N
X
C
Hal
O
N
S
P Si
Natural
Products
Solvents
C
C
C
C C
C
8.11 Carbonyl Compounds
421
8.11.2 Ketones
Aliphatic Ketones
Fragmentation: Cleavage of the bond next to CO is the most important primary
fragmentation. The charge can remain on either side. The acyl ions then lose CO.
McLafferty rearrangement giving rise to C n H 2n O +· ions (m/z 58, 72, 86, …).
Consecutive rearrangements occur if both alkyl chains contain a γ-H atom. Ketoenol tautomerism of the first rearrangement product is not a prerequisite for the
second rearrangement to occur. Oxygen is sometimes indicated by weak signals at
[M-18] +· and m/z 31, 45, 59. Fragmentation of the hydrocarbon chain similar to that
in the corresponding alkanes.
ESI-MS/MS: Water elimination from [M+H] + to [M-OH] + is a common process.
The cleavage of the C–C(=O)-Bond produces either an alkyl cation or, after alkane
elimination, an acyl ion. The sum of the masses of the two products is M (M+H-1).
Ion series: Dominating fragments of the series C n H 2n+1 and C n H 2n-1 O (in both
cases m/z 29, 43, 57, …, but often distinguishable by the intensity of the 13 C
isotope signal), with maxima due to cleavage at the CO group to give acyl ions
and their decarbonylation products. Weaker signals in the series C n H 2n-1 (m/z
41, 55, 69, …). Even-mass maxima, C n H 2n O (m/z 58, 72, 86, …), due to alkene
elimination (McLafferty rearrangement). Usually, m/z 43 (CH 3 CO + ) is strong if an
unsubstituted α-CH 2 group is present.
Intensities: Intensive peaks mainly in the lower mass range.
Molecular ion: Relatively abundant, weak in long-chain and branched ketones.
Unsaturated Ketones
Fragmentation: Cleavage of the bond next to CO, more favorably on the saturated
side, is the most important primary fragmentation. The acyl ion then loses CO. The
McLafferty rearrangement occurs neither when the unsaturated substituents are in
α,β-position nor when the only available γ-hydrogen atom is attached to a doublebonded carbon.
Molecular ion: Relatively abundant.
X
C
Hal
O
N
S
P Si
Natural
Products
Solvents
C
C
C
C C
C
8.11 Carbonyl Compounds
421
8.11.2 Ketones
Aliphatic Ketones
Fragmentation: Cleavage of the bond next to CO is the most important primary
fragmentation. The charge can remain on either side. The acyl ions then lose CO.
McLafferty rearrangement giving rise to C n H 2n O +· ions (m/z 58, 72, 86, …).
Consecutive rearrangements occur if both alkyl chains contain a γ-H atom. Ketoenol tautomerism of the first rearrangement product is not a prerequisite for the
second rearrangement to occur. Oxygen is sometimes indicated by weak signals at
[M-18] +· and m/z 31, 45, 59. Fragmentation of the hydrocarbon chain similar to that
in the corresponding alkanes.
ESI-MS/MS: Water elimination from [M+H] + to [M-OH] + is a common process.
The cleavage of the C–C(=O)-Bond produces either an alkyl cation or, after alkane
elimination, an acyl ion. The sum of the masses of the two products is M (M+H-1).
Ion series: Dominating fragments of the series C n H 2n+1 and C n H 2n-1 O (in both
cases m/z 29, 43, 57, …, but often distinguishable by the intensity of the 13 C
isotope signal), with maxima due to cleavage at the CO group to give acyl ions
and their decarbonylation products. Weaker signals in the series C n H 2n-1 (m/z
41, 55, 69, …). Even-mass maxima, C n H 2n O (m/z 58, 72, 86, …), due to alkene
elimination (McLafferty rearrangement). Usually, m/z 43 (CH 3 CO + ) is strong if an
unsubstituted α-CH 2 group is present.
Intensities: Intensive peaks mainly in the lower mass range.
Molecular ion: Relatively abundant, weak in long-chain and branched ketones.
Unsaturated Ketones
Fragmentation: Cleavage of the bond next to CO, more favorably on the saturated
side, is the most important primary fragmentation. The acyl ion then loses CO. The
McLafferty rearrangement occurs neither when the unsaturated substituents are in
α,β-position nor when the only available γ-hydrogen atom is attached to a doublebonded carbon.
Molecular ion: Relatively abundant.