N
X
C
O
N
S
Hal
Solvents
Natural
Products
P Si
C
C
C C
C
C
392
8 Mass Spectrometry
8.8 Alcohols, Ethers, and Related Compounds [1,2]
8.8.1 Alcohols and Phenols
Aliphatic Alcohols [3]
Fragmentation: Elimination of water from M +· and from fragments. Strong for
primary alcohols. If an aliphatic H atom can be transferred in a 6-ring process, it
is involved in the water elimination in 90% of the investigated cases. If a CH 2 CH 2
group is attached to the O-bearing C atom, water elimination is often followed by
loss of ethylene. Water elimination is dominant for long-chain alcohols, rendering
their spectra similar to those of alkenes.
HO
H
R
1
R
2
H 2 O
R
1
R
2
+
R
1
R
2
+
– H 2 O
R
1
R
2
– CH 2 CH 2
+ .
+ .
.
.
Cleavage of bonds next to the OH-bearing C atom to form oxonium ions, then
elimination of water and of alkenes. The α-cleavage is often dominant. Usually, its
importance increases with increasing branching at the α-carbon atom. The larger
substituent is lost most readily.
Consecutive H 2 O and alkene eliminations in longer-chain primary alcohols lead to
[M-46] +· , [M-74] +· , [M-102] +· , …. In particular, branched alcohols frequently show
a typical series of fragments at [M-15] + , [M-18] +· , and [M-33] + .
Water elimination from [M+H] + is also a common process in ESI-MS/MS.
Ion series: Dominant alkene ions corresponding to C n H 2n-1 (m/z 41, 55, 69, …),
C n H 2n (m/z 42, 56, 70, …), accompanied by weaker fragments, C n H 2n+1 O (m/z 31,
45, 59, …), with one or more local maxima in the latter series (m/z 31 dominates in
primary alcohols).
Intensities: Intensive peaks in the lower mass range, local maxima among alkenetype fragment ions of the type C n H 2n+1 O + .
Molecular ion: Mostly weak, often missing, especially in tertiary and long-chain
alcohols. Indirect determination of M +· is often possible from the fragments at
[M-15] + , [M-18] +· and [M-33] + . [M+1] + is often significant. In primary and
secondary alcohols also [M-1] + can usually be seen. Sometimes, [M-2] +· is formed
because of oxidation to carbonyl compounds during sample introduction.
Alicyclic Alcohols
Fragmentation: Elimination of water from M +· , followed by loss of alkyl or alkenyl
residues. Ring cleavage at the O-bearing C atom, followed by loss of alkyl residues
after H rearrangement (see scheme).
R
2
C OH
R
1
R
3
– R
3
C OH
+
R
2
R
1
+ .
[CH 2 OH] + (m/z 31) for primary alcohols (R 1 , R 2 : H)
[30 + R 1 ] + (m/z 45, 59, 73, …) for secondary alcohls (R 2 : H)
[29 + R 1 + R 2 ] + (m/z 59, 73, 87, …) for tertiary alcohols
X
C
O
N
S
Hal
Solvents
Natural
Products
P Si
C
C
C C
C
C
392
8 Mass Spectrometry
8.8 Alcohols, Ethers, and Related Compounds [1,2]
8.8.1 Alcohols and Phenols
Aliphatic Alcohols [3]
Fragmentation: Elimination of water from M +· and from fragments. Strong for
primary alcohols. If an aliphatic H atom can be transferred in a 6-ring process, it
is involved in the water elimination in 90% of the investigated cases. If a CH 2 CH 2
group is attached to the O-bearing C atom, water elimination is often followed by
loss of ethylene. Water elimination is dominant for long-chain alcohols, rendering
their spectra similar to those of alkenes.
HO
H
R
1
R
2
H 2 O
R
1
R
2
+
R
1
R
2
+
– H 2 O
R
1
R
2
– CH 2 CH 2
+ .
+ .
.
.
Cleavage of bonds next to the OH-bearing C atom to form oxonium ions, then
elimination of water and of alkenes. The α-cleavage is often dominant. Usually, its
importance increases with increasing branching at the α-carbon atom. The larger
substituent is lost most readily.
Consecutive H 2 O and alkene eliminations in longer-chain primary alcohols lead to
[M-46] +· , [M-74] +· , [M-102] +· , …. In particular, branched alcohols frequently show
a typical series of fragments at [M-15] + , [M-18] +· , and [M-33] + .
Water elimination from [M+H] + is also a common process in ESI-MS/MS.
Ion series: Dominant alkene ions corresponding to C n H 2n-1 (m/z 41, 55, 69, …),
C n H 2n (m/z 42, 56, 70, …), accompanied by weaker fragments, C n H 2n+1 O (m/z 31,
45, 59, …), with one or more local maxima in the latter series (m/z 31 dominates in
primary alcohols).
Intensities: Intensive peaks in the lower mass range, local maxima among alkenetype fragment ions of the type C n H 2n+1 O + .
Molecular ion: Mostly weak, often missing, especially in tertiary and long-chain
alcohols. Indirect determination of M +· is often possible from the fragments at
[M-15] + , [M-18] +· and [M-33] + . [M+1] + is often significant. In primary and
secondary alcohols also [M-1] + can usually be seen. Sometimes, [M-2] +· is formed
because of oxidation to carbonyl compounds during sample introduction.
Alicyclic Alcohols
Fragmentation: Elimination of water from M +· , followed by loss of alkyl or alkenyl
residues. Ring cleavage at the O-bearing C atom, followed by loss of alkyl residues
after H rearrangement (see scheme).
R
2
C OH
R
1
R
3
– R
3
C OH
+
R
2
R
1
+ .
[CH 2 OH] + (m/z 31) for primary alcohols (R 1 , R 2 : H)
[30 + R 1 ] + (m/z 45, 59, 73, …) for secondary alcohls (R 2 : H)
[29 + R 1 + R 2 ] + (m/z 59, 73, 87, …) for tertiary alcohols