Recent Progress in Stable Silylene Chemistry

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CHEM 344
Organic Chemistry Lab
Spectroscopy of Organic Compounds Lecture 3
Infrared and Mass Spec
September 9th and 10th 2008
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1
2
Infrared radiation excites vibrational energy
levels of a molecule Bond stretching, bending,
twisting, rocking etc. IR spectroscopy most
useful for functional group assignment IR bands
reported in wavenumbers (cm-1)
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100
Transmittance ()
N H O H C H (sp gt sp2 gt sp3)
CC CN
CC CN CO
Fingerprint region
C-C C-O C-X C-N
Alkynes Nitriles
Alkenes Imines Carbonyls
Not used in CHEM 344
0
4000 3000
2000 1600
400 Wavenumber
(cm-1)
IR correlation tables in lab manual, textbook
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2o amine R2NH
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1o amine RNH2
Sym and asym N-H bands typical of 1o amine RNH2
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Terminal alkyne
sp C-H
3315 cm-1
2120 cm-1
CC
sp3 C-H
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Nitrile
2260 cm-1
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CO
1698 cm-1
3639 cm-1
O-H
C-O band in fingerprint region
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CO and C-C in same region (1700 1600 cm-1)
which is which?
sp2 sp3 C-H
CO usually most intense band in IR spectrum
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2800 - 2600 cm-1
Broad band typical of RCO2H
1716 cm-1
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Mass Spectrometry
Uses high energy electron beam (70 eV), sample in
gas phase Ionization potential for most organic
molecules 8-15 eV Gives info on molecular
weight of compound by m/z Gives info on
connectivity of molecule by fragmentation
pattern
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M. (also called the parent peak) gives the
mass of the molecule
Fragments give info on connectivity (i.e.
structure) of the molecule
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Mass Spectrum of methanol CH3OH
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Base peak
m/z mass/charge ratio
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M.
In CHEM 344, we only consider singly charged
species (i.e. z1)
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The most intense peak is referred to as the base
peak
All other peaks measured relative to base peak
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Radicals are not detected by MS
Molecular Ion Cation radical
Cation stability more important than radical
stability
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Mass Spectrum of methanol CH3OH
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CH2OH Base peak
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CH3OH.
M.
CHO
29
CH3
15
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Mass Spectrum of 2,2,4-trimethylpentane
57
3o carbocation
43
2o carbocation
No M. detected
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Mass Spectrum of 1-butanol
56
31
43
via loss of H2O
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No M. detected
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Mass Spectrum of 2-methyl-2-butanol
59
73
55
43
70
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Fragmentation pattern of 2-methyl-2-butanol
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Mass Spectrum of 2-octanone
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43
McLafferty rearragement
via a-cleavage
via a-cleavage
M.
113
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a-cleavage at a CO group
m/z 43
Molecular ion m/z 128
m/z 113
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McLafferty rearrangement of 1-octanone
?
ß
a
1-pentene
Carbonyl compound must possess gamma H-atoms to
undergo McLafferty rearrangement Process always
eliminates a neutral alkene, hence McLafferty
product is a radical cation
McLafferty rearrangement of a carboxylic acid
ethene
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Mass Spectrum of Benzamide
77
105
M.
121
via a-cleavage
via a-cleavage
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Mass Spectrum of Toluene
92
91
Loss of H. Benzyl cation???
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Tropylium cation m/z 91
Toluene radical cation m/z 92
m/z 91 common to all alkyl benzenes