N
X
C
O
N
S
Hal
Solvents
Natural
Products
P Si
C
C
C C
C
C
380
8 Mass Spectrometry
8.4 Alicyclics [1]
Cyclopropanes [2,3]
Fragmentation: Generally, spectra of cyclopropanes and alkenes are very similar
because at 70 eV ionization, the ring readily isomerizes to the corresponding alkene
radical cations.
Reaction coordinate
kJ/m ol
C 3 H 5
+
+ H
.
42
≈167
0
205
+ .
+ .
Preferred primary fragmentation by bond cleavage at branched C atoms. Loss
of alkyl residues and of neutral alkenes dominates. The ring of monosubstituted
cyclopropanes is opened exclusively at the 1,2- and not at the 2,3-bond. The
primarily formed double bond is predominantly (for R: OCH 3 ) or exclusively (for
R: H, alk, COOCH 3 ) found in the β,γ-position (even for COOCH 3 , where the α,βunsaturation is thermodynamically more stable).
Molecular ions of cyclopropyl cyanide, allyl cyanide, methacrylonitrile, and pyrrole
rearrange to one common radical cation, most likely that of pyrrole [4].
Ion series: Consecutive maxima corresponding to C n H 2n-1 (m/z 41, 55, 69, 83, …),
accompanied by alkyl and alkenyl ions of the type C n H 2n+1 (m/z 43, 57, 71, 85, …)
and C n H 2n (m/z 42, 56, 70, 84, …), mostly of lower intensity.
Intensities: Dominant peaks in the low mass range, peaking around C 4 . Local evenmass maxima due to alkene eliminations if the resulting double bond is highly
substituted.
Molecular ion: Significant, but not necessarily strong.
Saturated Monocyclic Alicyclics [5]
Fragmentation: Preferred primary fragmentation by bond cleavage at branched C
atoms, followed by loss of alkyl residues and alkenes.
Ion series: Consecutive maxima corresponding to C n H 2n-1 (m/z 41, 55, 69, 83, …),
accompanied by C n H 2n+1 (m/z 43, 57, 71, 85, …) and C n H 2n (m/z 42, 56, 70, 84, …)
of lower intensities. In general, the maxima are so similar to those of alkenes that no
clear distinction is possible.
Intensities: Overall distribution of peaks maximizing in the lower mass range,
around C 4 or C 5 . Local maxima can result from branching effects.
Molecular ion: Significant, mostly of medium intensity.
R
1
2
3
H
R
H
H
+
H
H
R
γ
β
α
+ .
+ .
.
X
C
O
N
S
Hal
Solvents
Natural
Products
P Si
C
C
C C
C
C
380
8 Mass Spectrometry
8.4 Alicyclics [1]
Cyclopropanes [2,3]
Fragmentation: Generally, spectra of cyclopropanes and alkenes are very similar
because at 70 eV ionization, the ring readily isomerizes to the corresponding alkene
radical cations.
Reaction coordinate
kJ/m ol
C 3 H 5
+
+ H
.
42
≈167
0
205
+ .
+ .
Preferred primary fragmentation by bond cleavage at branched C atoms. Loss
of alkyl residues and of neutral alkenes dominates. The ring of monosubstituted
cyclopropanes is opened exclusively at the 1,2- and not at the 2,3-bond. The
primarily formed double bond is predominantly (for R: OCH 3 ) or exclusively (for
R: H, alk, COOCH 3 ) found in the β,γ-position (even for COOCH 3 , where the α,βunsaturation is thermodynamically more stable).
Molecular ions of cyclopropyl cyanide, allyl cyanide, methacrylonitrile, and pyrrole
rearrange to one common radical cation, most likely that of pyrrole [4].
Ion series: Consecutive maxima corresponding to C n H 2n-1 (m/z 41, 55, 69, 83, …),
accompanied by alkyl and alkenyl ions of the type C n H 2n+1 (m/z 43, 57, 71, 85, …)
and C n H 2n (m/z 42, 56, 70, 84, …), mostly of lower intensity.
Intensities: Dominant peaks in the low mass range, peaking around C 4 . Local evenmass maxima due to alkene eliminations if the resulting double bond is highly
substituted.
Molecular ion: Significant, but not necessarily strong.
Saturated Monocyclic Alicyclics [5]
Fragmentation: Preferred primary fragmentation by bond cleavage at branched C
atoms, followed by loss of alkyl residues and alkenes.
Ion series: Consecutive maxima corresponding to C n H 2n-1 (m/z 41, 55, 69, 83, …),
accompanied by C n H 2n+1 (m/z 43, 57, 71, 85, …) and C n H 2n (m/z 42, 56, 70, 84, …)
of lower intensities. In general, the maxima are so similar to those of alkenes that no
clear distinction is possible.
Intensities: Overall distribution of peaks maximizing in the lower mass range,
around C 4 or C 5 . Local maxima can result from branching effects.
Molecular ion: Significant, mostly of medium intensity.
R
1
2
3
H
R
H
H
+
H
H
R
γ
β
α
+ .
+ .
.