Structure of the toxaphene compound 2,5-endo,6-exo,8,9,9,10,10- octachlorobornene-2: A temperature-dependent formation of two rotamers

Article abstract of DOI:10.1021/es040075t

The irradiation of 2,2,3-exo,5-endo,6-exo,8,9,9,10,10-decachlorobornane in n-hexane at 254 nm leads to a spontaneous Cl₂ elimination as the major reaction pathway. This results finally in the main product 2,5-endo,6-exo,8,9,9,10,10-octachlorobornene-2, of which the structure could be elucidated with the help of X-ray, ¹H and ¹³C NMR, IR, and MS. Temperature-dependent ¹H NMR spectroscopic investigations have shown that the -CHCl₂ groups located at C1 and C7 are able to rotate slowly under normal circumstances. If such measurements, however, are exerted at low temperatures (-10 to -60 °C), so can be seen that two rotamers are formed due to the hindrance of the free rotation about the bonds C1-C10, C7-C8, and C7-C9, which for the first time could be revealed for a toxaphene compound. Furthermore, as all ¹H NMR chlorobornane spectra known so far show only sharp and clear signals, it can be assumed that chlorobornane compounds as main toxaphene components have fixed bonds, which requires to indicate chlorine atoms within the tentacles such as "a", "b", and "c" for characterizing their correct position. Those fixed tentacles are probably the reason that many toxaphene congeners remain very stable in environmental compartments, and particularly the biotic and abiotic transformation may strongly be hindered by the inflexibility of the tentacles.

Full text of DOI:10.1021/es040075t

Environ. Sci. Technol. 2005, 39, 1736-1740
insecticides that are found worldwide because of their
Structure of the Toxaphene
Compound 2,5-endo,6-
persistency and spacious transportability in different eco-
systems (1-6). Therein, many of these substances remain
rather stable under specific environmental conditions,
whereas others decay relative easily. This is ascribable to
microbial and photolytic effects, where the predominating
mechanism is a reductive dechlorination or dehydrochlo-
rination and an oxidative decay, which may occur as well
(7-10). Especially the top predating species of the Northern
Hemisphere are affected due to effective bioaccumulation
(11-18). Toxaphene is mutagenic when applied to the Ames
test and is above all very toxic to fish. To date, there is little
known about the mode of action and the toxicity of some
single components (19-23).
exo,8,9,9,10,10-Octachlorobornene-2:
A Temperature-Dependent Formation
of Two Rotamers
H A R U N P A R L A R , * , † , ‡
J U¨ R G E N B U R H E N N E , §
M E H M E T C O E L H A N , ‡ A N D
W A L T E R V E T T E R |
Department of Chemical-Technical Analysis and Chemical
Food Technology, Technical University of Munich,
Weihenstephaner Steig 23,
D-85354 Freising-Weihenstephan, Germany, Research Center
Weihenstephan for Brewing and Food Quality,
Alte Akademie 3, D-Freising-Weihenstephan, Germany,
Department of Internal Medicine VI, Clinical Pharmacology
and Pharmacoepidemiology, University of Heidelberg,
Im Neuenheimer Feld 410, D-69120 Heidelberg, Germany, and
Institute of Food Chemistry, University of Hohenheim,
Garbenstrasse 28, D-70599 Stuttgart, Germany
Toxaphene is distinguished in terms of its dispersion
tendency. Relatively high amounts of residues were found
in different samples taken at areas far away from the region
of application. This shows the high transportability of this
substance, and the most important way of transportation is
obviously via air (24, 25). The toxaphene content in the
atmosphere at areas of Great Lakes in Canada and the United
States has been determined frequently, which has shown
that these regions are strongly affected (26-28).
The production of toxaphene occurs via camphene, which
in turn is the result of a catalytic isomerization of R-pinene.
A permanent supply of chlorine for 8 h to a solution
containing carbon tetrachloride and 20% camphene is leading
under UV irradiation to a final solution, which contains 67-
69% of chlorinated terpenes. Since 1973, work has been
carried out to isolate and to identify single components from
technical toxaphene (29-34). Until now, more than 60
compounds have been identified and elucidated in their
structures. The main products are chlorobornanes, followed
by chlorocamphenes and chlorobornenes (35, 36).
The irradiation of 2,2,3-exo,5-endo,6-exo,8,9,9,10,10-
decachlorobornane in n-hexane at 254 nm leads to a
spontaneous Cl2 elimination as the major reaction pathway.
This results finally in the main product 2,5-endo,6-exo,8,9,9,-
10,10-octachlorobornene-2, of which the structure could
1
be elucidated with the help of X-ray, H and 13C NMR, IR,
1
and MS. Temperature-dependent H NMR spectroscopic
In past years, there has been a controversial discussion
about whether rotating toxaphene isomers can occur due to
hindered rotation about the bonds C1-C10, C7-C8, and
C7-C9. This has also led to the nomenclature “a”, “b”, and
“c” of the valences located at C1, C8, and C9 in order to
express the exact structure (37). For answering correctly this
question, it was necessary to show with the help of an example
the existence of two or more rotamers derived from the same
compound. In this work, spectroscopic data of the toxaphene
compound 2,5-endo,6-exo,8,9,9,10,10-octachlorobornene-2
(Figure 1, compound 2) were used to show that two rotamers
occur at low temperatures.
investigations have shown that the -CHCl2 groups located
at C1 and C7 are able to rotate slowly under normal
circumstances. If such measurements, however, are exerted
at low temperatures (-10 to -60 °C), so can be seen
that two rotamers are formed due to the hindrance of the
free rotation about the bonds C1-C10, C7-C8, and C7-
C9, which for the first time could be revealed for a toxaphene
1
compound. Furthermore, as all H NMR chlorobornane
spectra known so far show only sharp and clear signals,
it can be assumed that chlorobornane compounds as main
toxaphene components have fixed bonds, which requires
to indicate chlorine atoms within the tentacles such as “a”,
“b”, and “c” for characterizing their correct position.
Those fixed tentacles are probably the reason that many
toxaphene congeners remain very stable in environmental
compartments, and particularly the biotic and abiotic
transformation may strongly be hindered by the inflexibility
of the tentacles.
Experimental Section
Materials. The toxaphene compound 2,2,3-exo,5-endo,6-
exo,8,9,9,10,10-decachlorobornane (Figure 1, compound 1)
was isolated in our laboratory by excessive chlorination of
2-exo,10-dichlorobornane (30). All solvents used were of
analytical grade. Petroleum ether (DAB quality, bp 60-80
°C) was purified by distillation over a glass column (1 m in
length). The purity was controlled by HRGC/ECD. Silica gel
(70-230 mesh) was obtained from Merck, Germany.
Introduction
Irradiation Experiments. Five portions weighing 146 mg
each were prepared out of 730 mg of 1 and dissolved in 80
mL of n-hexane. The solutions have been freed from oxygen
by passing through a gentle stream of nitrogen for 10 min
prior to irradiation. In total, 16 quartz tubes (5 mm i.d., 35
cm in length) each containing 5 mL of the sample were placed
circularly in a distance of 2 cm around the light source (Hg
low-pressure lamp, Type Vycor 250 mA; Graentzel, Germany)
and irradiated for 3 h at λmax ) 254 nm.
Toxaphene, a technical mixture of more than 200 polychlo-
rinated C10-terpenes, belongs to those organochlorine
* Corresponding author phone: +49-(0)8161-71-3283; fax: +49-
(0)8161-71-4418; e-mail: parlar@wzw.tum.de.
† Technical University of Munich.
‡ Research Center Weihenstephan for Brewing and Food Quality.
§ University of Heidelberg.
| University of Hohenheim.
9
1736 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 6, 2005
10.1021/es040075t CCC: $30.25
2005 American Chemical Society
Published on Web 02/01/2005

FIGURE 1. Photoinduced reaction of 2,2,3-exo,5-endo,6-exo,8,9,9,10,10-decachlorobornane (1) to 2,5-endo,6-exo,8,9,9,10,10-octachlorobornene-2
(2) and compounds 3 and 4.
X-ray Spectroscopy. An Enray-Nonivs CAD4 V 5.0 was
used; data: XCAD4PC; SHELXS-93, graphic: SCHAKAL 92:
λ ) 0.71073 nm Å Mo KR.
Spectroscopic Data of 1. EI-MS (m/z): 407, 359, 263, 243,
241, 229, 227, 195, 161; FTIR (cm-1): 3100-2900, 1470-1440,
1
1390-1370; H NMR (ppm) (400 MHz): 3.21 (d), 4.44 (d),
5.00 (d), 5.15 (d), 5.34 (s), 5.38 (d), 6.74 (s), 7.05 (d); 13C NMR:
41.9, 57.2, 63.8, 65.3, 66.5, 67.9, 69.7, 71.9, 73.5, 97.3; X-ray,
P1: a ) 8.191, b ) 14.153, c ) 15.130, R ) 109.93°, â ) 93.50°,
γ ) 94.06°, z ) 4, typ: triclin, V ) 1638.0, Mo KR; X-ray,
ω-scan: (3.20 + 0.35 + 0)°, length: 2.44 < 0 < 24, 97°, -9
e h e 0, -16 e k e 16, -17 e l e 17, t ) 30 s, no. of
reflections: 6162.
Spectroscopic Data of 2. EI-MS (m/z): 325, 289, 277, 243,
241, 209, 207, 173; FTIR (cm-1): 3100-2900, 1597, 1470-
1440, 1390-1370; 1H NMR (ppm) (400 MHz): details in Table
2; 13C NMR: 40.0, 55.2, 63.0, 66.2, 67.8, 67.9, 72.0, 73.1, 131.2,
FIGURE 2. HRGC/EI-MS total ion chromatogram of the irradiated
132.0; X-ray, P21/c: a ) 13.890, b ) 7.640, c ) 13.980, R )
solution after 180 min. Retention times: Rt (2) ) 30.2, Rt (3) ) 34.9,
90°, â ) 98.32°, γ ) 90°, z ) 4, typ: monoclin, V ) 1469.3,
Rt (4) ) 36.3.
Mo KR; X-ray, ω-scan: (1.50 + 0.35 + 0)°, length: 1.48 < 0
< 24.97°, -16 e h e 10, -5 e k e 9, -16 e l e 12, t ) 30
Isolation of Photoproducts. The course of photoreaction
was monitored by analyzing one of the samples every 45 min
by HRGC/EI-MS. After 180 min, 1 decayed into traces (Figure
1). Thereafter, all 16 samples were unified, and n-hexane
s, no. of reflections: 3469.
HRGC-FTIR Spectroscopy. A Hewlett-Packard HRGC-
FTIR (5890-5965) for generating the spectra has been applied.
The FTIR system used the same column as in HRGC, and the
was evaporated using a rotary evaporator. For the separation
of the three photoproducts (2-4), a silica gel glass column
(1 m in length, 3 cm i.d., petroleum ether flow rate: 30 mL
min-1) was used, and the initial run of 2000 mL was discarded.
Compound 3 eluted first, of which the elution started at 2250
mL rotation volume and lasted to above 2900 mL. It partially
coeluted with 2, and the elution of this substance started at
2400 mL and finished at 3000 mL. Thereafter, 4 followed,
which eluted completely at 3700 mL. As a result, yields of 30
mg were achieved for 3 and 4 with purities above 95%, as
well as 50 mg for 2 with 99% purity (Figure 2).
experiments were performed under the same conditions.
Results and Discussion
Three compounds (2-4) were formed upon irradiation of 1
at 254 nm in n-hexane and then separated in pure form by
middle-pressure liquid column chromatography. Their struc-
tures were then elucidated with certainty by spectroscopic
methods (1H and 13C NMR, IR, MS) and X-ray structure
analysis (Figure 5). Compound 2 was the result of an
intramolecular Cl2 elimination, whereupon a double bond
is formed between C2 and C3.
The formation of the photodechlorinated products 3 and
4, however, is an intermolecular reaction, where at the first
step a chlorine radical is eliminated and a hydrogen atom
is spontaneously abstracted from the solution. The deha-
logenation takes place at the C2-atom, which is having a
geminal C-Cl2 group. In the irradiated solution, 3 and 4
occur in a molar ratio of ca. 1:1 (Figure 2); therefore, the
radicals formed in the first step can capture a hydrogen radical
in the endo-position as well as in the exo-position from the
n-hexane solution.
High-Resolution Gas Chromatography-Mass Spectros-
copy (HRGC-MS/EI). A Hewlett-Packard (series 5890) gas
chromatograph was used in the following mode: splitless
(0.5 min) to split at 230 °C; injection volume: 1-2 µL; HP
Ultra column: 25 m in length and 0.2 mm i.d.; 0.33 µm;
helium gas flow rate: 1 mL min-1; temperature program:
GC oven initially set at 140 °C, held for 3 min, programmed
at a rate of 4 °C/min to 250 °C, held for 20 min; MS: HP 5988
A; interface: 280 °C; EI ion source: 200 °C with 70 eV.
1H and 13C NMR Spectroscopy. A Bruker AC-400 spec-
trometer has been applied with tetramethylsilane (TMS) as
standard and deuterated chloroform (CDCl3) as the solvent.
Both chemicals were purchased from Aldrich, Germany. All
chemical shifts were referenced to the solvent peak and
recalculated with respect to TMS, δ(1H) )7.24 ppm for CHCl3.
The EI-MS spectrum of 2 does not show M+, M - Cl+, and
M - HCl+ fragments. At first, a group of peaks is shown with
a six-chlorine cluster and a relative intensity of 70% at a
mass of 325. The base peak (100%) lies at mass 277 with five
chlorine atoms. This fragment results from a retro-Diels-
Alder splitting, which is typical for unsaturated six-ring
9
VOL. 39, NO. 6, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 1737

FIGURE 3. 1H NMR spectra of compound 2,5-endo,6-exo,8,9,9,10,10-octachlorobornene-2 (2) in CDCl3 solution, taken at different temperatures.
systems. Thereafter, fragments follow at the masses 241 (13%),
207 (12%), and 173 (8%), deriving from a fully aromatized,
substituted system. Intensive fragments are further registered
at the masses 193 (36%) and 159 (42%), which can be assigned
to chlorinated dihydrotropylium cations. Rather typical as
well is the fragment CHCl2+, observed at the mass 83 and at
the intensity of 87%. The FTIR spectrum of 2 shows above
3000 cm-1 a C-C-H absorption (double bond between
carbon atoms) and below it shows C-H valencies, jutting
out to 2900 cm-1. Typical is the band at 1597 cm-1, which
unambiguously relates to a C-C group. The location of this
band is typical for a monochlorine-substituted double bond
within the ring system. Absorptions between 1470 and 1440
cm-1 as well as 1390 and 1370 cm-1 derive from C-H bonds.
The signals shown in the discoupled 13C NMR spectra (in
total 10 signals) can be definitely assigned to the single carbon
atoms of the bornene system. Furthermore, the spectra, which
are not discoupled, show as well the structure of the bornene
systems (40.0, triplet; 55.2, doublet; 63.0, doublet; 66.2,
doublet; 67.8, doublet; 67.9, doublet; 72.0, doublet; 73.1,
doublet). In the spectra not discoupled are also the olefinic
carbon atoms clearly recognizable, which have been regis-
tered at 132.0 ppm as a singlet and at 131.2 ppm as a doublet.
The 1H NMR spectra of 2 at 23 °C depicts, besides of
sharp olefinic H3 protons, unresolved groups of peaks,
indicating that the bonds between C1-C10, C7-C8, and C7-
C9 are able to rotate still freely although impeded. It can
already be recognized at these temperatures that this solution
contains rotamers, occurring in a molar ratio of 3:5 (Figure
3, Table 1).
To weaken the rotation barrier, the 1H NMR spectra have
been recorded at temperatures of 30 and 45 °C (Figure 3,
Table 1). Looking at these spectra, it is ascertainable that the
peaks become slightly sharper and that the chemical shifts
of the single peak groups lie in the middle of those peaks
being registered at 23 °C. Therefore, it can be concluded that
it must be a single compound, where the protons are definitely
1
assignable. If H NMR spectra are recorded at deep tem-
peratures, from -10 to -30 °C and at -60 °C, then the rotation
becomes more blocked and all protons from both rotamers
are clearly being registered and give sharp signals. It is also
4
worth mentioning that both rotamers do not show the J-
coupling between the protons located at the atoms C8
and C9, which are usually found for this class of sub-
stances.
The olefinic protons localized at C3 of both rotamers are
sharply registered (δ ) 6.43 ppm) at higher temperatures. At
deeper temperatures, however, this signal splits into two
groups of peaks, registered at δ ) 6.45 and 6.47 ppm, which
can be ascribed to both rotamers (Table 1, Figure 3). All other
protons emitted further signals at -10 °C, and their assess-
ment is not easy. Looking at the chemical shifts, some of
these protons are assignable with some restrictions. From
-10 °C onward, the existence of both rotamers 2x and 2y is
clearly recognizable, and some signals can be determined
without problems. It is particularly worth mentioning that
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1738 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 6, 2005

TABLE 1. 1H NMR Chemical Shifts and Coupling Constants According to Temperature of
2,5-endo,6-exo,8,9,9,10,10-Octachlorobornene-2 (2) and Its Rotamers 2x and 2ya
temperature (°C)
Hb Cc
45
30
23
-10
-30
-60
H3
2
6.43 (d)
2x
2y
2
2x
2y
6.43 (d); JH3H4 ) 3.99 6.43 (d); JH3H4 ) 3.99 6.44 (d); JH3H4 ) 3.99
6.43 (d); JH3H4 ) 3.99 6.43 (d); JH3H4 ) 3.99 6.44 (d); JH3H4 ) 3.99
6.47 (d); JH3H4 ) 3.99
6.45 (d); JH3H4 ) 3.99
6.47 (d); JH3H4 ) 3.99
6.45 (d); JH3H4 ) 3.99
H4
H5
3.72 (w)
4.80 (w)
3.78 (w)
3.45 (w)
3.80 (w)
3.65 (w)
3.81 (dd); JH4H3 ) 3.99 3.81 (dd); JH4H3 ) 3.99 3.81 (dd); JH4H3 ) 3.99
6.63 (dd); JH3H4 ) 3.99, 3.63 (dd); JH4H3 ) 3.99, 3.63 (dd); JH4H5 ) 2.85
JH4H5 ) 2.85
JH4H5 ) 2.85
2
2x
4.72 (w)
4.82 (w)
4.69 (w)
4.80 (w)
4.67 (dd); JH5H4 ) 2.85, 4.67 (dd); JH5H4 ) 2.85, 4.67 (dd); JH5H4 ) 2.85,
JH5H6 ) 3.00 JH5H6 ) 3.00 JH6H5 ) 3.00
2y
4.82 (dd); JH5H4 ) 2.85, 4.82 (dd); JH5H4 ) 2.85, 4.82 (dd); JH5H4 ) 2.85,
JH5H6 ) 3.00
JH5H6 ) 3.00
JH5H6 ) 2.00
H6
2
4.34 (w)
4.72 (w)
4.78 (w)
6.32 (w)
6.04 (w)
2x
2y
2
2x
2y
2
2x
2y
2
2x
2y
2
4.35 (w)
4.50 (w)
4.35 (w)
4.28 (w)
4.49 (d); JH4H5 ) 3.00
4.78 (hw)
4.49 (d); JH6H5 ) 3.00
4.28 (d); JH6H5 ) 3.0
4.49 (d); JH6H5 )3.00
H8
4.65 (w)
4.87 (w)
4.65 (w)
4.57
4.60 (d); JH8H8 ) 11.4
4.53 (d); JH8H8 ) 11.4
4.60 (d); JH8H8 ) 11.4
4.53 (d); JH8H8 ) 11.4
4.60 (d); JH8H8 ) 11.4
4.53 (d); JH8H8 ) 11.4
H8′
H9
4.35 (w)
4.70 (w)
4.38 (w)
4.75 (w)
4.24 (w)
4.52; JH8H8 ) 11.4
4.24 (w)
4.52; JH8H8 ) 11.4
4.24 (d); JH8H8 ) 11.4
4.52; JH8H8 ) 11.4
6.35 (w)
6.25 (w)
6.38 (w)
6.28 (w)
6.38 (s)
6.27 (s)
6.38 (s)
6.27 (s)
6.38 (s)
6.27 (s)
H10
2x
2y
5.95
6.05 (w)
5.35 (w)
6.05 (w)
5.86 (s)
6.03 (s)
5.86 (s)
6.03 (s)
5.86 (s)
6.03 (s)
a Presented in the following order: chemical shift δ in ppm (peak characteristic); coupling constant J in Hz. At wide and small peaks, δ and J
are uncertain. s ) singlet, d ) doublet, dd ) double doublet, w ) wide, hw ) half wide. b Hydrogen atom. c Compound.
FIGURE 4. Suggested structures of two rotamers from 2,5-endo,6-
1
exo,8,9,9,10,10-octachlorobornene-2 (2) on the basis of H NMR
spectroscopic data.
TABLE 2. Chemical Shift Differences of Some Protons of the
Rotamers 2x and 2y
protons
∆δ values (Hz)
protons
∆δ values (Hz)
H32x-H32y
H42x-H42y
H52x-H52y
H62x-H62y
+8.55
+79.80
-74.10
-45.71
H82x-H82y
H8′2x-H8′2y
H92x-H92y
H102x-H102y
-114.0
-62.70
+37.05
-68.40
the H6 protons and one of the 2x isomer’s H8 protons start
first to emit sharp signals at -60 °C. Above this temperature,
the signals are broad and not resolvable, so that the coupling
constants belonging to them cannot be measured.
FIGURE 5. Structure of compound 2,5-endo,6-exo,8,9,9,10,10-oc-
tachlorobornene-2 (2) obtained from X-ray measurements.
By taking all findings from the temperature-dependent
1H NMR measurements into consideration as well as the
spacious need of the chlorine atoms within the bornane
system, the structures of the two rotamers as to Figure 4 can
be suggested. The structure of the rotamer 2x is almost similar
with the one being derived according to the X-ray measure-
ment. Provided that the bond between C7-C8 is more flexible
than those between C1-C10 and C7-C9 and that it reaches
first the rotation barrier at -60 °C, it can be assumed that
the two rotamers are only formed when the bond between
C7-C8 is fixed at two different positions where the volu-
minous Cl atoms are as far away from each other as possible.
The correctness of the structures of the rotamers is
significantly supported by 1H NMR spectroscopy. Looking at
the chemical shifts of some single protons, then significant
differences in the δvalues can be recognized, which reinforces
the suggested structures of 2x and 2y (Table 2). In the
structure of 2x, the group C8-Cl is located at position 8c, but
in the isomer 2y, it is located at 8b. Due to this change of
position, the protons H4 and H9 receive a positive ∆δ effect,
9
VOL. 39, NO. 6, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 1739

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∆δ effect. These differences can well be explained with the
help of a simple 3D model by looking on the proximity or
distance of these protons to the C8-Cl group (Figure 5).
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Received for review July 7, 2004. Revised manuscript received
October 20, 2004. Accepted October 20, 2004.
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