N
X
C
Hal
O
N
S
P Si
Natural
Products
Solvents
C
C
C
C C
C
8.8 Alcohols, Ethers, and Related Compounds
395
8.8.3 Ethers
Aliphatic Ethers [5,6]
Fragmentation: Homolysis of the C–C bond next to the O atom to yield oxygencontaining fragments. Preferably, the bond at the highest substituted C atom breaks
and the larger alkyl group is lost.
O
R
1
R
3
R
2
O CH
R
1
+
R
2
O CH
R
1
+
R
2
C n H 2n+ 1 O
+
, m/z 31, 45, 59, ...
– R
3
+ .
.
This homolysis is followed by the elimination of alkenes, aldehydes, or, less importantly, of water.
O CH
R
1
+
R
2
H
– R
1
CH =CH 2
H O=CH R
2
[30 + R
2
]
+
+
O CH
R
1
+
R
2
– R
2
CH =O
R
1
CH 2 CH 2
+
m/z 29, 43, 57, ...
As a competing process, especially with increasing molecular weight, heterolysis
at the O atom takes place to yield strong alkyl ion signals. The larger as well as the
branched alkyl rests are fragmented preferably. The base peak often arises from
heterolysis of the C–O bond:
O
R
1
R
3
R
2
m/z 29, 43, 57, ...
– R
1
CH 2 CH 2 O
.
H 2 C
R
3
R
2
+
+ .
In contrast to the H 2 O elimination from alcohols, the H transfer involved in the
elimination of RCH 2 CH 2 OH from ethers is nonspecific:
O
R
1
R
3
R
2
H
O
R
1
R
3
R
2
+
H
– R
1
CH 2 CH 2 O H
R
3
CH=R
2
m/z 28, 42, 56, ...
+ .
+ .
.
ESI-MS/MS: Alcohol elimination after cleavage of the C–O bond is a common
process. Depending on the proton affinities of the products, it results either in a
carbocation or, by alkene elimination, in a protonated alcohol:
[R 1 OR 2 + H] +
[R 1 ] + + R 2 OH
[R 1 OR 2 + H] +
[R 1 OH + H] + + [R 2 - H]
If both products can be observed, the sum of their masses is M+2 (M+H+1).
Ion series: Alkyl fragments, C n H 2n+1 (m/z 29, 43, 57, …), with maxima due to
cleavage of the C–O bond. Alkene ion series, C n H 2n (m/z 28, 42, 56, …), due
to elimination of alcohol. Oxygen-containing fragments, C n H 2n+1 O (m/z 31, 45,
59, …), with maxima due to cleavage of the C–C bond next to the oxygen.
Intensities: Intensive peaks mainly in the lower mass range.
Molecular ion: Significant or weak. Decreasing with increasing chain length and
branching.
X
C
Hal
O
N
S
P Si
Natural
Products
Solvents
C
C
C
C C
C
8.8 Alcohols, Ethers, and Related Compounds
395
8.8.3 Ethers
Aliphatic Ethers [5,6]
Fragmentation: Homolysis of the C–C bond next to the O atom to yield oxygencontaining fragments. Preferably, the bond at the highest substituted C atom breaks
and the larger alkyl group is lost.
O
R
1
R
3
R
2
O CH
R
1
+
R
2
O CH
R
1
+
R
2
C n H 2n+ 1 O
+
, m/z 31, 45, 59, ...
– R
3
+ .
.
This homolysis is followed by the elimination of alkenes, aldehydes, or, less importantly, of water.
O CH
R
1
+
R
2
H
– R
1
CH =CH 2
H O=CH R
2
[30 + R
2
]
+
+
O CH
R
1
+
R
2
– R
2
CH =O
R
1
CH 2 CH 2
+
m/z 29, 43, 57, ...
As a competing process, especially with increasing molecular weight, heterolysis
at the O atom takes place to yield strong alkyl ion signals. The larger as well as the
branched alkyl rests are fragmented preferably. The base peak often arises from
heterolysis of the C–O bond:
O
R
1
R
3
R
2
m/z 29, 43, 57, ...
– R
1
CH 2 CH 2 O
.
H 2 C
R
3
R
2
+
+ .
In contrast to the H 2 O elimination from alcohols, the H transfer involved in the
elimination of RCH 2 CH 2 OH from ethers is nonspecific:
O
R
1
R
3
R
2
H
O
R
1
R
3
R
2
+
H
– R
1
CH 2 CH 2 O H
R
3
CH=R
2
m/z 28, 42, 56, ...
+ .
+ .
.
ESI-MS/MS: Alcohol elimination after cleavage of the C–O bond is a common
process. Depending on the proton affinities of the products, it results either in a
carbocation or, by alkene elimination, in a protonated alcohol:
[R 1 OR 2 + H] +
[R 1 ] + + R 2 OH
[R 1 OR 2 + H] +
[R 1 OH + H] + + [R 2 - H]
If both products can be observed, the sum of their masses is M+2 (M+H+1).
Ion series: Alkyl fragments, C n H 2n+1 (m/z 29, 43, 57, …), with maxima due to
cleavage of the C–O bond. Alkene ion series, C n H 2n (m/z 28, 42, 56, …), due
to elimination of alcohol. Oxygen-containing fragments, C n H 2n+1 O (m/z 31, 45,
59, …), with maxima due to cleavage of the C–C bond next to the oxygen.
Intensities: Intensive peaks mainly in the lower mass range.
Molecular ion: Significant or weak. Decreasing with increasing chain length and
branching.