Transcript Organic Chemistry - City University of New York

William H. Brown
Christopher S. Foote
Brent L. Iverson
Eric Anslyn
http://academic.cengage.com/chemistry/brown
Chapter 12
Infrared Spectroscopy
William H. Brown • Beloit College
12-1
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
Region of the
Electromagneti c Frequency
S pectrum
(hetz)
Radio frequency 3 x107-9x10 8
Infrared
Ul travi oletvisibl e
1 x1013-1x1014
Absorpti on of
Electromagneti c
Type of
Radiation Results
i n Transi ti on Between
Spectroscopy
Nuclear m agnetic Nuclear spin states
resonance
Vi brati onal energy level s
Infrared
2.5 x1014-1.5x1015 Ultravioletvisible
Electronic energy levels
12-2
Molecular vibrations
 Fundamental
stretching and bending vibrations
for a methylene group.
12-3
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
12-4
Infrared Spectroscopy
 Infrared
spectrum of 3-methyl-2-butanone.
C-H Stretching
Fingerprint Region:
Highly Complex and
Unique for Every Molecule
12-5
Correlation Tables
 Infrared
stretching frequencies of selected
functional groups.
Less Valuable
Almost all organic
molecules have C-H
bonds
C=C often too weak or
encroaching on
fingerprint region
Buried in fingerprint
region
Bond
O-H
N-H
C-H
C=C
C=O
C-O
Stretching
Frequency (cm-1)
3200-3650
3100-3550
2700-3300
1600-1680
1630-1820
1000-1250
More Valuable
Intensity
Potentially difficult to
weak to strong
distinguish between oneanother, but good
medium
indication of heteroatomhydrogen bond
weak to medium
weak to medium
Signature Stretch.
strong
Very strong, very
identifiable
strong
12-6
Carboxylic acids
 Infrared
spectrum of pentanoic acid.
12-7
Amines
 Infrared
spectrum of 1-butanamine, a 1° amine.
12-8
Hydrocarbons-Table 12.5
Hydr ocar bon
Alkane
C-H
Difficult to
distinguish
between oneanother
CH2
CH3
C-C
Alkene
C-H
C= C
Alkyne
C-H
C C
Ar ene
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 inter pr etation - too many bands
Stretching 3000 - 3100
Stretching 1600 - 1680
Weak to medium
Weak to medium
Stretching 3300
Str etching 2100-2250
Medium to str ong
Weak
Stretching 3030
Stretching 1450-1600
Bending
690-900
Weak to medium
Medium
Str ong
12-9
Aromatics
 Infrared
spectrum of toluene.
12-10
Hydrocarbons-Table 12.5
Hydr ocar bon
Alkane
C-H
CH2
CH3
C-C
Alkene
C-H
C= C
Alkyne
C-H
C C
Ar ene
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 inter pr etation - too many bands
Stretching 3000 - 3100
Stretching 1600 - 1680
Weak to medium
Weak to medium
Stretching 3300
Str etching 2100-2250
be useful.
Medium to str ong Can
In fairly distinct
regions
Weak
Stretching 3030
Stretching 1450-1600
Bending
690-900
Weak to medium
Medium
Str ong
12-11
Alkynes
 Infrared
spectrum of 1-octyne.
12-12