Introduction
Modern techniques for structure determination of
organic compounds include:
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Mass spectrometry
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Infrared spectroscopy
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•
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Size and formula of the compound
Functional groups present in the compound
Ultraviolet spectroscopy
•
Conjugated p electron system present in the compound
Nuclear magnetic resonance spectroscopy
•
Carbon-hydrogen framework of the compound
10.1 Mass Spectrometry of Small Molecules:
Magnetic-Sector Instruments
Mass spectrometry (MS) measures the mass and molecular
weight (MW) of a molecule
• Provides structural information by finding the masses of
fragments produced when molecules break apart
• Three basic parts of mass spectrometers:
Mass Spectrometry of Small Molecules:
Magnetic-Sector Instruments
Electron-impact, magneticsector instrument
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•
•
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Sample is vaporized into
ionization source
Bombarded by electron beam
(70 eV) dislodging valence
electron of sample producing
cation-radical
Most cation-radicals fragment
and are separated in magnetic
field according to their mass-tocharge ratio (m/z)
Since z = 1 for most ions the
value of m/z is mass of ion
Fig. 15-5, p. 619
Table 15-3, p. 620
Interpreting Mass Spectra
Mass spectrum of hexane (C6H14; MW = 86)
• All carbon-carbon bonds of hexane are electronically
similar and break to a similar extent
• Mass spectrum contains mixture of ions
Interpreting Mass Spectra
M+ = 86 for hexane
•
•
•
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m/z = 71 arises from loss of
methyl radical from hexane
cation radical
m/z = 57 arises from loss of
ethyl radical from hexane
cation radical
m/z = 43 arises from loss of
propyl radical from hexane
cation radical
m/z = 29 arises from loss of
butyl radical from hexane
cation radical
Fig. 15-6, p. 621
Fig. 15-7, p. 622
Fig. 15-8, p. 626
10.3 Mass Spectrometry of Some Common
Functional Groups
Alcohols
• Fragment by two pathways
•
Alpha cleavage
•
Dehydration
Mass Spectrometry of Some Common Functional
Groups
Amines
• Aliphatic amines undergo characteristic a cleavage
Mass Spectrometry of Some Common Functional
Groups
Carbonyl compounds
• Ketones and aldehydes with C-H three atoms away from
carbonyl group undergo McLafferty rearrangement
Mass Spectrometry of Some Common Functional
Groups
• Ketones and aldehydes also undergo a cleavage of bond
between carbonyl group and neighboring carbon
Worked Example 10.2
Identifying Fragmentation Patterns in a Mass
Spectrum
The mass spectrum of 2-methylpentan-3-ol is shown below.
What fragments can you identify?
Worked Example 10.2
Identifying Fragmentation Patterns in a Mass
Spectrum
Strategy
• Calculate the mass of the molecular ion, and
identify the functional groups in the molecule.
Then write the fragmentation processes you might
expect, and compare the masses of the resultant
fragments with the peaks present in the spectrum.
Worked Example 10.2
Identifying Fragmentation Patterns in a Mass
Spectrum
Solution
•
2-Methylpentan-3-ol, an open-chain alcohol, has M+ = 102 and might be
expected to fragment by a cleavage and by dehydration. These
processes would lead to fragment ions of m/z = 84, 73, and 59. Of the
three expected fragments, dehydration is not observed (no m/z = 84
peak), but both a cleavages take place (m/z = 73, 59).
Interpreting Mass Spectra
Mass spectrum of 2,2-dimethylpropane (MW = 72)
• 2,2-dimethylpropane fragments so easily that no M+ peak
observed when electron-impact ionization is used
• “Soft” ionization methods can prevent fragmentation of
molecular ion
Interpreting Mass Spectra
• Base peak in mass spectrum of 2,2-dimethylpropane is
at m/z = 57
•
•
m/z = 57 corresponds to t-butyl cation
Molecular ion fragments to give most stable carbocation
Mass Spectrometry in Biological Chemistry:
Time-of-Flight (TOF) Instruments
MALDI-TOF mass spectrum of chicken egg-white lysozyme
• Peak at 14,307.7578 daltons (amu) is due to the monoprotonated protein