N
X
C
Hal
O
N
S
P Si
Natural
Products
Solvents
C
C
C
C C
C
8.11 Carbonyl Compounds
425
Ion series: C n H 2n+1 (m/z 29, 43, 57, …) for the alkyl groups at the ester oxygen
(except for methyl esters). C n H 2n-1 (m/z 27, 41, 55, …). C n H 2n-1 O 2 (m/z 59, 73,
87, ...), exhibiting maxima for n = 4, 8, 12, … (m/z 87, 143, 199, …) in the case
of methyl esters of long-chain acids. Even-mass maxima for C n H 2n O 2 (m/z 60, 74,
88, …) due to alkene elimination via McLafferty rearrangements on both sides of
the carboxyl group. C n H 2n (m/z 28, 42, 56, …) as H rearrangement product from
the alcohol side.
Intensities: Intensive peaks due to the above mentioned ions in the lower mass
range.
Molecular ion: Often of low abundance. Easily protonated to [M+H] + .
Esters of Unsaturated Carboxylic Acids
α,β-Unsaturated esters: Loss of alk–O · followed by CO elimination is the dominant
fragmentation path. Also, loss of the δ-substituent yields a 6-membered oxonium
ring:
O
O CH 3
R
O
O CH 3
– R
.
m/z 113
+
+ .
Significant difference between Z and E isomers of long-chain α,β-unsaturated
esters: Single H rearrangement occurs with Z esters, and double H rearrangements
(leading to protonated acids) have been found for E esters.
β,γ-Unsaturated esters: Only slight qualitative, but significant quantitative differences have been observed as compared to α,β-unsaturated esters (e.g., less intensive
signals for M +· of β,γ- than of α,β-unsaturated esters).
γ,δ-Unsaturated esters: Loss of the alcohol chain as a radical, R · , followed by
ketene elimination.
Aliphatic enol esters and aryl esters: Formation of alk–CO + (m/z 43, 57, 71, …).
Elimination of a ketene to give the enol or phenol radical cation. The rearrangement
occurs prodominantly, but not exclusively, through a 4-membered transition state:
O
H
R
O
– RCH=C=O
O
[M –42]
+ . for R: H
H
+ .
+ .
Esters of Aromatic Carboxylic Acids
Fragmentation: Dominant loss of RO · to form the benzoyl ion, followed by
decarbonylation (Δm 28) and further loss of acetylene (Δm 26). Ethyl esters also
eliminate C 2 H 4 (Δm 28) to give the acid radical cation, which then loses OH ·
to yield the benzoyl ion. In higher alkyl esters, besides the acid, the protonated
acid is formed (double H rearrangement). In ortho-substituted aryl esters with
an α-hydrogen atom on the substituent, an alcohol is eliminated from M +· .
X
C
Hal
O
N
S
P Si
Natural
Products
Solvents
C
C
C
C C
C
8.11 Carbonyl Compounds
425
Ion series: C n H 2n+1 (m/z 29, 43, 57, …) for the alkyl groups at the ester oxygen
(except for methyl esters). C n H 2n-1 (m/z 27, 41, 55, …). C n H 2n-1 O 2 (m/z 59, 73,
87, ...), exhibiting maxima for n = 4, 8, 12, … (m/z 87, 143, 199, …) in the case
of methyl esters of long-chain acids. Even-mass maxima for C n H 2n O 2 (m/z 60, 74,
88, …) due to alkene elimination via McLafferty rearrangements on both sides of
the carboxyl group. C n H 2n (m/z 28, 42, 56, …) as H rearrangement product from
the alcohol side.
Intensities: Intensive peaks due to the above mentioned ions in the lower mass
range.
Molecular ion: Often of low abundance. Easily protonated to [M+H] + .
Esters of Unsaturated Carboxylic Acids
α,β-Unsaturated esters: Loss of alk–O · followed by CO elimination is the dominant
fragmentation path. Also, loss of the δ-substituent yields a 6-membered oxonium
ring:
O
O CH 3
R
O
O CH 3
– R
.
m/z 113
+
+ .
Significant difference between Z and E isomers of long-chain α,β-unsaturated
esters: Single H rearrangement occurs with Z esters, and double H rearrangements
(leading to protonated acids) have been found for E esters.
β,γ-Unsaturated esters: Only slight qualitative, but significant quantitative differences have been observed as compared to α,β-unsaturated esters (e.g., less intensive
signals for M +· of β,γ- than of α,β-unsaturated esters).
γ,δ-Unsaturated esters: Loss of the alcohol chain as a radical, R · , followed by
ketene elimination.
Aliphatic enol esters and aryl esters: Formation of alk–CO + (m/z 43, 57, 71, …).
Elimination of a ketene to give the enol or phenol radical cation. The rearrangement
occurs prodominantly, but not exclusively, through a 4-membered transition state:
O
H
R
O
– RCH=C=O
O
[M –42]
+ . for R: H
H
+ .
+ .
Esters of Aromatic Carboxylic Acids
Fragmentation: Dominant loss of RO · to form the benzoyl ion, followed by
decarbonylation (Δm 28) and further loss of acetylene (Δm 26). Ethyl esters also
eliminate C 2 H 4 (Δm 28) to give the acid radical cation, which then loses OH ·
to yield the benzoyl ion. In higher alkyl esters, besides the acid, the protonated
acid is formed (double H rearrangement). In ortho-substituted aryl esters with
an α-hydrogen atom on the substituent, an alcohol is eliminated from M +· .