ir spectroscopy ch12
TRANSCRIPT
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Organic Chemistry II
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Outline
Electromagnetic Radiation
Molecular Spectroscopy
Infrared Spectroscopy Interpreting Infrared Spectra
Solving Infrared Spectral
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Objectives
Understand the concept of electromagneticradiation, and other forms of radiant energy and the
relationship to wavelength and frequency.
Understand the features of an IR spectrum. Understand and learn the characteristic absorption
patterns to identify various functional groups such as
alkanes, alkenes, alkynes, alcohols, ethers, amines,
aldehydes and ketones and carboxylic acids.
Learn to solve infrared spectral problems.
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Introduction
Spectroscopy is an analytical technique
which helps determine structure.
It destroys little or no sample.
The amount of light absorbed by the
sample is measured as wavelength is
varied.
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IR Spectroscopy
measures the bond vibration frequencies
in a molecule and is used to determine
the functional group.
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Electromagnetic Spectrum
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V I B G Y O R
the higher the frequency, the greater the energy of the radiation
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Electromagnetic Spectrum
Frequency(n) and wavelength (l) are inverselyproportional.
c= nl, where cis the speed of light. Energy per photon, E = hn, where his Plancks
constant.
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n - a Greek symbol called nul - a Greek symbol called lamda
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The IR Region
Just below red in the visible region. Wavelengths usually 2.5-25 mm. More common units are wavenumbers, or cm-1,
the reciprocal of the wavelength in centimeters. Wavenumbers are proportional to frequency and
energy.
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cm-1 called reciprocal centimetre
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The Spectrum and
Molecular Effects
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Molecular Vibrations
Covalent bonds vibrate at only certain
allowable frequencies.
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Stretching Frequencies
Frequency decreases with increasing atomic weight.
Frequency increases with increasing bond energy.
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Vibrational Modes
Nonlinear molecule with n atoms usually has 3n - 6
fundamental vibrational modes.
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For linear molecules with n atoms, there are 3n 5 allowed
fundamental vibrations
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Fingerprint of Molecule
Whole-molecule vibrations and bendingvibrations are also quantized.
No two molecules will give exactly the same
IR spectrum (except enantiomers). Simple stretching: 1600-3500 cm-1.
Complex vibrations: 600-1400 cm-1, called
the fingerprint region.
N.B.: Quantized- only specific vibrational energy levels are allowed
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IR-Active and Inactive
A polar bond is usually IR-active.
A nonpolar bond in a symmetrical molecule
will absorb weakly or not at all.
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An Infrared Spectrometer
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FT-IR Spectrometer
Uses an interferometer.
Has better sensitivity.
Less energy is needed from source.
Completes a scan in 1-2 seconds.
Takes several scans and averages them.
Has a laser beam that keeps the instrumentaccurately calibrated. =>
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Carbon-Carbon Bond Stretching
Stronger bonds absorb at higher frequencies:
C-C 1200 cm-1
C=C 1660 cm-1
CC 2200 cm-1 (weak or absent if internal) 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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Carbon-Hydrogen Stretching
Bonds with more s character absorb at a higher
frequency.
sp3
C-H, just below 3000 cm-1
(to the right) sp2 C-H, just above 3000 cm-1 (to the left)
sp C-H, at 3300 cm-1
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An Alkane IR Spectrum
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An Alkene IR Spectrum
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An Alkyne IR Spectrum
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O-H and N-H Stretching
Both of these occur around 3300 cm-1, but
they look different.
Alcohol O-H, broad with rounded tip.
Primary amine (RNH2), broad with two sharp
spikes.
Secondary amine (R2NH), broad with one sharpspike.
No signal for a tertiary amine (R3N) =>
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An Alcohol IR Spectrum
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broad with rounded tip
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A 1 AmineIR Spectrum
broad with two sharp spikes
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A 2 AmineIR Spectrum
broad with one sharp spike
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Carbonyl Stretching
The C=O bond of simple ketones,aldehydes, and carboxylic acids absorb
around 1710 cm-1
. Usually, its the strongest IR signal.
Carboxylic acids will have O-H also.
Aldehydes have two C-H signals around2700 and 2800 cm-1.=>
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A Ketone
IR Spectrum
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An Aldehyde
IR Spectrum
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O-H Stretch of a
Carboxylic AcidThis O-H absorbs broadly, 2500-3500 cm-1, due to
strong hydrogen bonding.
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Variations in
C=O Absorption
Conjugation of C=O with C=C lowers the
stretching frequency to ~1680 cm-1.
The C=O group of an amide absorbs at an evenlower frequency, 1640-1680 cm-1.
The C=O of an ester absorbs at a higher
frequency, ~1730-1740 cm
-1
. Carbonyl groups in small rings (5 Cs or less)
absorb at an even higher frequency. =>
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An Amide
IR Spectrum
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Carbon - Nitrogen Stretching
C - N absorbs around 1200 cm-1.
C = N absorbs around 1660 cm-1and is
much stronger than the C = C absorption inthe same region.
C N absorbs strongly just above2200 cm-1. The alkyne C C signal is much weakerand is just below2200 cm-1 .
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A Nitrile
IR Spectrum
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Summary of IR
Absorptions
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Strengths and Limitations
IR alone cannot determine a structure.
Some signals may be ambiguous.
The functional group is usually indicated. The absence of a signal is definite proof that
the functional group is absent.
Correspondence with a known samples IRspectrum confirms the identity of the
compound.
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Solving Infrared Spectral Problems
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