N
X
C
Hal
O
N
S
P Si
Natural
Products
Solvents
C
C
C
C C
C
8 Mass Spectrometry
8.1 Alkanes [1]
Unbranched Alkanes [2,3]
Fragmentation: Larger alkyl fragments (with C n > 4 ) are chiefly formed by
direct cleavage. They dehydrogenate and undergo substantial H and skeleton
rearrangements. Smaller alkyl fragments (C 2 to C 4 ) are mainly formed by secondary
decomposition of higher alkyl fragments. Eliminations of groups from within the
chain (and recombination of its ends) also occur.
Ion series: Consecutive peaks corresponding to C n H 2n+1 (m/z 29, 43, 57, 71, …),
accompanied by C n H 2n-1 (m/z 27, 41, 55, 69, …) and C n H 2n (m/z 28, 42, 56, 70, …)
of lower intensity.
Intensities: Maximum intensity at m/z 43 or 57; with increasing masses, intensity of
local maxima smoothly decreasing to a minimum at [M-15] + .
Molecular ion: Medium intensity.
Branched Alkanes
Fragmentation: In most cases, apparently simple bond cleavages, preferably at
branched C atoms. The positive charge remains mainly on the branched C atom.
Mechanistically, many H and skeleton rearrangements take place. This is reflected
by the fact that no specific localization of heavy isotopes is possible.
CH CH 2 R
2
R
3
R
1
CH CH 2 R
2
+
R 1
C CHR
2
H
R
1
+
– R
3
– R
3
H
.
+ .
.
ESI-MS/MS: Alkane elimination from [M+H] + :
[R 1 R 2 CHR 3 +H] +
R 3 H + [R 1 R 2 CH] +
[R 1 R 2 R 3 CR 4 +H] +
R 4 H + [R 1 R 2 R 3 C] +
Ion series: Consecutive peaks corresponding to C n H 2n+1 (m/z 29, 43, 57, 71, …),
accompanied by C n H 2n-1 (m/z 27, 41, 55, 69, …) and C n H 2n (m/z 28, 42, 56, 70,
…) of lower intensity.
Intensities: Local intensity maxima at those masses that result from cleavage at
branched C atoms if the charge is localized there. Both C n H 2n+1 and (often more
characteristically) C n H 2n show this tendency.
Molecular ion: Intensity decreasing with increasing degree of branching. No M +· is
observed in highly branched systems.
© The Author(s), under exclusive license to Springer-Verlag GmbH, DE,
part of Springer Nature 2020
E. Pretsch et al., Structure Determination of Organic Compounds,
https://doi.org/10.1007/978-3-662-62439-5_8
375
X
C
Hal
O
N
S
P Si
Natural
Products
Solvents
C
C
C
C C
C
8 Mass Spectrometry
8.1 Alkanes [1]
Unbranched Alkanes [2,3]
Fragmentation: Larger alkyl fragments (with C n > 4 ) are chiefly formed by
direct cleavage. They dehydrogenate and undergo substantial H and skeleton
rearrangements. Smaller alkyl fragments (C 2 to C 4 ) are mainly formed by secondary
decomposition of higher alkyl fragments. Eliminations of groups from within the
chain (and recombination of its ends) also occur.
Ion series: Consecutive peaks corresponding to C n H 2n+1 (m/z 29, 43, 57, 71, …),
accompanied by C n H 2n-1 (m/z 27, 41, 55, 69, …) and C n H 2n (m/z 28, 42, 56, 70, …)
of lower intensity.
Intensities: Maximum intensity at m/z 43 or 57; with increasing masses, intensity of
local maxima smoothly decreasing to a minimum at [M-15] + .
Molecular ion: Medium intensity.
Branched Alkanes
Fragmentation: In most cases, apparently simple bond cleavages, preferably at
branched C atoms. The positive charge remains mainly on the branched C atom.
Mechanistically, many H and skeleton rearrangements take place. This is reflected
by the fact that no specific localization of heavy isotopes is possible.
CH CH 2 R
2
R
3
R
1
CH CH 2 R
2
+
R 1
C CHR
2
H
R
1
+
– R
3
– R
3
H
.
+ .
.
ESI-MS/MS: Alkane elimination from [M+H] + :
[R 1 R 2 CHR 3 +H] +
R 3 H + [R 1 R 2 CH] +
[R 1 R 2 R 3 CR 4 +H] +
R 4 H + [R 1 R 2 R 3 C] +
Ion series: Consecutive peaks corresponding to C n H 2n+1 (m/z 29, 43, 57, 71, …),
accompanied by C n H 2n-1 (m/z 27, 41, 55, 69, …) and C n H 2n (m/z 28, 42, 56, 70,
…) of lower intensity.
Intensities: Local intensity maxima at those masses that result from cleavage at
branched C atoms if the charge is localized there. Both C n H 2n+1 and (often more
characteristically) C n H 2n show this tendency.
Molecular ion: Intensity decreasing with increasing degree of branching. No M +· is
observed in highly branched systems.
© The Author(s), under exclusive license to Springer-Verlag GmbH, DE,
part of Springer Nature 2020
E. Pretsch et al., Structure Determination of Organic Compounds,
https://doi.org/10.1007/978-3-662-62439-5_8
375