N
X
C
O
N
S
Hal
Solvents
Natural
Products
P Si
C
C
C C
C
C
420
8 Mass Spectrometry
8.11 Carbonyl Compounds [1–4]
8.11.1 Aldehydes
Aliphatic Aldehydes [5]
Fragmentation: Cleavage of the bond next to CO. The fragmentation of the
hydrocarbon chain is similar to that in corresponding alkanes. McLafferty
rearrangement with localization of the charge on either side, giving rise to C n H 2n
+·
(m/z 28, 42, 56, …) and, often less important, to C n H 2n O +· ions (m/z 44, 58, 72,
…). At least one product (often both) is significant. Elimination of water from the
molecular ion to give [M-18] +· , occasionally very pronounced.
Ion series: Dominating fragments of the series of C n H 2n+1 and C n H 2n-1 O (in both
cases: m/z 29, 43, 57, …). Weaker signals of the series C n H 2n-1 (m/z 41, 55, 69, …)
and rearrangement products, C n H 2n (m/z 28, 42, 56, …).
Intensities: Intensive peaks concentrated in the lower mass range. Local evenmass maxima from McLafferty-type reactions ([M-44] +· when the aldehyde is not
substituted in α-position).
Molecular ion: Only strong for molecules of low molecular weight; very weak for
C n > 9 . [M-1] + may be more relevant than M +· .
Unsaturated Aliphatic Aldehydes
Fragmentation: Cleavage of the bond next to CO, leading to [M-1] + (more significant
than in saturated aldehydes), [M-29] + , and m/z 29. No McLafferty rearrangement
occurs if the γ-hydrogen atom is attached to a double-bonded carbon or if there is a
double bond in α,β-position.
Ion series: Fragments of the series of C n H 2n-1 and C n H 2n-3 O (in both cases, m/z
41, 55, 69, …).
Molecular ion: Stronger than in saturated aldehydes. Usually, [M-1] + is relevant.
Aromatic Aldehydes
Fragmentation: Characteristic H · loss to yield the corresponding benzoyl ion,
[M-1] + , followed by decarbonylation to a phenyl ion, [M-1-28] + , of lower intensity.
To a small extent also decarbonylation of the molecular ion, leading to [M-28] +· .
Weak signal at m/z 29 (CHO + ).
ESI-MS/MS: Decarbonylation of [M+H] + is common.
Ion series: Aromatic fragments corresponding to C n H n and C n H n±1 (m/z 39, 51–53,
63–65, 75–77, …).
Intensities: Intensive peaks predominantly in the molecular ion region.
Molecular ion: Usually prominent. [M-1] + is strong.
X
C
O
N
S
Hal
Solvents
Natural
Products
P Si
C
C
C C
C
C
420
8 Mass Spectrometry
8.11 Carbonyl Compounds [1–4]
8.11.1 Aldehydes
Aliphatic Aldehydes [5]
Fragmentation: Cleavage of the bond next to CO. The fragmentation of the
hydrocarbon chain is similar to that in corresponding alkanes. McLafferty
rearrangement with localization of the charge on either side, giving rise to C n H 2n
+·
(m/z 28, 42, 56, …) and, often less important, to C n H 2n O +· ions (m/z 44, 58, 72,
…). At least one product (often both) is significant. Elimination of water from the
molecular ion to give [M-18] +· , occasionally very pronounced.
Ion series: Dominating fragments of the series of C n H 2n+1 and C n H 2n-1 O (in both
cases: m/z 29, 43, 57, …). Weaker signals of the series C n H 2n-1 (m/z 41, 55, 69, …)
and rearrangement products, C n H 2n (m/z 28, 42, 56, …).
Intensities: Intensive peaks concentrated in the lower mass range. Local evenmass maxima from McLafferty-type reactions ([M-44] +· when the aldehyde is not
substituted in α-position).
Molecular ion: Only strong for molecules of low molecular weight; very weak for
C n > 9 . [M-1] + may be more relevant than M +· .
Unsaturated Aliphatic Aldehydes
Fragmentation: Cleavage of the bond next to CO, leading to [M-1] + (more significant
than in saturated aldehydes), [M-29] + , and m/z 29. No McLafferty rearrangement
occurs if the γ-hydrogen atom is attached to a double-bonded carbon or if there is a
double bond in α,β-position.
Ion series: Fragments of the series of C n H 2n-1 and C n H 2n-3 O (in both cases, m/z
41, 55, 69, …).
Molecular ion: Stronger than in saturated aldehydes. Usually, [M-1] + is relevant.
Aromatic Aldehydes
Fragmentation: Characteristic H · loss to yield the corresponding benzoyl ion,
[M-1] + , followed by decarbonylation to a phenyl ion, [M-1-28] + , of lower intensity.
To a small extent also decarbonylation of the molecular ion, leading to [M-28] +· .
Weak signal at m/z 29 (CHO + ).
ESI-MS/MS: Decarbonylation of [M+H] + is common.
Ion series: Aromatic fragments corresponding to C n H n and C n H n±1 (m/z 39, 51–53,
63–65, 75–77, …).
Intensities: Intensive peaks predominantly in the molecular ion region.
Molecular ion: Usually prominent. [M-1] + is strong.