To Do’s
•  Read Chapters 8 & 9.
•  Complete the end-of-chapter problems,
8-2, 8-3, 8-4 and 8-5
•  Complete the end-of-chapter problems,
9-2 and 9-3
Answer Keys are available in CHB204H
MS Fragmentation
Molecular ion must be capable of producing the
important ions in the high-mass region of the
spectrum by loss of logical neutral species
What are the important ions?
Structural Analysis and
Fragmentation Data
M
Other data
NMR, IR, UV etc
MW and MF
Predict fragments
computer
Possible
structures
Correct
structure
43
C5H10O MW = 86
71
OH
O
43
59
M+•
71
58
O
85
M+•
M+•
MS Fragmentation
•  Simple bond cleavages and rearrangements
•  Primary and secondary fragmentations
•  Both charged and neutral fragments are
formed
M
e
A B
A
[A B]+
A + B
M
observable
loss of neutral fragment (B)
Neutral fragments are not directly observed.
What Influences Fragmentation?
•  Bond strength (C-H > C-C > C-X)
•  ΔG# (free energy of activation)
•  Stability of fragments
CH3
CH3
A
H3C
C
+
CH3
+
CH3
CH3
?
H3C C CH3
CH3
CH3
B
H3C
C
CH3
Stability of Cations and Radicals
H
H
H
In gas phase, the difference in energy between
ions are more important than neutral species.
37
E
(kcal/mol)
H
Dominant fragmentation pathways often produce
stable cations.
H
H
H
28
18
3.5
3.5
H
H
Stevenson s Rule
[A•B]+•
A• + B+
Consider ionization potential (IE) for A• and B•.
A•
A+ +
electron
IP(A)
B•
B+ +
electron
IP(B)
If IE(A) > IE(B), B will take the positive charge.
A+ + free electron
IP (ionization energy)
A•
MS of Isooctane
57 (t-butyl cation)
CH3
H3C
C
CH3
CH2
H
C CH3
CH3
99 (M-CH3)
MS of 2-Methylhexane
Stability of Cations
N
F
>
N
F
Odd- and Even-electron Ions
O
O
CH3
+
CH3
even
odd
+
OH
odd
H2O
odd
O
+
CO
even
even
odd (with some exceptions)
Br
Br
- Br
- Br
Br
Common Fragment Ions
Common Neutral Fragments
Alkanes
43
CH3
H3C
MW = 114
57
29
71
85 (M-Et)
M+•
•  Sequesntial loss of [CH2]
•  For long-chain alkanes, [M-CH3] is often absent due to the instability of CH3 radial.
Branched Alkanes
[M - CH3]
•  Branching
M+• intensity
Cycloalkanes
83 (M-CH3)
56 (M-28)
M+•
MW = 98
MW = 84
M+•
•  Intense M+•
•  cleavage of side chains
•  loss of ethylene (C2H4 (28))
Charge Localization
•  Suggested by C. Djerrassi to help
interpreting fragmentation pathways
e
HOMO
M
•  Electron is lost from HOMO
M+•
O
O
Cl
Cl
Alkenes and Alkynes
e
C
C
C
fragmentation
C
Loss of CH3, C2H5 etc
M+•
•  It is difficult to locate double bonds because of rapid migration
of charge and radical centers.
•  Alkynes behave similarly.
Notable fragments:
Allyl cation m/z = 41
H C
C CH2
Propagyl cation m/z = 39
C
C R
Hexenes
Hexynes
67 (M-CH3)
MW = 82
67 (M-CH3)
M+•
H
53 (M-Et)
39
39
M-1
CH3
MW = 82
Aromatic Compounds
•  Intense M+• due to their stability
CH3
CH2
+
H
H
H
H
Tropylium cation
m/z = 91
H
Toluene and Propylbenzene
91
91
CH3
MW = 92
M+•
MW = 120
M+•
Tropylium cation
M/z = 91
MS of Isopropylbenzene
105
MW = 120
M+•
CH3
- CH3
m/z = 105
MS of n-Hexylbenzene
91
92
M+•
MW = 162
CH2
H
H
H
H
m/z = 92
+
MS of m-Xylene
CH3
91
M+•
CH3
MW = 106
105
CH3
-H
CH3
m/z = 105
CH3
CH3
- CH3
m/z = 91
?
Tropylium cation
Summary of Hydrocarbons
Alkyl and Aryl Halides (R-X)
(a)  Alkyl Halides
(1)  Loss of X (X = Br and I)
H
(2)  Loss of HX (X = F and Cl)
Bond dissociation energy
C-H (413 kJ/mol) vs
R
X
R
+ HX
H-F (565 kJ/mol)
H-Cl (427 kJ/mol)
H-Br (363 kJ/mol)
X
(3) Formation of
(b) Aryl Halides
Loss of X (Cl, Br and I)
(X = Cl and Br)
X
+
X
MS of Bromooctane
43
57
Br
71
Br
135
29
137
MW = 192
Bromohexane and Iodohexane
85
Br
85
M+•
135 137
I
Br
MW = 164
MW = 212
MS of 2-Chloroheptane
Cl
MW = 134
(loss of HCl)
98
105
Cl
MS of Bromobenzene
77
Br
M+•
MW = 156
M+2
Chlorobenzene and Fluorobenzene
M+•
Cl
77
MW = 112
M+•
F
MW = 96
Summary of Alkyl and Aryl
Halides
Amines
•  α-Cleavage
Relatively low electronegativity of nitrogen
α
β
Stabilized by resonance
N, N-Dimethyl-2-phenylethylamine
CH3
H2C
N
CH3
CH2
M+•
The Nitrogen Rule and
Fragmentation
(a)  Molecules with even #, including zero, nitrogens
MW is an even number, and M+• has even m/z value
Closed shell (even-electron) fragments have odd m/z values
(b)  Molecules with odd # of nitrogens
MW is an odd number, and M+• has odd m/z value
Closed shell (even-electron) fragments have even m/z values
N, N-Diethylaniline
CH2
N
- C 2H 4
CH2
NH
M+•
N-Isopropyl-N -phenyl-p-phenylenediamine
211 (M-CH3)
H
N
H
N
HN
HN
- C 2H 4
MW = 226
H
N
HN
183
M+•
Summary of Amines
Alcohols, Ethers and Phenols
(1) Alcohols
Oxygen is more electronegative than nitrogen
•  α-Cleavage
R'
R'
R
C
+
R
OH
C
R''
R''
R'
R
C
R''
OH
R'
OH
R
+
C
OH
R''
•  Dehydration (M-H2O), both parent ion and fragments
3,3-Dimethyl-2-butanol
OH
- CH3
OH
- H 2O
M+•
Mechanism
OH
- H2O
CH3 +
OH
OH
m/z = 87
m/z = 69
+
OH
OH
allyl cation (m/z = 41)
m/z = 45
H transfer
+
OH
- CH3
and H transfer
OH
m/z = 57
+
OH
m/z = 56
MS of n-Hexanol
OH
MW = 102
H
H
OH
31
- CH3
69
M - H 2O
84
n-Hexyl alcohol and Hexene
OH
Butanols (MW = 74)
31
H
H
H3C
OH
H
45
OH
OH
OH
59
H
59
H3C
H3C
OH
OH
OH
Cyclohexanol
M - H 2O
Alcohols, Ethers and Phenols
(2) Ethers
•  α-Cleavage
R1
R
O
R1
R2
C
R
O
R3
O
R2
C
R3
R2
R1
R
+
C
R1
R
O
R3
+
C
R3
R2
•  C-O cleavage
R1
R
O
C
R2
R3
R
O
R1
+
R2
C
R3
Methyl 2-phenylethyl ether
M+•
O
H
CH2
H H
CH2
Mechanism
+
O
O
m/z = 45
O
O
+
m/z = 91
H H
O
H
O
CH2
m/z = 105
CH2
m/z = 104
+
HO
Ethyl tert-pentyl ether
- C 2H 2
- H 2O
- C 2H 2
- C 2H 2
Alcohols, Ethers and Phenols
(3) Phenols
H
M+•
H H
H H
OH
O
- CO
O
H
H
OH
- HCO
66
65
H H
4-Ethylphenol
OH
M+•
Anisole
M+•
OCH3
65
78
- CO
O
93
Mechanism
O
- CH3
O
O
H
H
- CO
CH3
m/z = 93
O
O
CH2
H
m/z = 65
H
H
CH2
- CH2O
H
H
m/z = 78
4-Ethylanisole
OCH3
M+•
Summary of Alcohols, Ethers
and Phenols
Ketones and Aldehydes
•  α - Cleavage
R
- CO
R
O
+
R'
C
R'
O
R'
•  McLafferty rearrangement
H H
O
R1
H
R3
R2 H
H
R4
O
H
+
CHR4
CHR3
R1
R2
A distonic radical cation (radical and cation are located
on different atoms
4-Octanone
allyl cation
- CO
O
O
- CO
- C 2H 4
ML
M+•
Mechanism
Mechanism - cont.
Decanones (MW = 156)
57
ML
43
ML
O
O
127
M+•
141
M+•
O
71
113
ML
M+•
Cyclohexanone
- C 3H 7
- C 2H 4
- C 2H 4
M+•
•  Intense M+•
•  No McLafferty rearrangement possible.
•  Most fragments are based on secondary fragmentation after α-cleavage.
Mechanism
Hexanal
44
O
56
H
MW = 100
O
H
29
72
- C 2H 4
M+•
Mechanism
H
O
+
H
H O
m/z = 44
H
H
O
+
H
m/z = 56
H O
+
H
H
H
H transfer H H
H O
H
H
H
H
H O
H
H
m/z = 72
Benzaldehyde, MW = 106
77
105
- CO
C
O
M+•
Summary of Ketones and
Aldehydes
Carboxylic Acid Derivatives
(1) Carboxylic acids
•  McLafferty rearrangement
O
H
R'
O
H
+
H
R'
HO
HO
R
R
•  Loss of OH, important for aromatic carboxylic acids
O
R
OH
C
OH
+
O
R
C
- CO
R
Octanoic acid (MW = 144)
60
73
86
85
87
(M-C3H7)
101
(M-C2H5)
115
M+•
Mechanism
H
O
+
OH
O
m/z = 60
OH
H
+
O
OH
m/z = 84
Benzoic acid MW = 122
105
77
C
O
M+•
Carboxylic Acid Derivatives
(2) Esters
•  McLafferty rearrangement
•  α - cleavage
•  C-O cleavage
Ethyl hexanoate, MW = 144
H
O
O
O
O
O
M-Pr
M-Et
Mechanism
Ethyl benzoate, MW = 150
O
- CO
O
H
O
M+•
Carboxylic Acid Derivatives
(3) Amides (similar to esters)
- McLafferty rearangement
- α-cleavage
Octanamide
NH2
H
O
(nitrogen rule!)
NH2
O
44
M+•
N, N-Dimethylbenzamide
105
O
O
N CH3
H3C
77
O
MW = 149
N CH3
- CO
H2C
M+•
Summary of Carboxylic Acid
Derivatives
Other Functional Groups
•  Nitro compounds
•  Sulfur-containing compounds
•  Silanes