N
X
C
O
N
S
Hal
Solvents
Natural
Products
P Si
C
C
C C
C
C
386
8 Mass Spectrometry
Molecular ion: Strong. Characteristic S isotope signal ([M+2] +· corresponds to
4.5% of M +· ). No pronounced tendency of protonation. Usually, [M-1] + is very
strong in methylthiophenes.
Pyrroles [5]
Fragmentation: HCN elimination (Δm 27) from M +· and from fragments. In
methylpyrroles, [M-1] + is dominant. Benzylic-type cleavage in C- and N-alkylpyrroles with or without (nonspecific) H rearrangements.
Ion series: Aromatic hydrocarbon fragments, C n H n and C n H n±1 (m/z 39, 51–53,
63–65, …).
Intensities: Dominant peaks for M +· and products of benzylic-type cleavage.
Molecular ion: Strong. Odd mass for odd number of N atoms in the molecule. No
tendency to protonate. In methyl-substituted pyrroles, [M-1] + is dominant.
Pyridines
Fragmentation: HCN elimination (Δm 27) from M +· and from fragments and the ion
H 2 CN + (m/z 28) are characteristic. Additional reactions in 2- or 6-methylpyridines
are CH 3 CN elimination (Δm 41) and the formation of CH 3 CNH + (m/z 42). Benzylic
cleavage is dominant for 3-alkyl-, strong for 4-alkyl-, and weak for 2-alkylpyridines.
Typical rearrangements with participation of the N atom in 2- and 6-alkylpyridine
derivatives.
Intramolecular N-alkylation in 2-alkyl derivatives:
– R
.
N
R
N +
N +
– R
.
N +
N R
– R
.
N +
N
R
m/z 106
m/z 120
m/z 134
+ .
+ .
+ .
McLafferty rearrangements are important in 2- and 4-alkylpyridines:
N H
R
1
R
2
R
2
R
1
–
N H
CH 2
+
+ .
.
Ion series: Aromatic hydrocarbon fragments, C n H n , C n H n±1 and C n H n±1 N (m/z
39–41, 51–54, 63–67, 75–80, …).
Intensities: Dominant peaks for M +· or, if possible, for products of benzylic-type
cleavage.
Molecular ion: Strong, except when benzylic-type cleavage is possible. Odd mass
for odd number of N atoms in the molecule. No tendency to protonate. [M-1] + is
usually present and is strong in alkyl-substituted pyridines.