Transcript Mass Spectrometry - HCC Learning Web

12. Structure Determination: Mass
Spectrometry
12.1 Mass Spectrometry (MS), Mass Spectrum
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Sample vaporized and bombarded by energetic electrons that
remove an electron, creating a cation-radical.
A radical cation carries a positive charge and has an unpaired
electron
Bonds in cation radicals begin to break (fragment)
The fragmentation pattern depends upon the class of
compounds.
The fragmentation processes favored are the ones that form
the most stable cations
Plot mass of ions (m/z) (x-axis) versus the intensity of the
signal (corresponding to the number of ions) (y-axis)
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Mass Spectrometer
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The Mass Spectrum
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Tallest peak is base peak (100%). Other peaks listed as the %
of that peak
Peak that corresponds to the unfragmented radical cation is
parent peak or molecular ion (M+): usually correspond to
the highest mass in the spectrum
The neutral fragments that may be lost in a fragmentation do
not appear in the mass spectrum but can be accounted for by
differences in masses between a peak and the molecular ion
peak
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Alkanes
a. n-alkanes
•For straight-chain alkanes, a molecular ion peak can be observed.
•Fragmentation occurs by breaking of Carbon Carbon bonds.
•As the chain length of an n-alkane increases, the chance of
finding the fragment corresponding to the loss of a CH3 unit
decreases.
•Alkanes fragment to form clusters of peaks which are separated
from the neighboring clusters by 14 mass units accounted for by a
CH2.
•Within a cluster, additional peaks are accounted for by losses of 1
or 2 H atoms
b. Branched alkanes
•The more branched the alkane, the lower the intensity of the
molecular ion peak. May be even absent
•Secondary and tertiary carbocation fragments
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MS Examples: Methane (a) and Propane (b)
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Methane produces a parent peak (m/z = 16)
and fragments of 15 and 14
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Mass Spectral Fragmentation of Hexane
Hexane (m/z = 86 for parent) has peaks at m/z = 71, 57, 43, 29
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2,2-Dimethylpropane
Molecular ion peak (m/z = 72) missing!
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Cycloalkanes
•Relatively intense molecular peak present
•Cleavage of two Carbon Carbon bonds required for
fragmentation
•The common fragmentation involves loss of a molecule of
ethene either from the parent molecule or from an intermediate
radical ion.
•In substituted cycloalkanes, a common fragmentation
involves loss of the substituent.
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Practice: methylcyclohexane or ethylcyclopentane?
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Mass Spectra of Alcohols
•Molecular ion peak is usually of low intensity (primary, secondary
alcohols) or absent (tertiary alcohols)
•One important mode of fragmentation is the loss of an alkyl
groupThe largest alkyl group is the one that is lost most easily
•Another fragmentation pattern is dehydration especially for
longer chain alcohols.
•The dehydration maybe a 1,2 elimination or involve a 1,4elimination involving a cyclic mechanism in longer chain alcohols.
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12.2 Interpreting Mass
Spectra
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Molecular weight from the mass of the molecular
ion
Double-focusing instruments provide highresolution “exact mass”
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0.0001 atomic mass units – distinguishing specific
atoms
Example MW “72” is ambiguous: C5H12 and
C4H8O but:
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C5H12 72.0939 amu exact mass C4H8O 72.0575 amu
exact mass
Result from fractional mass differences of atoms 16O
= 15.99491, 12C = 12.0000, 1H = 1.00783
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Use of Heavier Isotope Peaks
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Peaks above the molecular weight appear as a result of
naturally occurring heavier isotopes in the sample
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 (M+1) peak: one mass unit heavier than the M peak.
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 (M+2) peak: two mass units heavier than the M peak.
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 If 1 Br is present in the formula, M+2 peak as large as M
peak
 If 1 Cl is present in the formula, M+2 peak a third as
large as M+ peak
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 A m/z peak at 127 due to I
 If 1 S is present in the formula, intensity of M+2 = 4% of
that of M+ peak
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 If 1 N is present in the formula, odd M peak
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Mass Spectral Cleavage of Amines
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Amines undergo -cleavage: the loss of an alkyl group.
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Fragmentation of Ketones and Aldehydes
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A C-H that is three atoms away leads to an internal transfer of
a proton to the C=O, called the McLafferty rearrangement
One important mode of fragmentation is the loss of an alkyl
group:  cleavage
Aldehydes also undergo -cleavage.
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