Organic Chemistry,
8th Edition
L. G. Wade, Jr.
Chapter 12
Infrared Spectroscopy
(IR)
© 2013 Pearson Education, Inc.
© 2013 Pearson Education, Inc.
Rizalia Klausmeyer
Baylor University
Waco, TX
Introduction
• The identification and characterization of the structures of unknown
substances are an important part of organic chemistry.
• Information about the compound: physical state and properties
(melting point, boiling point, solubility, odor, color, etc.), elemental
analysis, and confirmatory tests for functional are very important in
identification of these compounds. Also, it is often possible to
establish the structure of a compound on the basis of Spectroscopic
techniques (IR, MS, NMR).
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Introduction
• Spectroscopy is a technique used to determine the
structure of a compound.
• Most techniques are nondestructive (destroys little or no
sample).
• Absorption spectroscopy measures the amount of light
absorbed by the sample as a function of wavelength.
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Types of Spectroscopy
• Infrared (IR) spectroscopy measures the bond vibration
frequencies in a molecule and is used to determine the
functional group.
• Mass spectrometry (MS) fragments the molecule and
measures the mass. MS can give the molecular weight of
the compound and functional groups.
• Nuclear magnetic resonance (NMR) spectroscopy analyzes
the environment of the hydrogens in a compound. This
gives useful clues as to the alkyl and other functional groups
present.
• Ultraviolet (UV) spectroscopy uses electronic transitions
to determine bonding patterns.
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Wavelength and Frequency
• The frequency (n) of a wave is the number of complete
wave cycles that pass a fixed point in a second.
• Wavelength (l) is the distance between any two peaks
(or any two troughs) of the wave.
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Electromagnetic Spectrum
• Frequency and wavelength are inversely
proportional.
c = ln
l = c/n
where c is the speed of light (3 × 1010 cm/sec).
• Energy of the photon is given by
E = hn
where h is Planck’s constant (6.62 × 10–37 kJ•sec).
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The Electromagnetic Spectrum
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Use spectrum (singular) and
spectra (plural) correctly: “This
spectrum….”
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The Infrared (IR) Region
• From right below the visible region to just above the
highest microwave and radar frequencies.
• Wavelengths are usually 2.5 x 10–4 to 25 x 10–4 cm. (0.25
micron-2.5 micron)
• More common units are wavenumbers,
or cm–1
(reciprocal centimeters, IR region = 400 - 4000 cm-1)
• Wavenumbers are proportional to frequency and energy.
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What happens when absorption of
IR occurs?
1.
Changes in the shape of molecules such as stretching of bonds,
bending of bonds, or internal rotation around single bonds.
2.
IR absorption only occurs when IR radiation interacts with a
molecule undergoing a change in dipole moment as it vibrates or
rotates.
3.
Infrared absorption only occurs when the incoming IR photon has
sufficient energy for transition to the next allowed vibrational state
to take place (E = hn).
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IR active and IR in active
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Molecular Vibrations
• If the bond is stretched, a restoring force pulls the two
atoms together toward their equilibrium bond length.
• If the bond is compressed, the restoring force pushes the
two atoms apart.
• If the bond is stretched or compressed and then released,
the atoms vibrate.
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Bond Stretching Frequencies
• Frequency decreases with increasing atomic mass.
• Frequency increases with increasing bond energy.
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Vibrational Modes
• A nonlinear molecule with n atoms has 3n – 6 fundamental
vibrational modes.
• Water has 3(3) – 6 = 3 modes. Two of these are stretching modes,
and one is a bending mode (scissoring).
https://www.youtube.com/watch?v=1PQqDfJKXvA
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Fingerprint Region of the Spectrum
• No two molecules will give exactly the same IR spectrum (except
enantiomers).
• Fingerprint region is between 600 and 1400 cm–1 and has the most
complex vibrations.
• The region between 1600 and 3500 cm–1 has the most common
vibrations, and we can use it to get information about specific
functional groups in the molecule.
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Effect of an Electric Field on a
Polar Bond
•
A bond with a dipole moment (as in HF, for example) is either stretched or
compressed by an electric field, depending on the direction of the field.
•
Notice that the force on the positive charge is in the direction of the electric
field (E) and the force on the negative charge is in the opposite direction.
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The Infrared Spectrometer
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FT–IR Spectrometer
Interferogram generated by n-octane
Infrared spectrum for n-octane
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FT–IR Spectrometer
• Has better sensitivity.
• Less energy is needed
from source.
• Completes a scan in 1 to
2 seconds.
• Takes several scans and
averages them.
• Has a laser beam that
keeps the instrument
accurately calibrated.
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IR spectroscopy of Hydrocarbons
Carbon–Carbon Bond Stretching
• Stronger bonds absorb at higher frequencies because
the bond is difficult to stretch:
C—C
C= C
C C
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1200 cm–1
1660 cm–1
<2200 cm–1
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IR spectroscopy of Hydrocarbons
Carbon–Carbon Bond Stretching
• Conjugation lowers the frequency:
▪ isolated C=C
1640–1680 cm–1
▪ conjugated C=C
1620–1640 cm–1
▪ aromatic C=C approx. 1600 cm–1
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IR spectroscopy of Hydrocarbons
Carbon–Hydrogen Stretching
• A greater percent of s character in the hybrid orbitals will make the
C—H bond stronger.
• The C—H bond of an sp3 carbon will be slightly weaker than the C—
H of an sp2 or an sp carbon.
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Interpretation of the IR Spectra of
Alkanes
• An alkane will show stretching and bending frequencies for C—H and C—C
only.
• The C—H stretching is a broad band between 2800 and 3000 cm–1, a band
present in virtually all organic compounds.
• The bands in the fingerprint region are due to the bending vibrations.
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Interpretation of the IR Spectra of
Alkenes
• The most important absorptions in the 1-hexene are the C═C
stretch at 1642 cm–1 and the unsaturated stretch at 3080 cm–1.
• Notice that the bands of the alkane are present in the alkene.
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Interpretation of the IR Spectra of
Alkynes
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