Download Lecture 1 - UW-Madison Chemistry

CHEM 344
Organic Chemistry Lab
Structural Determination of
Organic Compounds
Lecture 1 – Mass Spectrometry
January 20th & 21st 2009
O
H
N
OH
O
OH
O
O
O
O
HO
Acetaminophen
O
O
Geranyl butyrate
Methyl Salicylate
Acetylsalicylic acid
O
O
N
O
DEET
O
N
N
HO
N
N
O
OH
Glutamic acid
O
O
N
H
O
NH2
Caffeine
H3N
O
Aspartame
OH
O
O
O
H
N
H2N
HN
O
HO
NH2
Tamiflu
O
O
H
O
S
N
O
N
CO2H
Amoxicillin
O
O
O
9-THC
Cocaine
MS
Connectivity/Weight
NMR
IR
Detailed connectivity
Functional groups
Structure
Determination
UV-vis, EPR
CD, AA, Raman
X-ray
diffraction
Mass Spectrometry
Uses high energy electron beam (70 eV), sample in gas phase
Ionization potential for most organic molecules 8-15 eV
Like shooting an egg with a shotgun......
and being able to predict the shape of the fragments!
Gives info on molecular weight and formula of compound (m/z, isotopes)
Gives info on connectivity of molecule (fragmentation pattern)
Molecular Ion [M].+
[M].+ gives the mass (m) of the molecule
Molecule
Molecular Fragments
Fragments give info on connectivity (i.e. structure) of the molecule
31
Base peak
m/z = mass/charge ratio
[M]+.
32
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
15
29
Where is the electron lost from?
Alkanes – sigma bond
e
Alkenes – pi bond
e
Heteroatom compounds (O, N, S, etc.) – non-bonding lone pairs
e
O
O
Cation
A
[A-B]
Molecular Ion
Cation radical
Radical
+
B
Radicals and neutral compounds
are NOT detected by MS
A
Radical
+
B
Cation
Mass Spectrum of Octane
43
29
57
71
85
[M].+
114
[CH3CH2CH2CH2CH2CH2CH2CH3]
CH3(CH2)5CH2 + [CH3]
CH3(CH2)5CH2 + [CH3]
mass = 99
m/z = 15
✗
m/z = 99
✗
mass = 15
Both pathwways involve formation of a methyl radical or a methyl cation
Neither are good…..
[CH3CH2CH2CH2CH2CH2CH2CH3]
CH3(CH2)4CH2 + [CH3CH2]
m/z = 85
✔
mass = 29
CH3(CH2)3CH2
m/z = 71 ✔
CH3(CH2)4CH2 + [CH3CH2]
mass = 85
CH3(CH2)2CH2
m/z = 57 ✔
m/z = 29 ✔
CH3CH2CH2
m/z = 43 ✔
Mass Spectrum of 2-methylpentane
43
71
29
[M].
+
86
CH3CH2CH2
mass = 43
[CH3]
mass = 15
Branched alkanes fragment at the branch points
H3C
C
H
CH3
m/z = 43 ✔
H3CH2CH2C
CH3
C
H
m/z = 71✔
Mass Spectrum of 1-hexene
41
43
[M].
+
84
e
+
m/z = 41
✔
Resonance-stabilized allyl cation – typical for alkenes
Mass Spectrum of 1-butanol
56
31
43
29
No [M].+ detected
OH
H2C OH
H2C OH +
m/z = 31 ✔
OH
CH2
H H
m/z = 56
✔
+ H2O
Mass Spectrum of 1-butanol
56
31
43
29
via loss of H2O
Mass Spectrum of 2-octanone
58
43
via a-cleavage
via a-cleavage
[M].+
113
a-cleavage at a C=O group
Me C O
+
m/z = 43
✔
O
Molecular ion
m/z =
m/z = 128
R
C
O
✔
C O
+ CH3
113
R
C
O
Resonance stabilized acylium cation
Mass Spectrum of 2-octanone
58
43
via a-cleavage
via McLafferty rearrangement
via a-cleavage
[M].+
113
McLafferty rearrangement of 2-octanone
H
H
O
m/z = 128
Carbonyl compound must possess
O
+
m/z = 58
g H-atoms to undergo a McLafferty rearrangement
Process always eliminates a neutral alkene, hence McLafferty product is a radical cation
Mass Spectrum of Ethylbenzene
91
106
Et
Et
CH2
+ CH3
m/z = 106
CH2
CH2
Benzylic carbocation
Tropylium carbocation (aromatic)
m/z = 91
Summary
Mass spectrometry – smashing electrons into a molecule
Molecular ion – radical cation; breaks up predictably; stability
Alkanes - “unzipping” (-14); branch points
Alkenes - allyl cation (m/z = 41)
Alcohols - eliminate H2O (-18); oxonium cation
Carbonyls - alpha cleavage; McLafferty if gamma protons
Alkyl benzene - tropylium cation (m/z = 91)
Resonance is good
Pages 418 - 435 Solomons & Fryhle UW Edition