N
X
C
O
N
S
Hal
Solvents
Natural
Products
P Si
C
C
C C
C
C
424
8 Mass Spectrometry
8.11.4 Carboxylic Acid Anhydrides
Fragmentation: In the case of linear anhydrides, abundant acyl ions due to cleavage
next to carbonyl group. For cyclic anhydrides, maxima due to decarboxylation (Δm
44), followed by decarbonylation.
Molecular ion: Weak or absent (especially in linear aliphatic anhydrides), easily
protonated to [M+H] + . Relatively strong for phthalic anhydrides.
8.11.5 Esters and Lactones
Esters of Aliphatic Carboxylic Acids
Fragmentation: Dominant fragmentation of the bonds next to the carbonyl C,
leading to alk-CO + (m/z 43, 57, 71, …; decreasing intensity with increasing length
of the alkyl chain) and followed by decarbonylation, as well as fragmentation to
COOR + (m/z 59, 73, 87, …) and to alk + (m/z 15, 29, 43, …). Alcohol elimination
to give C n H 2n-2 O (m/z 42, 56, 70, …), followed by decarbonylation (Δm 28) or
ketene elimination (Δm 42). Alkene elimination from the acid side via McLafferty
rearrangements, leading to C n H 2n O 2 (m/z 60, 74, 88, …). The larger alkyl group
participates in the rearrangement if several γ-H atoms are available. In the following
example, the alternative process leading to [M-C 2 H 4 ] +· is negligible:
O
H
O CH 3
– C 5 H 10
OH
O CH 3
–
. CH 3
O
O CH 3
+
[M –70]
+ .
+ .
+ .
Nonspecific H rearrangements on the alcohol side (from M +· or the McLafferty
product) lead to C n H 2n O 2 and to the corresponding alkene, C n H 2n (m/z 28, 42,
56, …). In methyl esters of long chain acids, the ions [(CH 2 ) 2+4n COOCH 3 ] + (m/z
87, 143, 199, …) correspond to maxima. For esters of higher alcohols (C n ≥ 3 ), double
H rearrangement to the protonated acid, C n H 2n+1 CO 2 H 2
+ (m/z 61, 75, 89, …).
α-Substituted esters may lose the substituent and then CO (Δm 28) via alkoxyl
rearrangement. Analogously, β-substituted esters may eliminate ketene (Δm 42).
Besides usual ester reactions, specific rearrangements can be observed in formates:
O
H
O
R
2
R
1
– CO , – R
2
(m/z 31 for R
1
: H )
O
H
R
1
H
+
+ .
.
ESI-MS/MS: Elimination of alcohol generates the acyl ion Alk–CO + , which
subsequently loses CO. The cleavage of the Alk–O bond is also an important
reaction. Depending on the proton affinity of the products it leads to the protonated
acid or to the alkyl cation:
[R–COOAlk + H] +
[RCOOH + H] + + [Alk - H]
[R–COOAlk + H] +
Alk + + RCOOH
X
C
O
N
S
Hal
Solvents
Natural
Products
P Si
C
C
C C
C
C
424
8 Mass Spectrometry
8.11.4 Carboxylic Acid Anhydrides
Fragmentation: In the case of linear anhydrides, abundant acyl ions due to cleavage
next to carbonyl group. For cyclic anhydrides, maxima due to decarboxylation (Δm
44), followed by decarbonylation.
Molecular ion: Weak or absent (especially in linear aliphatic anhydrides), easily
protonated to [M+H] + . Relatively strong for phthalic anhydrides.
8.11.5 Esters and Lactones
Esters of Aliphatic Carboxylic Acids
Fragmentation: Dominant fragmentation of the bonds next to the carbonyl C,
leading to alk-CO + (m/z 43, 57, 71, …; decreasing intensity with increasing length
of the alkyl chain) and followed by decarbonylation, as well as fragmentation to
COOR + (m/z 59, 73, 87, …) and to alk + (m/z 15, 29, 43, …). Alcohol elimination
to give C n H 2n-2 O (m/z 42, 56, 70, …), followed by decarbonylation (Δm 28) or
ketene elimination (Δm 42). Alkene elimination from the acid side via McLafferty
rearrangements, leading to C n H 2n O 2 (m/z 60, 74, 88, …). The larger alkyl group
participates in the rearrangement if several γ-H atoms are available. In the following
example, the alternative process leading to [M-C 2 H 4 ] +· is negligible:
O
H
O CH 3
– C 5 H 10
OH
O CH 3
–
. CH 3
O
O CH 3
+
[M –70]
+ .
+ .
+ .
Nonspecific H rearrangements on the alcohol side (from M +· or the McLafferty
product) lead to C n H 2n O 2 and to the corresponding alkene, C n H 2n (m/z 28, 42,
56, …). In methyl esters of long chain acids, the ions [(CH 2 ) 2+4n COOCH 3 ] + (m/z
87, 143, 199, …) correspond to maxima. For esters of higher alcohols (C n ≥ 3 ), double
H rearrangement to the protonated acid, C n H 2n+1 CO 2 H 2
+ (m/z 61, 75, 89, …).
α-Substituted esters may lose the substituent and then CO (Δm 28) via alkoxyl
rearrangement. Analogously, β-substituted esters may eliminate ketene (Δm 42).
Besides usual ester reactions, specific rearrangements can be observed in formates:
O
H
O
R
2
R
1
– CO , – R
2
(m/z 31 for R
1
: H )
O
H
R
1
H
+
+ .
.
ESI-MS/MS: Elimination of alcohol generates the acyl ion Alk–CO + , which
subsequently loses CO. The cleavage of the Alk–O bond is also an important
reaction. Depending on the proton affinity of the products it leads to the protonated
acid or to the alkyl cation:
[R–COOAlk + H] +
[RCOOH + H] + + [Alk - H]
[R–COOAlk + H] +
Alk + + RCOOH