chem 344 organic chemistry lab january 20 th & 21 st 2009 structural determination of organic...
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CHEM 344Organic Chemistry Lab
January 20th & 21st 2009
Structural Determination of Organic Compounds
Lecture 1 – Mass Spectrometry
HN
HO
O
Acetaminophen
N
N N
N
O
O
Caffeine
O
N
DEET
H3NNH
O
O
O
O
O
Aspartame
NH2
O
HN
O
O
O
Tamiflu
O
OH
O
Methyl Salicylate
NO
O O
O
Cocaine
O
HO
9-THC
H2N
O
HN
N
S
OH
O
H
CO2H
Amoxicillin
O
O OH
O
Acetylsalicylic acid
HO OH
NH2
O
Glutamic acid
O
O
Geranyl butyrate
Structure Determination
Connectivity/WeightMS
X-ray diffraction
NMRDetailed
connectivity
IRFunctional groups
UV-vis, EPR CD, AA, Raman
Mass SpectrometryUses high energy electron beam (70 eV), sample in gas phase
Ionization potential for most organic molecules 8-15 eV
Gives info on molecular weight and formula of compound (m/z, isotopes)
Gives info on connectivity of molecule (fragmentation pattern)
Like shooting an egg with a shotgun......
and being able to predict the shape of the fragments!
M + e [M] + 2 e
Molecular Fragments
Molecule
Molecular Ion [M].+
[M].+ gives the mass (m) of the molecule
Fragments give info on connectivity (i.e. structure) of the molecule
32
31
29
15
Base peak
[M]+.m/z = mass/charge ratio
We will only consider singly charged species (i.e. z =1)
The most intense peak is referred to as the base peak
All other peaks measured relative to base peak
Where is the electron lost from?
O
e
O
eAlkanes – sigma bond
Heteroatom compounds (O, N, S, etc.) – non-bonding lone pairs
Alkenes – pi bond
e
Molecular Ion
Cation radical
Cation Radical
CationRadical
Radicals and neutral compounds
are NOT detected by MS[A-B]
A B+
A B+
Mass Spectrum of Octane
43
7185
5729
MW = 114 Da
Octane
[M].+
114
[CH3CH2CH2CH2CH2CH2CH2CH3]MW = 114 Da
Octane
CH3(CH2)5CH2 + [CH3]
mass = 99 m/z = 15
CH3(CH2)5CH2 + [CH3]
m/z = 99 mass = 15
Both pathwways involve formation of a methyl radical or a methyl cationNeither are good…..
✗ ✗
[CH3CH2CH2CH2CH2CH2CH2CH3]MW = 114 Da
Octane
CH3(CH2)4CH2 + [CH3CH2]
m/z = 85 mass = 29
CH3(CH2)4CH2 + [CH3CH2]
mass = 85 m/z = 29
CH3(CH2)3CH2 CH3(CH2)2CH2
m/z = 71 m/z = 57
CH3CH2CH2
m/z = 43
✔✔
✔✔✔
Mass Spectrum of 2-methylpentane
43
71
[M].
+86
2-methylpentane
MW = 86 Da
29
2-methylpentane
MW = 86 Da
Branched alkanes fragment at the branch points
CH3CH2CH2 CH3C CH3
H
m/z = 43mass = 43 ✔
[CH3] CH3CH2CH2C CH3
Hmass = 15 m/z = 71✔
Mass Spectrum of 1-hexene
41
84
43
[M].
+
1-hexene
MW = 84 Da
e
+
m/z = 41
1-hexene
MW = 84 Da
Resonance-stabilized allyl cation – typical for alkenes
✔
Mass Spectrum of 1-butanol
56
31
43
29
CH3CH2
OH
C4H10O MW = 74
No [M].+ detected
OHHH
CH2 + H2O
m/z = 56 ✔
OH H2C OH H2C OH
m/z = 31
+
✔
Mass Spectrum of 1-butanol
56
31
43
29
CH2=OH
via loss of H2OCH3CH2
OH
C4H10O MW = 74
Mass Spectrum of 2-octanone
OC8H16OMW = 128
113
43
58
O
O
via -cleavage
via -cleavage
[M].+
Molecular ion
m/z = 128
-cleavage at a C=O group
C OC O RR Resonance stabilized acylium cation
O
m/z = 43
+C OMe
✔
m/z = 113
CH3+C O✔
Mass Spectrum of 2-octanone
OC8H16OMW = 128
113
43 OH
58
O
O
via -cleavage
via -cleavage
[M].+
via McLafferty rearrangement
McLafferty rearrangement of 2-octanone
Carbonyl compound must possess H-atoms to undergo a McLafferty rearrangement
Process always eliminates a neutral alkene, hence McLafferty product is a radical cation
OH O
+
m/z = 58m/z = 128
H
Mass Spectrum of Ethylbenzene
91
106
Et
EthylbenzeneMW = 106 Da
Tropylium carbocation (aromatic)
m/z = 91
CH2
Benzylic carbocation
m/z = 106
Et Et CH2
+ CH3
CH2
Summary
Mass spectrometry – smashing electrons into a molecule Molecular ion – radical cation; breaks up predictably; stability
Alkanes - “unzipping” (-14); branch pointsAlkenes - allyl cation (m/z = 41) Alcohols - eliminate H2O (-18); oxonium cation
Carbonyls - alpha cleavage; McLafferty if gamma protonsAlkyl benzene - tropylium cation (m/z = 91)
Resonance is good
Pages 418 - 435 Solomons & Fryhle UW Edition