Organic
Chemistry
William H. Brown
Christopher S. Foote
Brent L. Iverson
12-1
Infrared
Spectroscopy
Chapter 12
12-2
Electromagnetic Radiation
 Electromagnetic
radiation: light and other forms
of radiant energy
 Wavelength (): the distance between
consecutive peaks on a wave
 Frequency (): 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”)
12-3
Electromagnetic Radiation
 Common
units used to express wavelength
Unit
Meter (m)
Millimeter (mm)
Micrometer (m)
Nanometer (nm)
Angstrom (Å)
Relation
to Meter
---1 mm = 10-3 m
1 m = 10-6 m
1 nm = 10-9 m
1 Å = 10-10 m

12-4
Molecular Spectroscopy
 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
• we study three types of molecular spectroscopy
Absorption of
Region of the
Electromagnetic
Electromagnetic Type of
Radiation Results
Spectrum
Spectroscopy
in Transition Between
Radio fequency Nuclear magnetic Nuclear spin states
resonance
Infrared
Vibrational energy levels
Infrared
Ultravioletvisible
Ultravioletvisible
Electronic energy levels
12-5
Infrared Spectroscopy
vibrational IR extends from 2.5 x 10-6 m (2.5
m) to 2.5 x 10-5 m (25 m)
 The
• the frequency of IR radiation is commonly expressed
in wavenumbers
- : the number of waves per centimeter,
• wavenumber()
with units cm-1 (read reciprocal centimeters)
• expressed in wavenumbers, the vibrational IR extends
from 4000 cm-1 to 400 cm -1
-2
-1
10
m•cm
 =
= 4000 cm-1
2.5 x 10-6 m
 =
10-2 m•cm -1
2.5 x 10-5 m
= 400 cm-1
12-6
Infrared Spectroscopy
 IR
spectrum of 3-methyl-2-butanone
12-7
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
12-8
Molecular Vibrations
 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 polar bonds
H3 C
CH3
C C
H3 C
CH3
H3 C- C C- CH3
2,3-Dimethyl-2-butene
2-Butyne
12-9
Molecular Vibrations
 For
a nonlinear molecule containing n atoms,
there are 3n - 6 allowed fundamental vibrations
 For even a relatively small molecule, a large
number of vibrational energy levels exist and
patterns of IR absorption can be very complex
 The simplest vibrational motions are bending
and stretching
12-10
Molecular vibrations
 Fundamental
stretching and bending vibrations
for a methylene group
12-11
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

= 4.12
K

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
 = reduced mass of the two atoms, (m1m2)/(m1 + m2),
12-12
where m is the mass of the atoms in grams
Molecular Vibrations

= 4.12
K

 From
this equation, we see that the position 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 of absorption depends primarily on the
polarity of the vibrating bond
12-13
Correlation Tables
 Table
12.4 Characteristic IR absorptions for the
types of bonds and functional groups we deal
with most often
Bond
O-H
N-H
C-H
C=C
C=O
C-O
Stretching
Frequency (cm-1) Intensity
3200-3650
weak to strong
medium
3100-3550
2700-3300
weak to medium
1600-1680
weak to medium
1630-1820
strong
1000-1250
strong
12-14
Hydrocarbons-Table 12.5
Hydrocarbon
Alkane
C-H
CH 3
C-C
Alkene
C-H
C=C
Alkyne
C-H
C C
Arene
C-H
C=C
C-H
Vibration
Frequency
(cm-1 )
Intensity
Stretching 2850 - 3000
Medium
1450-1475
Medium
Bending
1375 and 1450 Weak to medium
Bending
(Not useful for interpretation - too many bands
Stretching
Stretching
3000 - 3100
1600 - 1680
Weak to medium
Weak to medium
Stretching
Stretching
3300
2100-2250
Medium to strong
Weak
Stretching
Stretching
Bending
3030
1450-1600
690-900
Weak to medium
Medium
Strong
12-15
Alkanes
 IR
spectrum of decane (Fig 12.4)
12-16
Alkenes
 IR
spectrum of cyclohexene (Fig 12.5)
12-17
Alkynes
 IR
spectrum of 1-octyne (Fig 12.6)
12-18
Aromatics
 IR
spectrum of toluene (Fig 12.7)
12-19
Alcohols
Bond
-1
Frequency, cm
Intensity
O-H (free)
3600-3650
Weak
O-H (H bonded)
C-O
3200 - 3500
1000 - 1250
Medium, broad
Medium
• IR spectrum of 1-hexanol (Fig 12.8)
12-20
Ethers
 IR
spectrum of dibutyl ether (Fig 12.9)
12-21
Ethers
 IR
spectrum of anisole (Fig 12.10)
12-22
Amines
 IR
spectrum of 1-butanamine (Fig 12.11)
12-23
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
12-24
IR of Molecules with C=O Groups
O
RCNH2
Amides
C=O
Stretching
1630-1680
N H
Stretching
3200, 3400
(1° amides have two N-H stretches)
(2° amides have one N-H stretch)
O
RCOR'
Carboxylic esters
C=O
Stretching
sp2 C O
Stretching
3
sp C O
O O
RCOCR
RC N
Stretching
Strong
Medium
1735-1800
1200-1250
Strong
Strong
1000-1100
Strong
Strong
Medium
Acid anhydrides
C=O
Stretching
C O
Stretching
1740-1760 and
1800-1850
900-1300
Nitriles
CN
Stretching
2200-2250
Strong
12-25
Aldehydes and Ketones
 IR
spectrum of menthone (Fig 12.12)
12-26
Carbonyl groups
 The
position of C=O stretching vibration is
sensitive to its molecular environment
• as ring size decreases and angle strain increases,
absorption shifts to a higher frequency
O
O
O
O
1715 cm-1
1745 cm-1
1780 cm-1
1850 cm-1
• conjugation shifts the C=O absorption to lower
frequency
O
O
O
H
-1
1717 cm
-1
1690 cm
-1
1700 cm
12-27
Carboxylic acids
 IR
spectrum of pentanoic acid (Fig 12.13)
12-28
Esters
 IR
of ethyl butanoate (Fig 12.14)
12-29
Infrared
Spectroscopy
End Chapter 12
12-30