N
X
C
Hal
O
N
S
P Si
Natural
Products
Solvents
C
C
C
C C
C
8.2 Alkenes
377
8.2 Alkenes [1–4]
Unbranched Alkenes
Fragmentation: Dominant loss of alkyl residues and neutral alkenes. The position
of highly substituted double bonds can be localized because in this case, alkene
eliminations are specific McLafferty-type reactions. Otherwise, double bonds can
be localized in derivatives, such as epoxides and glycols, or by means of low energy
ionization techniques. Branching effects are less characteristic than in isoalkanes.
Alicyclic compounds exhibit very similar spectra.
Ion series: Consecutive peaks corresponding to C n H 2n-1 (m/z 41, 55, 69, 83, …),
accompanied by alkyl and alkene ions, 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 maxima in the lower mass range, peaking around C 4 .
Local even-mass maxima due to alkene eliminations if the double bond is highly
substituted.
Molecular ion: Significant, but not necessarily strong.
Branched Alkenes
Fragmentation: Highly substituted double bonds are less easily displaced than the
unsubstituted ones and give rise to specific alkene eliminations of the McLafferty
type, resulting in significant local maxima corresponding to C n H 2n (see scheme).
The latter may allow to localize the double bond. With unsubstituted double bonds,
no reliable localization is possible and only moderately useful branching effects can
be observed. The branching position is more easily determined after reduction to
an alkane (in situ in GC/MS with H 2 as carrier gas and heated Pt wool as catalyst).
H
R
2
R
1
R
3
R
2
R
1
+
R
3
+ .
+ .
Ion series: Maxima of the alkene type (C n H 2n-1 ; m/z 41, 55, 69, 83, …), accompanied
by weaker alkyl fragments, C n H 2n+1 (m/z 43, 57, 71, 85, …), in the low mass range
and more significant alkene ions, C n H 2n (m/z 42, 56, 70, 84, …).
Intensities: Intensive peaks in the lower mass range. Diagnostically important local
maxima of even mass, frequently also in the higher mass range.
Molecular ion: Usually significant.
X
C
Hal
O
N
S
P Si
Natural
Products
Solvents
C
C
C
C C
C
8.2 Alkenes
377
8.2 Alkenes [1–4]
Unbranched Alkenes
Fragmentation: Dominant loss of alkyl residues and neutral alkenes. The position
of highly substituted double bonds can be localized because in this case, alkene
eliminations are specific McLafferty-type reactions. Otherwise, double bonds can
be localized in derivatives, such as epoxides and glycols, or by means of low energy
ionization techniques. Branching effects are less characteristic than in isoalkanes.
Alicyclic compounds exhibit very similar spectra.
Ion series: Consecutive peaks corresponding to C n H 2n-1 (m/z 41, 55, 69, 83, …),
accompanied by alkyl and alkene ions, 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 maxima in the lower mass range, peaking around C 4 .
Local even-mass maxima due to alkene eliminations if the double bond is highly
substituted.
Molecular ion: Significant, but not necessarily strong.
Branched Alkenes
Fragmentation: Highly substituted double bonds are less easily displaced than the
unsubstituted ones and give rise to specific alkene eliminations of the McLafferty
type, resulting in significant local maxima corresponding to C n H 2n (see scheme).
The latter may allow to localize the double bond. With unsubstituted double bonds,
no reliable localization is possible and only moderately useful branching effects can
be observed. The branching position is more easily determined after reduction to
an alkane (in situ in GC/MS with H 2 as carrier gas and heated Pt wool as catalyst).
H
R
2
R
1
R
3
R
2
R
1
+
R
3
+ .
+ .
Ion series: Maxima of the alkene type (C n H 2n-1 ; m/z 41, 55, 69, 83, …), accompanied
by weaker alkyl fragments, C n H 2n+1 (m/z 43, 57, 71, 85, …), in the low mass range
and more significant alkene ions, C n H 2n (m/z 42, 56, 70, 84, …).
Intensities: Intensive peaks in the lower mass range. Diagnostically important local
maxima of even mass, frequently also in the higher mass range.
Molecular ion: Usually significant.