N
X
C
Hal
O
N
S
P Si
Natural
Products
Solvents
C
C
C
C C
C
8.6 Heteroaromatics
385
8.6 Heteroaromatic Compounds [1,2]
General Characteristics
Fragmentation: Mostly fragments of aromatic character with specific eliminations
including heteroatoms, e.g., elimination of HCN, CO, CHO, CS, and CHS from
M +· , and of HCN, CO, and CS from fragments. In the case of alkyl-substituted
heteroaromatics, occurrence of benzylic-type cleavage and McLafferty
rearrangements of substituents with C n > 1 as well as specific rearrangements
including heteroatoms, especially in N aromatics.
Ion series: Aromatic fragments, C n H n and C n H n±1 (m/z 39, 51–53, 63–65, …), in the
lower mass range if the necessary number of C atoms is present (no such fragments,
e.g., in pyrazine). Ions including heteroatoms like HCN +· (m/z 27), CH 3 CNH + (m/z
42), and CS +· (m/z 44).
Intensities: Intensive peaks mainly in the higher mass range.
Molecular ion: Generally strong. [M-1] + is often relevant in alkyl-substituted
heteroaromatics.
Furans [3]
Fragmentation: Oxygen can be lost from M +· together with the neighboring C
as CHO (Δm 29). In 2- or 6-methylfurans, CH 3 CO + (m/z 43) can be seen (base
peak in 2,5-dimethylfuran). As in aromatic methyl ethers, [M-43] + is a product of a
two-step reaction: (M +· - CH 3
· - CO). Furans substituted with an alkyl group (C n > 1 ):
benzylic-type cleavage (to pyrylium ion, C 5 H 5 O + , m/z 81), followed by loss of CO.
Ion series: Mainly aromatic hydrocarbon fragments, C n H n and C n H n±1 (m/z 39,
51–53, 63–65, …).
Intensities: Intensive peaks mainly in the higher mass range. The fragments are
usually more important than in purely aromatic hydrocarbons.
Molecular ion: Strong. No pronounced tendency to protonate. Usually, [M-1] + is
very strong in methylfurans.
Thiophenes [4]
Fragmentation: Sulfur can be lost from M +· together with the neighboring C as
CHS (Δm 45) or CS (Δm 44). Typical for thiophenes substituted with an alkyl
group (C n > 1 ) is benzylic-type cleavage followed by loss of CS (Δm 44). Protonated
thiophene (m/z 85) is a characteristic product of monoalkylated thiophenes.
Ion series: Aromatic hydrocarbon fragments, C n H n and C n H n±1 (m/z 39, 51–53,
63–65, …). Besides the isotope peak at [M+2] +· , the signals at m/z 44 and 45 (CS +·
and CHS + ) are indicators of sulfur.
Intensities: Dominant peaks for M +· and products of benzylic-type cleavage.
X
C
Hal
O
N
S
P Si
Natural
Products
Solvents
C
C
C
C C
C
8.6 Heteroaromatics
385
8.6 Heteroaromatic Compounds [1,2]
General Characteristics
Fragmentation: Mostly fragments of aromatic character with specific eliminations
including heteroatoms, e.g., elimination of HCN, CO, CHO, CS, and CHS from
M +· , and of HCN, CO, and CS from fragments. In the case of alkyl-substituted
heteroaromatics, occurrence of benzylic-type cleavage and McLafferty
rearrangements of substituents with C n > 1 as well as specific rearrangements
including heteroatoms, especially in N aromatics.
Ion series: Aromatic fragments, C n H n and C n H n±1 (m/z 39, 51–53, 63–65, …), in the
lower mass range if the necessary number of C atoms is present (no such fragments,
e.g., in pyrazine). Ions including heteroatoms like HCN +· (m/z 27), CH 3 CNH + (m/z
42), and CS +· (m/z 44).
Intensities: Intensive peaks mainly in the higher mass range.
Molecular ion: Generally strong. [M-1] + is often relevant in alkyl-substituted
heteroaromatics.
Furans [3]
Fragmentation: Oxygen can be lost from M +· together with the neighboring C
as CHO (Δm 29). In 2- or 6-methylfurans, CH 3 CO + (m/z 43) can be seen (base
peak in 2,5-dimethylfuran). As in aromatic methyl ethers, [M-43] + is a product of a
two-step reaction: (M +· - CH 3
· - CO). Furans substituted with an alkyl group (C n > 1 ):
benzylic-type cleavage (to pyrylium ion, C 5 H 5 O + , m/z 81), followed by loss of CO.
Ion series: Mainly aromatic hydrocarbon fragments, C n H n and C n H n±1 (m/z 39,
51–53, 63–65, …).
Intensities: Intensive peaks mainly in the higher mass range. The fragments are
usually more important than in purely aromatic hydrocarbons.
Molecular ion: Strong. No pronounced tendency to protonate. Usually, [M-1] + is
very strong in methylfurans.
Thiophenes [4]
Fragmentation: Sulfur can be lost from M +· together with the neighboring C as
CHS (Δm 45) or CS (Δm 44). Typical for thiophenes substituted with an alkyl
group (C n > 1 ) is benzylic-type cleavage followed by loss of CS (Δm 44). Protonated
thiophene (m/z 85) is a characteristic product of monoalkylated thiophenes.
Ion series: Aromatic hydrocarbon fragments, C n H n and C n H n±1 (m/z 39, 51–53,
63–65, …). Besides the isotope peak at [M+2] +· , the signals at m/z 44 and 45 (CS +·
and CHS + ) are indicators of sulfur.
Intensities: Dominant peaks for M +· and products of benzylic-type cleavage.