Ir by d i brahmbhatt

1. INFRARED SPECTROSCOPY
Dr D. I. Brahmbhatt
Professor of Organic Chemistry
Department of chemistry
Sardar Patel University
Vallabh Vidyanagar – 388 120
Gujarat
Email: drdibrahmbhatt@gmail.com
Mob: 9427547665

2. Introduction
 Infrared spectroscopy is one of the important
spectroscopies for the assignment of the structure of an
unknown compound.
 All the compounds having covalent bond, whether
organic or inorganic, absorb various frequencies of
electromagnetic radiation in the infrared region.
 The absorption of infrared radiations can be expressed
either in terms of wavelength (λ) or in wave number
(v).
 In terms of wavelength (λ) the IR region lies between
2.5 μm to 25 μm ( 1 μm = 10⁻⁶ cm , 1nm = 10⁻⁹cm ).

3. 14000 cm¯¹ 200 cm¯¹
600 cm¯¹
4000 cm¯¹
Near IR IR Far IR
 The relationship between wavelength and wave number
is as follows:
1
Wavelength (cm)
Wave number v =
Thus it has unit cm¯¹.

4.  Mostly infrared spectra of organic compounds are
plotted as percentage transmittance against wave
number (4000-600 cm¯¹).
100%
0%
% T
Wave number
4000 600

5. Transmittance = radiant power transmitted by the sample/ radiant power incident on the sample
Absorbance (A) = log10 (1/T)

6. Molecular Vibrations:
 When infrared light is passed through the sample, the
vibrational and rotational energies of molecules are
increased.
 There are two types of molecular vibrations:
1) Stretching vibrations.
2) Bending vibrations.
 1) Stretching vibrations: There is increase or decrease in
bond length between two atoms, called stretching
process and vibrations arise due to this process is called
stretching vibration.

7.  Stretching vibrations are of two types:
a) Symmetrical stretching.
b) Asymmetrical stretching.
B
A A
B B
B
Symmetrical stretching Asymmetrical stretching

8.  Bending vibrations: Change in bond angle with respect to
common atom.
 Bending vibrations are of four types:
a) Scissoring bending
b) Rocking bending
c) Wagging bending
d) Twisting bending

9. Scissoring bending
Bonds come close to each other or
away to each other simultaneously
there by change in angle.
Rocking bending
Motion of both bonds in same
direction.
Wagging bending
With respect to central atom both
atoms move behind or above the
plane simultaneously.
Twisting bending
With respect to central atom, one
atom moves behind plane and
another moves above plane.

10. Non linear and linear molecules:
 Non linear molecules: 3n-6 Fundamental vibrations
 Linear molecules: 3n-5 Fundamental vibrations
 Example:
1) CO₂ :
Linear molecule.
therefore , 3n-5 = 3(3)-5 = 4. Fundamental vibrations

11. Requirement for the IR active transitions:
 Only those vibrations that result in change in dipole
moment of molecule are absorbed in IR.
Symmetrical stretching in CO₂
does not generate dipole moment.
Therefore IR inactive.
Asymmetrical stretching in CO₂
generate dipole moment.
Therefore IR active.
Gives band at 2350 cm¯¹.
two more bending vibrations, scissoring bending, is doubly degenerated
appear as peak at 666 cm¯¹ ( due to equal energy).

12. (+) (-)
(-)
Scissoring motions
Doubly degenerated at 666 cm¯¹
How to calculate frequency associated with
particular vibration?
 C-H stretching appears at around ~ 2900-3000 cm¯¹.
x Y

13. ν =
ƒ
MX MY / (MX+MY)
Where, ν = Vibrational frequency (cm ¯¹)
c = Velocity of light (cm/sec)
ƒ= Force constant of bond (dyne/cm)
1
2πc
MX and MY = Mass in gram for atom X and Y.
 ƒ = 5Χ10⁵ dyne/cm for single bond, two and three
times double for double and triple bonds
respectively.
MX MY
MX+MY
= µ ( Reduced mass)

14.  Example: C-H stretching frequency:
c = Velocity of light (3 Χ10¹⁰cm/sec)
ƒ = 5 Χ 10¹⁰ dyne/cm (single bond)
MX = 12 amu or 20 Χ10¯²⁴ gm.
MY = 1 amu or 1.6 Χ10¯²⁴ gm.
Calculate frequency will not be the exact value but
close to observed value (experimental value).
MX MY
MX+MY
µ = =
12 Χ 1
12 + 1
= 0.92 amu
= 0.92
6.02 Χ 10²³
gm = 1.53 Χ 10¯²⁴ gm.

15. ν = 1
2 Χ 3.14 Χ 3 Χ10¹⁰
5Χ10⁵
1.53 Χ 10¯²⁴
= 3032 cm¯¹
Characteristic group absorption:
Functional group region (4000-1300 cm-1);
Fingerprint region; and Aromatic region (900-650 cm-1)
(A) Alkanes:
 C- H and C-C bonds are present in alkanes.
 C-C bond:
 C- C bending vibrations are observed below 500
cm¯¹ and therefore we do not observed in IR.

16.  C- C stretching vibrations are weak and are observed
between 800- 1200 cm¯¹and have no much more
importance due to little contribution for identification
of compound.
 C- H bond:
(a) C-H stretching:
 C-H stretching in alkane are observed in the region
2800-3000 cm¯¹.
Group Type of stretching Frequency
CH₃ Symmetric
Asymmetric
2872 cm¯¹
2962 cm¯¹
CH₂
Symmetric
Asymmetric
2852 cm¯¹
2926 cm¯¹

17. (b) C-H bending:
Group Types of bending Frequency
CH₃
In phase Bending (symmetric) 1375 cm¯¹
Out of phase Bending (asymmetric) 1450 cm¯¹
CH₂
Scissoring
Rocking
Twisting and Wagging (weak)
1465cm¯¹
720cm¯¹
1350-1150 cm¯¹
 Cyclic alkanes:
 CH₂ stretching:
CH₂ stretching of unstrained cyclic alkanes have similar
band as saturated aliphatic alkanes.

18.  Increase in strain moves C-H frequency on higher
side
CH₂ and C-H absorb in the region
2900-3100 cm¯¹

 C-H bending vibrations:
CH₂ scissoring decrease in cyclic hydrocarbons
n-Hexane
Cyclohexane
Cyclopropane
1468 cm¯¹
1452 cm¯¹
1442 cm¯¹

19. (B) Alkenes:
 Important vibrations are:
1) C=C stretching.
2) C-H stretching of C=C
H
3) C-H out of plane and in plane vibrations.
 C=C stretching vibration in unconjugated linear alkenes:
 This vibration is observed in the region
1640-1667cm¯¹ (moderate/ weak)
 Mono substituted alkene i.e. vinyl group (CH₂=CH-)
will be observed at 1640 cm¯¹ with moderate intensity.

20.  Disubstituted alkene with trans geometry (w), trisubstituted
alkene and tetrasubstituted alkene give band at
~ 1670 cm¯¹. (Less intensity due to symmetry of
trans structure)
 Disubstituted cis alkene absorb at ~ 1650 cm¯¹.
1754 cm¯¹ 1786 cm¯¹
Abnormally shows high frequency

21.  As the angle strain increases in cycloalkenes, C=C
frequency moves on lower side in IR spectrum.
Cyclobutene
Cyclopropene
1566 cm¯¹
1641 cm¯¹
 Cycloalkenes:
 In unstrained cycloalkene the value is same as it is
for cis isomer in acyclic system i.e. 1650 cm¯¹.

22. 2) Symmetrical diene
at 1600 cm¯¹
 Conjugated alkenes:
 Conjugated dienes : Only unsymmetrical diene gives
two IR bands while, symmetrical diene gives only one
band.
e.g.
1) 1,3-Pentadiene.
at 1650 & 1600 cm¯¹

23. `
 Conjugation of C=C with aromatic ring or with
carbonyl group:
Frequency decreases .
The absorption observed at 1625 cm¯¹
S-cis S-trans
In case of S-cis Absorption is as
strong as carbonyl ,while in case
of S-trans the absorption is
weaker than S-cis.
Frequency is
lowered down
by 30 cm¯¹.

24.  Cummulated double bond:
e.g. Allene.
C=C stretching observed between 1900-2000 cm¯¹

25.  Alkene :C-H stretching vibrations:
 In general C-H stretching bands above 3000 cm¯¹
results from aromatic, heteroatomic, alkyne and
alkene.
 Alkene: C-H bending:
 Out of plane C-H bending at 650-1000 cm¯¹
 Only out of plane bending vibrations are important
and they give very strong band.
 In alkene structure, most reliable bands are those of
vinyl group.
~ 850 cm¯¹ arising from = CH₂ wagging.
~ 1416 cm¯¹ arising from scissoring.

26. (c) Alkynes:
 Two stretching vibrations and one bending vibration
(for acetylene and mono substituted alkynes).
stretching
C-H stretching
C-H bending
 Disubstituted alkynes: (with different substitution):
stretching 2190-2260 cm¯¹

If both substituents are same , this band is not observed.
 Terminal gives strong band than
internal
 The intensity of band increases when it has
conjugation with carbonyl group.

27.  C-H stretching in alkynes:
 C-H stretching band in monosubstituted alkynes
appear in the region 3267-3333 cm¯¹
 This is strong and sharp band than hydrogen bonded
-OH or –NH bands appearing in the same region.
 C-H bending in alkynes:
 Monosubstituted alkynes give a strong band in
region of 610-700 cm¯¹.
 The first overtone of C-H bending appears as a weak
and broad band in region 1220-1370 cm¯¹.

28. 1400 1200 700 600 cm¯¹
1300 cm¯¹ C-H bending in alkyne
at 650 cm¯¹
First overtone appear at double the
frequency of C-H bending in alkyne.

29. ( D) Aromatic hydrocarbons:
 The most prominent and most informative bands
in aromatic compounds occur in low frequency
range 675-900 cm¯¹.
 The strong absorption bands observed in this
region results from out of plane bending of the
ring C-H bond.
 In plane bending vibrations are observed
between 1000-1300 cm¯¹.

30.  Aromatic C-H bending observed in region
675-900 cm¯¹.It gives information about substitution
pattern in aromatic ring.
 Skeletal vibration:
C=C stretching vibration observed in the region
1585-1600 cm¯¹ and 1400-1500 cm¯¹.
 Aromatic C-H stretching :
 It is observed between 3000-3100 cm¯¹.
 In aromatic compounds, weak combination and
Overtone bands are observed in the region
1650-2000 cm¯¹.

31. 3100 3000 2000 1500 900 600 cm¯¹
C-H Stretching
-C=C- Stretching C-H Bending
Fig: Pattern for aromatic compound
 Out of plane C-H bending vibrations :
Substitution No. of bands Frequency
Monosubstituted benzene Two 690-710 cm¯¹
730-770 cm¯¹
o- Disubstituted benzene One 735-770 cm¯¹
m- Disubstituted benzene Two
680-725 cm¯¹
750-810 cm¯¹
p-Disubstituted benzene One 800-860 cm¯¹
1.
2.
3.
4.

32. (E) Absorption in alcohols and phenols:
 Important vibrations are:
O-H stretching and C-O stretching .
These vibrations are sensitive to hydrogen bonding.
 O-H stretching vibrations:
 O-H non bonded or “free” hydroxyl group of
alcohols and phenols absorb strongly in the
region 3584-3700 cm¯¹. These sharp, “free” hydroxyl
bands are observed in the vapour phase or ,in dilute
solution in non-polar solvents or –OH sterically
hindered.

33.  Intramolecular hydrogen bonding increases as the
concentration increases, and additional band start to
appear at lower frequency 3200-3550 cm¯¹.
Cyclohexanol:
Strong and sharp band
at 3623 cm¯¹.
Free –OH group
(0.03 M, low concentration)
3623 cm¯¹
(Free –OH)
3333 cm¯¹
(H-bonded)
As concentration of alcoholic
compound increases ,its peak
position shift to lower wave
number.
( 1M solution in CCl₄)
Here intermolecular H- bonding
occur but concentration is very low.

34. Intramolecular H- bonding occur in o-hydroxy
acetophenone.
3077 cm¯¹
Broad band
(independent of concentration)
 o-hydroxy acetophenone and p-hydroxy acetophenone:
3600 cm¯¹
(in dilute CCl₄ solution)
Here intermolecular H- bonding occur but
concentration is very low. Hence band
look like as it is free –OH group.

35.  C-O stretching vibrations:
 C-O stretching vibrations in alcohols and phenols
are observed between 1000-1260 cm¯¹.
 Alcoholic C-O stretching:
Alcohol type Absorption range
Saturated tertiary
Secondary, highly symmetrical
Saturated secondary
α-Unsaturated or cyclic tertiary
Secondary, α-unsaturated
Secondary, alicyclic five or six membered ring
Saturated primary
Tertiary, highly α-unsaturated
1.
2.
1.
2.
1.
2.
3.
1.
1124-1205 cm¯¹
1087-1124 cm¯¹
1050-1085 cm¯¹
< 1050 cm¯¹

36.  Phenols show C-O stretching at 1220 cm¯¹.
 In 1˚and 2˚alcohols, unsaturation on adjacent carbons
or a cyclic structure lowers the frequency of
C-O absorption.
1100 1070 cm¯¹ 1100 1070 cm¯¹ 1100 1060 cm¯¹
1050 1017 cm¯¹ 1050 1030 cm¯¹
1o
2o

37.  O-H bending vibration:
 The O-H in plane bending vibration occurs in the
region of 1330-1420 cm¯¹.
 The O-H out of plane bending vibration is observed
in the region of 650-769 cm¯¹.

38. (F) Ketones:
 Ketones, aldehydes, carboxylic acids, lactones, esters,
acid chlorides, anhydrides, amides and lactum show
strong –C=O stretching band in region 1540-1870 cm¯¹.
 It is very easy to recognize –C=O stretching band
in IR due to the following reasons:
1) Relatively constant position.
2) High intensity.
3) Free from other interfering bands.
 Saturated aliphatic ketones (neat sample) show a band
at 1715 cm¯¹.This is taken as normal position.

39.  Factors affecting the position of –C=O stretching
vibration:
1) Physical state.
2) Electronic and mass effect of neighbouring
substituents.
G = electronegativity of group or its mass,
affect the position of –C=O peak.
3) Conjugation.
4) Hydrogen bonding (inter and intra molecular).
5) Ring strain.

40.  Change in normal environment of carbonyl group can
either lower or raise the frequency from normal value
1715cm¯¹.
 Compared to neat , the value increase when sample is
dissolved in non-polar solvent.
 Replacement of alkyl group of saturated aliphatic
ketone by heteroatom X, shifts the –C=O absorption.
The shift will depend upon whether the predominent
effect is inductive or resonance.
Inductive effect Resonance

41.  The inductive effect reduces the –C=O bond length,
increases the force constant and hence increases the
frequency.
 Conjugation with -C=C- results in a delocalization of π
electrons of carbonyl group, reduces the double bond
character of the carbonyl and lowers the frequency.
 Conjugation with alkene or phenyl ring shifts the
absorption in lower side and is observed between
1666-1685 cm¯¹.

42. or
 Additional conjugation also reduces the frequency but
effect will be not much more as it is in case of first
conjugation.
It causes slight effect.
 Steric effect reduces the co-planarity, results in reduction
in the conjugation and increase in frequency.

43. e.g. Benzal acetone
S-trans
1674 cm¯¹
S-cis
Steric hinderance
Co-planarity loss
1699 cm¯¹
 Intermolecular H-bonding between ketone and hydroxyl
solvents such as methanol causes a decrease in carbonyl
frequency.
e.g. methyl ethyl ketone 1715 cm¯¹ (neat)
1706 cm¯¹ (10% solution in
methanol)

44.  β- Diketones usually exist as a mixture of tautomeric
keto-enol forms.
The enolic form does not show normal absorption of
conjugated ketone. Here a broad band is observed
between 1580-1640 cm¯¹.
1685- 1666 cm¯¹
keto enol (1580-1640cm¯¹)
β- diketone

45.  Cyclic ketones: Frequency increases as the ring size
decreases.
1715 cm¯¹ 1751 cm¯¹ 1775 cm¯¹

46. (G) Aldehyde:
 Important vibrations in aldehyde:
(1) –C=O stretching
(2) C-H stretching
(3) C-H bending
 -C=O stretching vibration of aldehyde is observed
in slightly higher frequency range than corresponding
methyl ketones.

47.  The region for aliphatic aldehyde is 1720-1740 cm¯¹.
 Factors affecting the position of –C=O stretching:
(1) Electro negative substituents at α-carbon increases
carbonyl frequency.
e.g. acetaldehyde 1730 cm¯¹
trichloroacetaldehyde 1768 cm¯¹
(2) α,β- unsaturated aldehyde and benzaldehyde absorb
at lower frequency 1685-1710 cm¯¹.
(3) Internal H- bonding lowers the frequency
e.g. Salicyldehyde.
1666 cm¯¹

48.  C-H stretching :
 Aldehyde C-H stretching is observed between
2695-2830 cm¯¹.
 Two moderately intensed bands are observed in this
region. These two bands are due to fermi resonance.
2720 cm¯¹
2830 2695 cm¯¹
Two moderately intensed bands

49.  C-H bending at 1390 cm¯¹(fundamental vibration),
double of this at 2780 cm¯¹ , first overtone ,which is
very close to C-H stretching fundamental vibration at
2830 cm¯¹.
 Fermi resonance: When fundamental vibration couples
with overtone is called fermi resonance.
 Overtone: In polyatomic molecules,overtones might
occur at frequencies approximately twice those of
corresponding fundamental vibration.

50.  Fundamental vibration at 2830 cm¯¹ couples with the
overtone of C-H bending at 2780 cm¯¹, results in
appearance of two bands ,one towards higher wave
number and other towards lower wave number side.
Higher side band between 2800-2860 cm¯¹.
Lower side band between 2700-2760 cm¯¹.
C-H bending
1390 cm¯¹
Over tone
2780 cm¯¹
Fundamental
vibration
2830 cm¯¹
Interaction
Fermi resonance
Appearance of
two bands,
one towards
higher wave number
side and other to
lower wave number
side

51. (H) Carboxylic acid:
 O-H stretching vibrations:
 In liquid or solid or in CCl₄ solution more than 0.01 M
concentration, acid exists in dimer form due to strong
H- bonding.
Dimer of acid
 Monomeric form, free –OH stretching observed near
3520 cm¯¹.
 Dimer shows a broad band between 2500-3300 cm¯¹
for –OH stretching.

52. 3300 3000 2500 cm¯¹
Dimer shows a very broad and intense peak in region
2500-3300 cm¯¹ and usually centered at 3000 cm¯¹.

53.  For aliphatic acid, C-H stretching of alkyl part is also
there, which is also superimposed upon broad –OH
band.
The C-H stretching bands are generally superimposed
upon broad –OH band .Such broad band also observed
in β-diketones at 2500-3000 cm¯¹ ,but this band is
less intense.
3000 cm¯¹

54.  Carboxylic acids can bond with ethers like dioxane
and THF, or other solvents which can act as proton
acceptor.
Such –OH shows a band at ~3100 cm¯¹
 -C=O stretching vibration:
 The –C=O stretching for monomer is at ~1760 cm¯¹.
It is more intense than ketone.
 Dimer of carboxylic acid has a center of symmetry,only
-C=O asymmetric stretching band is observed.This band
appear at lower frequency than monomer, due to
H- bonding and resonance.

55.  Dimer of aliphatic carboxylic acid absorb in the region
1706-1720 cm¯¹.
 Intramolecular H-bonding have more effect than inter
molecular H-bonding.
Intramolecular H-bonding
Salicylic acid
Absorb at 1665 cm¯¹.
Intermolecular H-bonding
p-Hydroxy benzoic acid
Absorb at 1680 cm¯¹.

56.  α ,β- unsaturated acid:
 α ,β- unsaturation in acid decreases the –C=O frequency
slightly.
α ,β- unsaturated acid and aryl conjugated acid show
absorption for dimer in the region between
1680-1710 cm¯¹.

X= halogen
-C=O frequency slightly increase
to 10-20 cm¯¹.
 C-O stretching and O-H bending:
Two bands arise from C-O stretching and O-H bending
They appear in the region 1210-1320 cm¯¹ and
1395-1440 cm¯¹.

57. (I) Esters and Lactones:
Ester δ- Lactone
 Two strong bands due to –C=O and C-O stretching.
 The presence of one more oxygen atom in esters
(R-COOR’) compared to ketone (RCOR’) raise the
wave number of absorption due to negative inductive
effect.

58.  The overlapping occurs between esters in which the
carbonyl frequency is lowered and ketone in which the
normal ketone frequency is raised.
when ester carbonyl frequency is reduced (1735 cm¯¹
1725 cm¯¹) and a ketone carbonyl frequency is
increased (1715 cm¯¹ 1725 cm¯¹) ,the overlapping
occurs.
 But ester can be distinguished from this overlapping
region with ketone due to presence of C-O stretching
band between 1000-1300 cm¯¹, which is not observed
for ketone.
 Ester can be distinguished from carboxylic acid due to
the absence of -OH stretching and bending vibrations.

59.  Factors affecting the position of –C=O stretching:
(1) –C=O stretching of saturated aliphatic ester fall in
region 1735-1750 cm¯¹.
(2) Formates, α ,β- unsaturated eaters and benzoates
absorb in the region 1715-1730 cm¯¹.Further
conjugation has little effect.
(3) Unsaturation attached to oxygen atom of ester:
-C=O stretching increased
but C-O stretching decreased

60. Ethyl acetate
Vinyl acetate
Phenyl acetate
-C=O stretching
1742 cm¯¹
1776 cm¯¹
1770 cm¯¹
Replacement of ethyl group by vinyl and phenyl ,
increase –C=O stretching frequency but decrease C-O
stretching frequency.

61. (4) Attachment of an electron withdrawing group to an
α- carbon raise the C=O stretching frequency.
-C=O stretching at
1770 cm¯¹
(5) β- Ketoester : Due to enolization ,-C=O frequency
decreased.
Enol (H-bonding)
at 1650 cm¯¹
Lowers the –C=O stretching

62.  Lactones:
δ- Lactone α ,β- unsaturation unsaturation on oxygen side
1735-1750 cm¯¹ 1720 cm¯¹ 1760 cm¯¹

63.  C-O stretching:
 The C-O stretching vibrations of ester actually consist
of two asymmetrically coupled vibrations:
 These bands occur in the region of 1000-1300 cm¯¹.
 The band of saturated esters, occurs
in the 1163-1210 cm¯¹ region.It is often broader
and stronger than –C=O stretching absorption.

64. (J) Amides:
 Important vibrations are:
(1) –C=O stretching (Amide-I band).
(2) N-H stretching.
(3) N-H bending (Amide-II band).
(4) C-N stretching (very weak band).
 Amide-I band (-C=O stretching)
 Appear at lower frequency than normal ketones.
 Primary amide (except acetamide - 1694 cm¯¹)
amide-I band appears ~1650 cm¯¹ in solid state
while in liquid state at 1690 cm¯¹.

65.  -C=O of tertiary amide is independent of physical state
shows band between 1630-1680 cm¯¹.
 -C=O of secondary amide appear at 1640 cm¯¹ in solid
state , while in dilute solution appear at ~1680 cm¯¹.
 N-H stretching:
 In dilute solution, in non-polar solvent, primary amides
shows two moderately intense bands for asymmetric
symmetric stretching.
~ 3520 cm¯¹ (asymmetric)
~ 3400 cm¯¹ (symmetric)
 In secondary amide, free N-H stretching at
3400-3500 cm¯¹.

66.  N-H bending vibration (amide-II)
 All primary amides show a sharp band in dilute solution
resulting from N-H bending at somewhat lower
frequency then –C=O band. This is called amide-II band.
 The band has intensity ½ or ⅓ of that –C=O
stretching of amide.
 In pellet : 1620-1655 cm¯¹ (normally under envelope
of amide-I band)
In dilute solution: 1590-1620 cm¯¹.(normally separated
from amide-I band)

67. 1620 cm¯¹
amide-II band
1695 cm¯¹
amide-I band

68. (K) Amines:
 Important vibrations in amines:
(1) N-H stretching
(2) N-H bending
(3) C-N stretching
 N-H stretching vibrations:
 Primary amines show two weak absorption bands ,
one at 3300-3400 cm¯¹ region and other at
3250-3300 cm¯¹ region. These bands represent
respectively, the “free” asymmetrical and symmetrical
stretching modes.
 Secondary amines show a single weak band in the
3310-3350 cm¯¹ region. Tertiary amine do not absorb
in this region.

69.  N-H stretching vibrations are sensitive to H- bonding.
 Aromatic primary amines absorb at slightly higher
frequencies .
 N-H bending vibrations:
 The N-H bending(scissoring) vibration of primary
amines is observed in the 1580-1650 cm¯¹ region.
 The N-H bending is seldom detectable in spectra of
aliphatic primary amines.
 Secondary aromatic amines absorb near 1515 cm¯¹.

70.  Liquid samples of primary and secondary amines show
broad absorption in the 666-906 cm¯¹ region ,arising
from N-H wagging.
 C-N stretching vibrations:
 The C-N stretching vibrations in primary, secondary
and tertiary aliphatic amines appear in the region of
1020-1250 cm¯¹.
 Aromatic amines show strong C-N stretching
absorption in the 1266-1342 cm¯¹ region. The
absorption at higher frequencies than the corresponding
aliphatic amines because resonance with aromatic
ring.

71. (L) Nitriles:
 Nitriles show stretching vibrations.
 Aliphatic nitriles absorb near 2240-2260 cm¯¹.
 Electron withdrawing atoms, such as oxygen or
chlorine, attached to carbon atom α to the
reduce the intensity of absorption.
 Aromatic nitriles absorb at 2222-2240 cm¯¹.

72. (M) Nitro:
 The presence of nitro group in compound is characterized
by the presence of two strong bands in its IR spectrum
which arise due to the symmetrical and asymmetrical
stretching modes of -N=O respectively ,
which occur in the region:
(i) 1275-1375 cm¯¹.
(ii) 1535-1620 cm¯¹.
Symmetrical stretching Asymmetrical stretching

73. X = electronegative group
Increases frequency of asymmetrical
NO₂ band and decreases the frequency
Of symmetrical NO₂ band.

 Aromatic nitro groups absorb near the same frequencies
as observed for the conjugated aliphatic nitro compounds.
Symmetric stretching - 1347 cm¯¹
Asymmetric stretching -1523 cm¯¹
Symmetric stretching - 1310 cm¯¹
Asymmetric stretching -1475cm¯¹
Vibration shifted to lower
frequency due to extended
resonance with electron
donating group.

74.  Nitro aromatic compounds show a C-N stretching
vibration near 870 cm¯¹.