Organic Lecture Series
Infrared
Spectroscopy
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Chap 12
Organic Lecture Series
Reactions will often give a mixture of products:
OH
H2SO4
+
Major
Minor
How would the chemist determine which product was formed?
Both are cyclopentenes; they are isomers.
Spectroscopy will provide the solution.
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Organic Lecture Series
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Organic Lecture Series
Electromagnetic Radiation
• Electromagnetic radiation: light and other
forms of radiant energy
• Wavelength (λ): the distance between
consecutive peaks on a wave
• Frequency (ν): the number of full cycles of
a wave that pass a given point in a second
• Hertz (Hz): the unit in which radiation
frequency is reported; s-1 (read “per
second”)
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Organic Lecture Series
Electromagnetic Radiation
• Common units used to express
wavelength
λ
Un it
Meter (m)
Millimeter (mm)
Micrometer (μm)
N anometer (nm)
An gs trom (Å )
Relation
to Meter
---1 mm = 10-3 m
1 μm = 10-6 m
1 nm = 10-9 m
1 Å = 10-10 m
c = λν
E = hν
E is kJ/mol
h= 3.99 X 10-13 kJzszmol-1
ν = frequency in Hz
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Organic Lecture Series
E=hν
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Organic Lecture Series
• Molecular spectroscopy: the study of which
frequencies of electromagnetic radiation are
absorbed or emitted by a particular substance
and the correlation of these frequencies with
details of molecular structure
– three types of molecular spectroscopy:
Region of the
Electromagnetic Type of
Spectroscopy
Spectrum
Radio fequency Nu clear magnetic
resonan ce
Infrared
Infrared
Ultravioletvisible
Ultravioletvisib le
Absorption of
Electromagnetic
Radiation Results
in Transition Between
Nuclear spin states
Vibrational energy levels
Electronic energy levels
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Organic Lecture Series
Infrared Spectroscopy
• The vibrational IR extends from 2.5 x 10-6 m (2.5
μm) to 2.5 x 10-5 m (25 μm)
– the frequency of IR radiation is commonly expressed in
wavenumbers
– Wavenumber ν: the number of waves per centimeter, with
units cm-1 (read reciprocal centimeters)
– expressed in wavenumbers, the vibrational IR extends
from 4000 cm-1 to 400 cm -1
-2
-1
-1
ν = 10 m•cm
= 4000 cm
2.5 x 10-6 m
ν =
10-2 m•cm -1
2.5 x 10-5 m
25 μm to 2.5 μm
-1
= 400 cm
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Organic Lecture Series
Sections of an IR Spectrum
This is the most common scale.
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Organic Lecture Series
• IR spectrum of 3-methyl-2-butanone
Strong
absorption
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Organic Lecture Series
Dispersive IR Spectrometer
(Not exam material)
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Organic Lecture Series
• IR spectrum of 3-methyl-2-butanone
C-H
Stretch
C=O
Stretch
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Organic Lecture Series
Molecular Vibrations
–atoms joined by covalent bonds undergo continual
vibrations relative to each other
–the energies associated with these vibrations are
quantized; within a molecule, only specific
vibrational energy levels are allowed
–the energies associated with transitions between
vibrational energy levels correspond to
frequencies in the infrared region:
4000 to 400 cm-1
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Organic Lecture Series
• For a molecule to absorb IR radiation
– the bond undergoing vibration must be polar and
– its vibration must cause a periodic change in the
bond dipole moment
• Covalent bonds which do not meet these
criteria are said to be IR inactive
– the C-C double and triple bonds of symmetrically
substituted alkenes and alkynes, for example, are
IR inactive because they are not polarized bonds
H3 C
CH3
C C
H3 C
CH3
H3 C- C C- CH3
2,3-Dimethyl-2-butene
2-Butyne
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Organic Lecture Series
Molecular Vibrations
• Consider two covalently bonded atoms as two
vibrating masses connected by a spring
– the total energy is proportional to the frequency of vibration
– the frequency of a stretching vibration is given by an
equation derived from Hooke’s law for a vibrating spring
ν
= 4.12
K
μ
K = a force constant, which is a measure of the bonds’
strength; force constants for single, double, and triple
bonds are approximately 5, 10, and 15 x 105 dynes/cm
μ = reduced mass of the two atoms, (m1m2)/(m1 + m2),
where m is the mass of the atoms in grams
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Organic Lecture Series
The simplest vibrational motions are bending
and stretching.
Here are the fundamental stretching and
bending vibrations for a methylene group:
http://en.wikipedia.org/wiki/Infrared_spectroscopy
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Organic Lecture Series
Symmetric
Asymmetric
Rotation
Wagging
Scissoring
Twisting
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Organic Lecture Series
Molecular Vibrations
ν
= 4.12
K
μ
• From this equation, we see that the position
(i.e. wavenumber) of a stretching vibration:
– is proportional to the strength of the vibrating bond
– is inversely proportional the masses of the atoms
connected by the bond
• The intensity (i.e. weak, s, m) of absorption
depends primarily on the polarity of the
vibrating bond
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Correlation Tables
Organic Lecture Series
Table 12.4 Characteristic IR absorptions for the
types of bonds and functional groups
encountered most often:
Bon d
O-H
N-H
C-H
C=C
C=O
C-O
Stretching
Frequ ency (cm -1)
3200-3650
3100-3550
2700-3300
1600-1680
1630-1820
1000-1250
Intens ity
w eak to s trong
mediu m
w eak to medium
w eak to medium
strong
strong
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Hydrocarbons-Table 12.5
Hydrocarbon
Alk ane
C-H
CH3
C-C
Alk ene
C-H
C=C
Alk yn e
C-H
C C
Arene
C-H
C=C
C-H
Vib ration
Stretchin g
Bend ing
Bend ing
(N ot useful
Frequen cy
-1
(cm )
Organic Lecture Series
Intens ity
2850 - 3000
Mediu m
1450-1475
Mediu m
1375 and 1450 Weak to medium
for interpretation - too man y b ands
Stretchin g
Stretchin g
3000 - 3100
1600 - 1680
Weak to medium
Weak to medium
Stretchin g
Stretchin g
3300
2100-2250
Mediu m to stron g
Weak
Stretchin g
Stretchin g
Bend ing
3030
1450-1600
690-900
Weak to medium
Mediu m
Strong
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Organic Lecture Series
Alkanes
• IR spectrum of decane (Fig 12.4)
3000
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Organic Lecture Series
Alkenes
• IR spectrum of cyclohexene (Fig 12.5)
3000
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Organic Lecture Series
Alkynes
• IR spectrum of 1-octyne (Fig 12.6)
3000
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Organic Lecture Series
Aromatics
• IR spectrum of toluene (Fig 12.7)
3000
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Alcohols
Bond
Frequency, cm-1
Inten sity
O-H (free)
3600-3650
Weak
O-H (H b ond ed)
C-O
3200 - 3500
1000 - 1250
Medium, broad
Medium
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Organic Lecture Series
Effect of Concentration Upon Hydrogen Bonding
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Effect of Concentration Upon Hydrogen Bonding
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Ethers
• IR spectrum of dibutyl ether (Fig 12.9)
3000
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Ethers
• IR spectrum of anisole (Fig 12.10)
3000
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Organic Lecture Series
Amines
• IR spectrum of 1-butanamine (Fig 12.11)
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Organic Lecture Series
IR of Molecules with C=O Groups
Carbonyl Group
Vibration
Frequency
(cm-1 )
Intensity
O
RCR'
Ketones
C=O
Stretching
1630-1820
Strong
O
RCH
Aldehydes
C=O
C-H
Stretching
Stretching
1630-1820
2720
Strong
Weak
Carboxylic acids
C=O
Stretching
O H
Stretching
1700-1725
2500-3300
Strong
Strong (broad)
O
RCOH
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Organic Lecture Series
IR of Molecules with C=O Groups
O
RCNH2
Amides
C=O
Stretchin g
1630-1680
N H
Stretchin g
3200, 3400
(1° amides h ave tw o N -H stretches )
(2° amides h ave one N -H stretch )
O
RCOR'
Carboxylic esters
C=O
Stretchin g
2
Stretchin g
sp C O
3
sp C O
Stretchin g
O O
RCOCR
Acid anhydrides
C=O
Stretchin g
RC N
Strong
Mediu m
1735-1800
1200-1250
1000-1100
Strong
Strong
Strong
Strong
Strong
Mediu m
C O
Stretchin g
1740-1760 and
1800-1850
900-1300
Nitriles
C≡N
Stretchin g
2200-2250
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Organic Lecture Series
Aldehydes and Ketones
• IR spectrum of menthone (Fig 12.12)
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Organic Lecture Series
Carboxylic acids
• IR spectrum of pentanoic acid (Fig 12.13)
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Amide
Organic Lecture Series
• IR of N-methylpropanamide (Fig 12.14)
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Esters
• IR of Ethyl butanoate (Fig 12.15)
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Organic Lecture Series
Strategies for IR Interpretation
Inspect the spectrum from left to right.
9If there is a strong, but broad band 3500 cm-1
then, OH is present. One or two weak peaks in this
area are indicative of amines (N—H stretch).
9Examine the 3000 cm-1 C—H aliphatic stretches
are to the right and C—H from alkenes & aromatics
are to the left.
9Aldehyde C—H stretch will be ~ 2720 cm-1
9Check the area from 1820 to 1630 cm-1. Strong
peaks in this area indicate C=O and this is often the
strongest peak in the spectrum.
9The area from 1250 to 1000 cm-1 are the C—O
stretches of ethers, esters, acids.
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