Download Infrared Spectroscopy

Review:
Infrared Spectroscopy,
Gas Chromatography and
Mass Spectrometry
Chemistry 2412 L
Dr. Sheppard
Spectroscopy
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
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 masses
Nuclear magnetic resonance (NMR) spectroscopy
– detects signals from hydrogen atoms and can be used to
distinguish isomers
Ultraviolet (UV) spectroscopy
– uses electron transitions to determine bonding patterns
Infrared Spectroscopy
IR Spectroscopy
Wavelengths usually 2.5-25 mm
More common units are wavenumbers, or
cm-1, the reciprocal of the wavelength in
centimeters (4000-400 cm-1)
Measures molecular vibrations
IR Spectrum
Baseline
Absorbance/
Peak
No two molecules will give exactly the same IR spectrum
(except enantiomers)
Simple stretching: 1600-3500 cm-1
Complex vibrations: 400-1400 cm-1, called the
“fingerprint region”
Interpretation
Looking for presence/absence of
functional groups
Correlation tables
– Klein: Ch. 15 and Padias: table 2-1
A polar bond is usually IR-active
A nonpolar bond in a symmetrical
molecule will absorb weakly or not at all
Summary of IR Absorptions
Padias
Correlation
Table
Another
Correlation
Table
Alkane and Alkene Spectra
Some Alkyne Spectra
Alcohol and Amine Spectra
Ketone and Aldehyde Spectra
Carboxylic Acid and Amide Spectra
Strengths and Limitations of
IR Spectroscopy
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 sample’s IR
spectrum confirms the identity of the compound
Gas Chromatography
and Mass Spectrometry
Chromatography
Separation of components of a mixture based
on affinity for stationary phase or mobile phase
Includes:
–
–
–
–
–
Thin-layer chromatography (TLC)
Paper chromatography
Column chromatography
Gas chromatography (GC)
High Performance Liquid Chromatography (HPLC)
Gas Chromatography
Mobile phase = stream of inert gas
Stationary phase = high boiling liquid film in
column
Affinity = vapor vs. liquid
Sample = injection from syringe (1 mL)
– Sometimes add 1 mL air
Separated components pass by detector, send
signal to recorder
Signal display = peaks
Area under peak = % of mixture
Analysis
– Retention time (min.), Area, % of mixture
Analysis of GC
How many components are in this mixture?
What is the percentage of each component in the
mixture?
Which component has the higher boiling point?
A GC from 2411L
Mass Spectrometry
Used with GC
– Mixture of compounds separated by gas chromatography,
then identified by mass spectrometry
Determines MW and provides information about structure
A beam of high-energy electrons breaks molecules into
ions (fragments)
M → M•+ + eM•+ → A+ + X
A+ → B + + Y
etc.
Ions are separated and detected; mass determined
The Mass Spectrum
Plot relative abundance vs. mass-to-charge
ratio
– Charge = +1
Isotopes
Present in their usual abundance
– Hydrocarbons contain 1.1% 13C, so there will be a
small M+1 peak
– If S is present, M+2 will be 4% of M+
– If Cl is present, M+2 is one-third of M+
– If Br is present, M+2 is equal to M+
– If I is present, peak at 127; large gap
81
Br
Mass Spectrum with Br
Mass Spectrum with I
Mass Spectra of Alkanes
More stable carbocations will be more
abundant
Mass Spectra of Alcohols
Alcohols usually lose a water molecule
M+ may not be visible
Mass Spectrum of
Methyl Benzoate
105
77
136
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