTURAL ELUCIDATION OF
ORGANIC COMPOUNDS
Submitted by
ANBU DINESH.J
M.PHARM (SEMESTER-1)
DEPARTMENT OF PHARMACEUTICAL CHEMISTRY
COLLEGE OF PHARMACY
SRI RAMAKRISHNA INSTITUTE OF PARAMEDICAL SCIENCES
COIMBATORE 2

3. CONTENTS
• INTRODUCTION
• CLASSIFICATION OF IR REGION
• TYPES OF VIBRATIONS
• HOOKE’S LAW
• REQUIREMENT OF A MOLECULE
• REGIONS OF IR SPECTRA
• FUNCTIONAL GROUPS AND IR TABLE
• INTERPRETATION OF IR SPECTRA
• DETERMINATION OF IR SPECTRA OF DRUGS
• CONCLUSION
• REFERENCE 3

4. INTRODUCTION
• Infrared radiation was discovered by Sir William Herschel in 1800 .
• Herschel was investigating the energy levels associated with the wavelengths of
light in the visible spectrum.
• Spectroscopy is the study of interaction of electromagnetic waves (EM) with matter
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5. CLASSIFICATION OF IR REGION
•0.7 µm to 2.5 µm
•14000-4000 cm-1
NEAR
INFRARED
SPECTRUM
•2.5 µm to 25 µm
•4000-400 cm-1
MID INFRARED
SPECTRUM
•25 µm to 300 µm
•400-10 cm-1
FAR INFRARED
SPECTRUM
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6. Study of overtones and
harmonic or combination
vibrations.
Fundamental vibrations
and the rotation-
vibration structure of
small molecules.
Low heavy atom
vibrations (metal-ligand
or the lattice vibrations).
NEAR IR
MID IR
FAR IR
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7. TYPES OF VIBRATIONS
ii) BENDING VIBRATION
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i) STRETCHING VIBRATION
1)SYMMETRICAL
STRETCHING
2)ASYMMETRICAL
STRETCHING
1) IN-PLANE BENDING
2) OUT-PLANE BENDING
In plane bending
Out plane bending
Scissoring Rocking Wagging Twisting

8. STRETCHING VIBRATION
• In this type of vibrations, the bond length is increased or decreased at regular intervals.
• There are two types of stretching vibrations.
1) Symmetrical stretching and
2) Asymmetrical stretching.
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1)Symmetrical stretching- Bond length increase or decrease symmetrically.
2)Asymmetrical stretching- Length of one bond increases and the other one
decreases.

9. BENDING VIBRATION
• A change in bond angle occurs between bonds with a common atom, or there is
a movement of a group of atoms with respect to the remainder of the molecule
without movement of the atoms in the group with respect to one another.
• The bending vibrations are also called as deformation vibrations. Deformation
vibrations are of two types.
• a) In-plane Bending vibrations
• b) Out-plane Bending vibrations
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10. INPLANE BENDING
• Bond angle changes occurs.
• This type of bending takes place within the same plane.
• In plane bending are of two types.
i. Scissoring - in which bond angle decrease
ii. Rocking - in which the bond angle is maintained but both bonds moves within the same plane.
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11. OUTPLANE BENDING
• This type of bending takes plane outside of the molecule.
i.Wagging -in which both atoms move to one side of the plane
ii.Twisting in which one atom is above the plane and the other is below the plane.
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12. HOOKE’S LAW
• According to Hooke’s Law , The Stretching frequency is related to the masses
of the atom and the force constant(a measure of resistance of a bond to
stretching) of a bond.
The stretching vibrational frequency of a bond can be calculated by treating the two
atoms and their connecting bond.
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13. REQUIREMENT OF A MOLECULE
In order to absorb the electromagnetic radiation for a molecule the frequency of the incident
radiation matches the natural frequency of the vibration, the IR photon is absorbed and the
amplitude of the vibration increases.
The dipole moment of the molecule must change as a result of a molecular vibration.
The change in the dipole moment allows interaction with the alternating electrical component
of the IR radiation wave.
Symmetric molecules (or bonds) do not absorb IR radiation since there is no dipole moment.
If the molecule vibrate asymmetrically, the change in its dipole moment takes place so
absorption spectra of this molecule can be obtained.
This is called Active IR spectra.
If the dipole moment of a molecule would not change i.e.as in Symmetric stretching the
absorption spectra of radiation cannot be obtained.
Such spectra is called as Forbidden or Inactive IR Spectra.
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14. IR SPECTRUM
• There are two type of IR Spectra from which we can obtained the information about the
quality of molecule .
1. The Functional Group region:
Identifies the functional group with the consequence of changing stretching
vibrations.
2. The Fingerprint region:
Identifies the exact molecule with the consequence of changing bending vibrations.
.
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Ranges from 4000 to 1600 cm-1
Ranges from 1600 to 625cm-1

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16. INTERPRETATION OF IR SPECTRA
• Structural information of a compound is derived from the presence or absence of
characteristics absorption bands of various functional groups in the IR Spectrum of the
compound.
• The band position of all the major structural bonding types have been determined in a tabular
form.
• Characteristic absorption position of some of the more important common functional groups
are presented in given table.
• This table is particularly useful for correlation when the spectrum of an unknown compound
has been obtained.
• It is always more useful to make direct comparison with the spectra of closely related
compound
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Bond Compound Type Frequency range, cm-1
C-H
Alkanes
2960-2850(s) stretch
1470-1350(v) scissoring and
bending
C-H Alkenes
3080-3020(m) stretch
1000-675(s) bend
C-H
Aromatic Rings 3100-3000(m) stretch
Phenyl Ring Substitution Bands 870-675(s) bend
Phenyl Ring Substitution Overtones
2000-1600(w) - fingerprint
region
C-H Alkynes
3333-3267(s) stretch
700-610(b) bend

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C=C Alkenes 1680-1640(m,w)) stretch
C≡C Alkynes 2260-2100(w,sh) stretch
C=C Aromatic rings 1600, 1500(w) stretch
C-O Alcohols, ethers, caboxylic acids, esters 1260-1000(s) stretch
C=O Aldehydes, ketones, caboxylic acids, esters 1760-1670(s) stretch
O-H
Monomeric -- Alcohols, Phenols 3640-3160(s,br) stretch
Hydrogen-bonded --Alcohols, Phenols 3600-3200(b) stretch
caboxylic acids 3000-2500(b) stretch
N-H Amines
3500-3300(m) stretch
1650-1580 (m) bend
C-N Amines 1340-1020(m) stretch
C≡N Nitriles 2260-2220(v) stretch
NO2 Nitro Compounds
1660-1500(s) asymmetrical stretch
1390-1260(s) symmetrical stretch

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INTERPRETATION OF IR SPECTRA

20. IR SPECTRUM OF ALKANES
• Alkanes have no functional groups.
• Their IR spectrum displays only C-C and C-H bond vibrations. Of these the most
useful are the C-H bands, which appear around 3000 cm-1.
• Since most organic molecules have such bonds, most organic molecules will display those
bands in their spectrum.
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21. IR SPECTRUM OF ALKENES
• IR spectrum of Alkenes shows that the band appearing around 3080 cm-1 can be
obscured by the broader bands appearing around 3000 cm-1
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22. IR SPECTRUM OF ALKYNES
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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. This can be an important diagnostic tool because very few organic compounds
show an absorption in this region.

23. IR SPECTRUM OF AN ALCOHOL
• The most prominent band in alcohols is due to the O-H bond ,and it appears as a strong,
broad band covering the range of about 3000 - 3700 cm-1.
• The sheer size and broad shape of the band dominate the IR spectrum and make
it hard to miss.
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24. IR SPECTRUM OF CARBOXYLIC ACIDS
• Carboxylic acids show a strong, wide band for the O–H stretch.
• Unlike the O–H stretch band observed in alcohols, the carboxylic acid O–H stretch
appears as a very broad band in the region 3300-2500 cm-1, centered at about
3000 cm-1.
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•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

25. IR SPECTRUM OF ALDEHYDES
• If a compound is suspected to be an aldehyde, a peak always appears around 2720 cm-1
which often appears as a shoulder-type peak just to the right of the alkyl C–H stretches.
• H–C=O stretch 2830-2695 cm-1
• C=O stretch aliphatic aldehydes 1740-1720 cm-1
• alpha, beta-unsaturated aldehydes 1710-1685cm1
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26. IR SPECTRUM OF KETONES
• The carbonyl stretching vibration band C=O of saturated aliphatic ketones appears:
• C=O stretch aliphatic ketones 1715 cm-1
• Alpha, beta-unsaturated ketones 1685-1666 cm-1
• Figure shows the spectrum of 2-butanone.
• This is a saturated ketone, and the C=O band appears at 1715.
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27. IR SPECTRA OF 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 resemble a molar tooth.
• Secondary amines have only one N-H bond, which makes them show only one
spike, resembling a canine tooth.
• Tertiary amines have no N-H bonds, and therefore this band is absent from
the IR spectrum all together. The spectrum below shows a secondary amine.
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28. IR SPECTRA OF AMINES
• The N–H stretches of amines are in the region 3300-3000 cm-1.
• These bands are weaker and sharper than those of the alcohol O–H stretches
which appear in the same region.
28The spectrum of aniline is shown above. This primary amine shows two N–H stretches (3442, 3360); note the
shoulder band, which is an overtone of the N–H bending vibration. The C–N stretch appears at 1281 rather
than at lower wavenumbers because aniline is an aromatic compound. Also note the N–H bend at 1619.

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• Secondary amines (R2NH) show only a single weak band in the 3300-3000 cm-1 region, since
they have only one N–H bond.
.
The spectrum of diethylamine is below. Note that this secondary amine shows only one N–
H stretch (3288). The C–N stretch is at 1143, in the range for non-aromatic amines (1250-
1020). Diethylamine also shows an N–H wag (733).

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Tertiary amines (R3N) do not show any band in this region since they do not have an N–H bond
Triethylamine is a tertiary amine and does not have an N–H stretch,
nor an N–H wag. The C–N stretch is at 1214 cm-1 (non-aromatic).

31. DETERMINATION OF IR SPECTRUM OF
DRUGS
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32. ASPIRIN
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33. ACETOPHENONE
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34. PARACETAMOL
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35. CONCLUSION
• Entire IR region is divided into two region. such as functional group region and
fingerprint region .
• The function group region is 4000-1500cm-1
• Range of group frequency occur in the region fingerprint region is above 1500cm-1
mainly used to identify the functional group present in the compound.
• The IR spectroscopy is the qualitative tool widely useful in pharmaceutical, chemical
industry to identify the functional group.
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36. REFERENCE
• 1) Principles of Instrumental Analysis by Skoog pg no: 430-480
• 2) The systematic Identification of organic compunds by Shriner pg no194-200
• 3) Figures of drugs are obtained from INDIAN PHARMACOPOEIA IP 2007 volume-1
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