Transcript Introduction to Organic Chemistry 2 ed William H. Brown

22
Introduction to
Organic
Chemistry
2 ed
William H. Brown
22-1
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
22
Infrared
Spectroscopy
Chapter 22
22-2
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
22 Infrared Spectroscopy
• The IR region covers
• 7.8 x 10-7 m (just above the visible region) to
• 2.0 x 10-3 m (just below the microwave region)
• Organic chemistry uses mainly the vibrational IR,
which covers
• 2.5 x 10-6 m (2.5 mm) to 2.5 x 10-5 m (25 mm)
• absorption of IR radiation in this region causes bonds
to change from a lower vibrational energy level to a
higher one
22-3
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
22 Infrared Spectroscopy
• The frequency of IR radiation is commonly
expressed in wavenumbers ()
• Wavenumber: the number of waves per
centimeter, cm-1 (read reciprocal centimeters)
• the vibrational IR extends from 4000 cm-1 to 400 cm -1

 =
10-2 m•cm-1
2.5 x 10-6 m
10-2 m•cm-1
2.5 x 10-5 m
= 4000 cm-1
= 400 cm-1
22-4
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22 Infrared Spectroscopy
Transmittance (%)
• For the IR spectra recorded in this text,
calibrations are
100 2.5
0
4000
Micrometers
Wavenumber (cm
20
-1
)
400
22-5
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
22 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 for most covalent bonds are
from 2 to 10 kcal/mol (8.4 to 42 kJ/mol)
22-6
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
22 Molecular Vibrations
• For a molecule to absorb IR radiation,
• the frequency of radiation must match the frequency of
a bond vibration
• the bond undergoing vibration must be a polar bond
• the greater the polarity of the bond, the more intense
the absorption
• 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, do not
absorb IR radiation because they are not polar bonds
22-7
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
22 Molecular Vibrations
• For a nonlinear molecule containing n atoms,
there are 3n -6 allowed fundamental vibrations
• thus, 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
22-8
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
22 Correlation Tables
• Characteristic IR absorptions for the types of
bonds and functional groups we deal with most
often
Bond
Frequency (cm-1)
O- H
3200-3500
Intensity
strong and broad
N- H
C- H
C= O
C= C
3100-3500
2850-3100
medium
medium to strong
1630-1800
1600-1680
strong
weak
C- O
1050-1250
strong
22-9
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
22 Correlation Tables
• Alkanes and alkenes
Hydrocarbon
Alkane
C- H
CH2
CH3
Alkene
C- H
C= C
Frequency
(cm-1)
Vibration
Intensity
stretching
bending
bending
2850 - 3000
1450
1375 and 1450
strong
medium
weak to medium
stretching
stretching
3000 - 3100
1600 - 1680
weak to medium
weak to medium
22-10
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
22 Correlation Tables
• Alcohols and ethers
Frequency, cm-1
Bond
O-H (hydrogen
bonded)
C-O
Intensity
3200 - 3500
medium, broad
1050 - 1250
medium
• Aromatic rings
Bond
Frequency , cm-1
Intensity
C- H
C= C
3030
1475 and 1600
medium to weak
strong to medium
22-11
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22 Correlation Tables
• Amines
Bond
Frequency, cm-1 Intensity
N-H
3100-3500
medium to strong
Note: 1° amines show two bands in this
region; 2° amines show only one band
22-12
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22 Correlation Tables
• Aldehydes and ketones
C=O
aldehyde & ketone
Frequency, cm-1 Intensity
1705-1780
strong
• because few other bond vibrations absorb energy in
this region, a peak here is a reliable means of
confirming the presence of a C=O group
• the C=O group, however, may also be that of a
carboxylic acid, anhydride, or ester
22-13
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22 Correlation Tables
• Acids, esters, and amides
Frequency
Compound
(cm-1)
1630-1680
N-H stretching at 3200 and 3400
(1° amides have two N-H peaks)
(2° amides have one N-H peak)
1700-1725
O-H stretching at 2400-3400
C-O stretching at 1210-1320
1735-1800
C-O stretching at 1000-1100
and 1200-1250
O
RCNH 2
O
RCOH
O
RCOR
Additional Absorptions
(cm-1)
22-14
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
22
Infrared
Spectroscopy
End Chapter 22
22-15
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.