Download 1 12.1 Mass Spectrometry (MS) Mass Spectrometry (MS) Pentane

Determining the Structure of an
Organic Compound
Structure Determination: Mass
Spectrometry
„ The analysis of the outcome of a reaction
requires that we know the full structure of the
products as well as the reactants
„ Physical methods now permit structures to be
determined directly. We will examine:
_____________________
_____________________
„ _____________________
„ _____________________
„
Chapter 12 – Part 1
„
12.1 Mass Spectrometry (MS)
„ Measures molecular weight
„ Information about the molecular formula and
some structural features
Diagram of a mass spectrometer.
Mass Spectrometry (MS)
Pentane
„ Sample vaporized and subjected to bombardment by
electrons (____) that remove an electron
„
Creates a cation-radical
„ Bonds in cation radicals begin to break (fragment)
„ Charge to mass ratio is measured (see Figure 12-1)
CH3CH2CH2CH2CH3
70eV
CH3CH2CH2CH2CH3
+ e-
cation radical
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The Mass Spectrum
„ Plot mass of ions (m/z) (x-axis) versus the
intensity of the signal (roughly corresponding
to the number of ions) (y-axis)
„
Note: _____________________.
„ Tallest peak is _____________________
„
Other peaks listed as the % of that peak
„ Peak that corresponds to the unfragmented
radical cation is _____________________
MS Examples: Methane and Propane
12.2 Interpreting Mass Spectra
„ Methane produces a parent peak (m/z = 16) and
„ Molecular weight from the mass of the molecular ion
„ Double-focusing instruments provide high-resolution
fragments of 15 and 14 (See Figure 12-2 a)
„ The MS of propane is more complex (Figure 12-2 b)
since the molecule can break down in several ways
“exact mass”
„
0.0001 atomic mass units – distinguishing specific
atoms
„ Example MW “72” is ambiguous: C5H12 and C4H8O
but:
„
„
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
„ Instruments include computation of formulas for each
peak
Pentane
Pentane
CH3CH2CH2
m/z = 43
CH3CH2CH2CH2CH3
M+ m/z = 72
+ CH3CH2
CH3CH2CH2 +
CH3CH2CH2CH2 +
m/z = 57
CH3CH2CH2CH2 +
CH3CH2
m/z = 29
CH3
CH3
m/z = 15
Fragments are identified by subtracting their
m/z value from the m/z value of the parent
ion.
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Pentane
12.3 Interpreting Mass-Spectral
Fragmentation Patterns
„ The mass spectrum of pentane shows a
„ The way molecular ions break down can produce
molecular ion (M+) peak with a mass to
charge ratio, m/z = 72.
„ The base peak is the peak of greatest
intensity (relative intensity of 100%)
„ The remaining peaks are shown as a
percentage of the base peak.
„ Note: _____________________
12.3 Interpreting Mass-Spectral
Fragmentation Patterns
„
„
„
„
characteristic fragments that help in identification
„ Serves as a “fingerprint” for comparison with
known materials in analysis (used in forensics)
„ Positive charge goes to fragments that best can
stabilize it
„ The relative height of the M+ peak is greatest for
straight chain compounds and decreases with the
degree of branching
„ The relative height of the M+ peak decreases with
increasing molecular weight in a homologous
series.
The spectrum of 2,2-dimethylpropane has no parent
peak! What is the molecular formula of the m/z = 57
ion?
Cleavage is favored at alkyl-substituted carbon
atoms
Double bonds, cyclic structures and aromatic rings
stabilize the molecular ion and increases it’s
fractional abundance
Double bonds favor allylic cleavage (especially for
cyclic alkenes
Saturated rings tend to lose alkyl side chains at
the α-bond.
Which spectrum corresponds to 2-methyl-pentene
and which corresponds to 2-hexene?
Consider
„ The fragmentation of two different
cyclohexanes both show a molecular ion
peak at m/z = 98.
One spectrum has a base peak at m/z = 69
The other shows a a base peak at m/z = 83.
„ Identify the cyclohexanes.
„
„
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12.4 Mass-Spectral Behavior of Some
Common Functional Groups
Mass Spectral Cleavage Reactions
of Alcohols
„ Functional groups cause common patterns of
„ Alcohols undergo _______ (at the bond next
cleavage in their vicinity
to the C-OH) as well as loss of H-OH to give
C=C
Mass Spectral Cleavage Reactions
of Alcohols
OH
CH3CH2CH2CH2CHCH3
-e-
2-Hexanol
OH
CH3CH2CH2CH2CHCH3
m/z = 102
α cleavage
CH3CH2CH2CH2 + CH3CH OH
m/z = 45
α cleavage
CH3CH2CH2CH2CH OH + CH3
m/z = 87
Mass Spectral Cleavage of Amines
Fragmentation of Ketones and
Aldehydes
„ Amines undergo _________, generating
„ A C-H that is three atoms away leads to an internal
radicals
transfer of a proton to the C=O, called the
______________ Carbonyl compounds can also
undergo _____
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Alkyl Halides
1- Bromopropane
CH3CH2CH2Br79
CH3CH2CH2Br
-e-
m/z = 122
CH3CH2CH2 + Br
+
m/z = 43
CH3CH2CH2Br81
m/z = 124
Alkyl Halides
2-Chloropropane
Cl35
CH3CHCH3
Cl
CH3CHCH3
m/z = 78
-e-
+
CH3CHCH3
+ Cl
m/z = 43
Cl37
CH3CHCH3
m/z = 80
α cleavage
Cl37
Cl35
CH3CH
m/z = 63
+
CH3CH
+
CH3
m/z = 65
Last Slide Chapter 12 - MS
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