N
X
C
O
N
S
Hal
Solvents
Natural
Products
P Si
C
C
C C
C
C
390
8 Mass Spectrometry
8.7 Halogen Compounds [1–3]
Saturated Aliphatic Halides
Fragmentation: Loss of halogen radical (I > Br > Cl > F) followed by elimination
of alkenes. Loss of alkyl radical followed by elimination of acid HX. Loss of acid
HX to give an alkene radical cation.
R CH 2 X
Important for F and Cl
W ith successive alkene elimination; im portant for
Br and I
Relevant for F and Cl com pounds of intermediate
chain length and for α-branching
W eak but characteristic halogen indicators
W eak but characteristic halogen indicators
R CH 2
CH 2 =X
+
+
X
+
Alkene
+ .
– HX
– X
.
– CH 2 X
.
– R
.
– RCH 2
.
R
+
+ .
ESI-MS/MS: Fluoro compounds eliminate HF from [M+H] + . Chloro compounds
eliminate HCl and to a lesser extent Cl · from [M+H] + . The loss of Br · is more
common in bromo compounds.
Ion series: The dominant hydrocarbon fragments are mainly alkenyl fragments
(C n H 2n-1 ) for F and Cl, mixed alkyl (C n H 2n+1 ) and alkenyl fragments (C n H 2n-1 ) for
Br, and mainly alkyl fragments (C n H 2n+1 ) for I.
Intensities: Intensive peaks mainly in the lower mass range. Characteristic maxima
for Cl and Br at C 4 H 8 X + (m/z 91, 93 and 135, 137, respectively), which has a cyclic
structure:
X
– R
.
X
+
R
m /z 91, 93 for X : Cl
m /z 135, 137 for X: Br
+ .
Alkyl substituents on the chain reduce the intensity of this fragment. If it is strong,
[M - X] + is weak. In the case of iodoalkanes, some I + and HI +· at m/z 127, 128 is
usually detectable.
Molecular ion: Strong for the smallest alkanes, with increasing intensity in the
sequence F, Cl, Br, I. Decreases rapidly with increasing mass and with increasing
branching. It is negligible for F and Cl if the n-alkyl chains are longer than pentyl,
and for Br and I if they are longer than heptyl and nonyl, respectively. Low tendency
to protonate. Characteristic isotope patterns for Cl and Br. Iodine can be detected
because of its high mass; the 13 C signals of M +· and its fragments are conspicuously
weak.
X
C
O
N
S
Hal
Solvents
Natural
Products
P Si
C
C
C C
C
C
390
8 Mass Spectrometry
8.7 Halogen Compounds [1–3]
Saturated Aliphatic Halides
Fragmentation: Loss of halogen radical (I > Br > Cl > F) followed by elimination
of alkenes. Loss of alkyl radical followed by elimination of acid HX. Loss of acid
HX to give an alkene radical cation.
R CH 2 X
Important for F and Cl
W ith successive alkene elimination; im portant for
Br and I
Relevant for F and Cl com pounds of intermediate
chain length and for α-branching
W eak but characteristic halogen indicators
W eak but characteristic halogen indicators
R CH 2
CH 2 =X
+
+
X
+
Alkene
+ .
– HX
– X
.
– CH 2 X
.
– R
.
– RCH 2
.
R
+
+ .
ESI-MS/MS: Fluoro compounds eliminate HF from [M+H] + . Chloro compounds
eliminate HCl and to a lesser extent Cl · from [M+H] + . The loss of Br · is more
common in bromo compounds.
Ion series: The dominant hydrocarbon fragments are mainly alkenyl fragments
(C n H 2n-1 ) for F and Cl, mixed alkyl (C n H 2n+1 ) and alkenyl fragments (C n H 2n-1 ) for
Br, and mainly alkyl fragments (C n H 2n+1 ) for I.
Intensities: Intensive peaks mainly in the lower mass range. Characteristic maxima
for Cl and Br at C 4 H 8 X + (m/z 91, 93 and 135, 137, respectively), which has a cyclic
structure:
X
– R
.
X
+
R
m /z 91, 93 for X : Cl
m /z 135, 137 for X: Br
+ .
Alkyl substituents on the chain reduce the intensity of this fragment. If it is strong,
[M - X] + is weak. In the case of iodoalkanes, some I + and HI +· at m/z 127, 128 is
usually detectable.
Molecular ion: Strong for the smallest alkanes, with increasing intensity in the
sequence F, Cl, Br, I. Decreases rapidly with increasing mass and with increasing
branching. It is negligible for F and Cl if the n-alkyl chains are longer than pentyl,
and for Br and I if they are longer than heptyl and nonyl, respectively. Low tendency
to protonate. Characteristic isotope patterns for Cl and Br. Iodine can be detected
because of its high mass; the 13 C signals of M +· and its fragments are conspicuously
weak.