Transcript General Chemistry - Chemistry Teaching Resources

Structural
Analysis 2
Gordon Watson
Chemistry Department, Kelso High School
Adv Higher Unit 3 Topic 4
KHS Chemistry
Unit 3.4 Structural Analysis
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Introduction
This topic continues to explore methods used in the Structural
Analysis of organic molecules including IR & NMR Spectroscopy
and X-Ray Crystallography.
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Unit 3.4 Structural Analysis
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Aspects of Spectroscopy
Spectroscopy was introduced in Unit 1 as a technique that uses the
interaction between Electromagnetic Radiation and particles to
help determine structure.
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Unit 3.4 Structural Analysis
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Visible Spectrum
Shorter Wavelength (l)
Longer Wavelength (l)
400 nm
750 nm
Visible Light
Higher Frequency (n )
Lower Frequency (n )
Higher Energy (E)
Lower Energy (E)
E=hn
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Unit 3.4 Structural Analysis
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Infra-Red Radiation
Just below red in
the visible region.
Wavelengths usually
2500-25000 nm.
Visible LightInfrared
Ultraviolet
IR Spectroscopy uses units called wavenumbers ( ), nu, the
reciprocal of the wavelength, (1/l), in centimeters (cm-1).
Wavenumbers usually
4000-400 cm-1
Wavenumbers are proportional to frequency and energy.
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Unit 3.4 Structural Analysis
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Molecular Vibrations
Qui ckTi me™ and a Graphics decompressor are needed to see this pictur e.
Molecules have a variety of
possible vibration states.
Some of these states are due to
Stretching: which changes the
distance between atoms in the
molecule.
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Some of these states are due to
Bending: which changes the
angle between atoms in the
molecule.
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Unit 3.4 Structural Analysis
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Stretching & Bending
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Unit 3.4 Structural Analysis
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Molecular Dipoles
Many of these vibrations can cause a change in the Molecular Dipole
- especially if the bond is polar.
The fluctuating electrical field produced
can interact with the electric field of
electromagnetic radiation
If the frequency of the radiation matches
the frequency of the vibration - then
energy will be absorbed.
Molecular vibrations are relatively low
energy - Infra Red
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Unit 3.4 Structural Analysis
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IR Active
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If a vibration has no effect on the dipole
of the molecule then it will be unable to
absorb radiation - IR inactive
The symmetric stretching of the O—H
bonds in water will be IR inactive
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The assymmetric stretching of the O—
H bonds in water will be IR active
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Unit 3.4 Structural Analysis
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Stretching & Bending
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Stretching vibrations are of higher
energy - 4000 - 1600 cm-1
Stretching vibrations tend to absorb
strongly to produce large distinct peaks.
4000 - 1600 cm-1 is where functional
groups can be identified.
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Bending vibrations tend to be of lower
energy - 1400 - 400 cm-1
Bending absorb weakly to produce
complex indistinct peaks.
1400 - 400 cm-1 is the fingerprint region
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Unit 3.4 Structural Analysis
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Frequencies 1
The spectra of Alkanes are among the ‘simplest’ and, since most
organic molecules contain alkyl groups, this is effectively the
background upon which other functional groups will appear.
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Unit 3.4 Structural Analysis
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Frequencies 1
Alcohols and amines display broad O-H and N-H stretching bands in
the region 3400-3100 cm-1. The O-H absorbtion is particularly strong.
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Unit 3.4 Structural Analysis
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Alcohols & Amines
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Unit 3.4 Structural Analysis
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Frequencies 2
Alkene and alkyne C-H bonds display sharp stretching absorptions
in the region 3100-3000 cm-1. The bands are of medium intensity
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Unit 3.4 Structural Analysis
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Alkene & Alkyne
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Unit 3.4 Structural Analysis
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Frequencies 3
Carbonyl stretching bands occur in the region 1800-1700 cm. The
bands are generally very strong and can be broad.
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Unit 3.4 Structural Analysis
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Aldehydes
Carbonyl band tends to be at lower end of 1800-1700 cm-1 region .
A characteristic double peak between 2700-2850 cm-1.
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Unit 3.4 Structural Analysis
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Ketones
Carbonyl band also tends to be at lower end of 1800-1700 cm-1
region . No characteristic double peak between 2700-2850 cm-1.
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Unit 3.4 Structural Analysis
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Esters
Carbonyl band tends to be slightly higher in 1800-1700 cm-1 region.
C—O stretches can sometimes be picked out (unreliable).
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Unit 3.4 Structural Analysis
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Acids
Carbonyl band tends to be typical.
Hydroxyl band tends to be even broader
than usual. This is due to strong hydrogen
bonding between molecules - often leads
to dimerisation
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Unit 3.4 Structural Analysis
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Amides
Carbonyl band tends to be much lower than normal .
N—H stretches also present.
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Unit 3.4 Structural Analysis
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Frequencies 4
Aromatic stretching bands occur in various places. They are often
difficult to pick out.
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Unit 3.4 Structural Analysis
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Aromatics 1
Sometimes the best indication that a molecule is aromatic is a
reasonable number of sharp bands in the fingerprint region
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Unit 3.4 Structural Analysis
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Aromatics 2
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Summary of IR Absorptions 1
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Unit 3.4 Structural Analysis
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Summary of IR Absorptions 2
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Unit 3.4 Structural Analysis
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Strengths & Limitations
IR alone cannot determine a structure.
Some signals may be ambiguous.
The functional group is usually indentifiable.
The absence of a signal is definite proof that the functional group is
absent.
Correspondence with a known sample’s IR spectrum confirms the
identity of the compound - fingerprint
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Unit 3.4 Structural Analysis
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Structural
Analysis 2
End of Topic 4
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Unit 3.4 Structural Analysis
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