Thermal decomposition and isomerization of cis-permethrin and β-cypermethrin in the solid phase

Article abstract of DOI:10.1002/ps.438

The stability to heat of cis-permethrin and β-cypermethrin in the solid phase was studied and the decomposition products identified. Samples heated at 210°C in an oven in the dark showed that, in the absence of potassium chlorate (the salt present in smoke-generating formulations of these pyrethroids), cis-permethrin was not isomerized, although in the presence of that salt, decomposition was greater and thermal isomerization occured. Other salts of the type KXO₃ or NaXO₃, with X being halogen or nitrogen, also led to a considerable thermal isomerization. Heating the insecticides in solution in the presence of potassium chlorate did not produce isomerization in any of the solvents assayed. Salt-catalysed thermal cis-trans isomerization was also found for other pyrethroids derived from permethrinic or deltamethrinic acid but not for those derived from chrysanthemic acid. The main thermal degradation processes of cis-permethrin and β-cypermethrin decomposition when potassium chlorate was present were cyclopropane isomerization, ester cleavage and subsequent oxidation of the resulting products. Permethrinic acid, 3-phenoxybenzyl chloride, alcohol, aldehyde and acid were identified in both cases, as well as 3-phenoxybenzyl cyanide from β-cypermethrin. A similar decomposition pattern occurred after combustion of pyrethroid fumigant formulations.

Full text of DOI:10.1002/ps.438

Pest Management Science
Pest Manag Sci 58:183±189 #online: 2001)
DOI: 10.1002/ps.438
Thermal decomposition and isomerization of
cis-permethrin and b-cypermethrin in the solid
phase
´
Paola Gonzalez Audino, Susana A Licastro and Eduardo Zerba*
Centro de Investigaciones de Plagas e Insecticidas (CIPEIN-CITEFA/CONICET), Zufriategui 4380, Villa Martelli (1603) Buenos Aires,
Argentina
Abstract: The stability to heat of cis-permethrin and b-cypermethrin in the solid phase was studied
and the decomposition products identi®ed. Samples heated at 210°C in an oven in the darkshowed
that, in the absence of potassium chlorate "the salt present in smoke-generating formulations of these
pyrethroids), cis-permethrin was not isomerized, although in the presence of that salt, decomposition
was greater and thermal isomerization occured. Other salts of the type KXO3 or NaXO3, with X being
halogen or nitrogen, also led to a considerable thermal isomerization. Heating the insecticides in
solution in the presence of potassium chlorate did not produce isomerization in any of the solvents
assayed. Salt-catalysed thermal cis±trans isomerization was also found for other pyrethroids derived
from permethrinic or deltamethrinic acid but not for those derived from chrysanthemic acid. The
main thermal degradation processes of cis-permethrin and b-cypermethrin decomposition when
potassium chlorate was present were cyclopropane isomerization, ester cleavage and subsequent
oxidation of the resulting products. Permethrinic acid, 3-phenoxybenzyl chloride, alcohol, aldehyde
and acid were identi®ed in both cases, as well as 3-phenoxybenzyl cyanide from b-cypermethrin. A
similar decomposition pattern occurred after combustion of pyrethroid fumigant formulations.
# 2001 Society of Chemical Industry
Keywords: cis-permethrin; b-cypermethrin; thermal stability; thermal decomposition
1
INTRODUCTION
tion and isomerization products to ensure ef®cient and
safe use of these formulations, as not all the isomers of
a pyrethroid are bioactive.5
Pyrethroids such as allethrin and furamethrin are
widely used as domestic insecticides in the form of
mosquito coils, sprays or vaporization units involving
heating a mat or a liquid. Thus, their pyrolysis
products have been well studied.1±4 The use of more
stable pyrethroids such as cis-permethrin and b-cyper-
methrin in smoke-generating formulations has been
studied.5 These smoke-generating formulations are
nowadays important tools for the control of the cone-
nose bugs #Triatoma infestans Klug), which are vectors
of Chagas' disease.6±10 They are being successfully
tested by now in Argentina in spatial treatments for
Aedes aegypti L, the mosquito vector of Dengue
fever.11
In this work, the stability to heat of cis-permethrin
and b-cypermethrin in the solid phase has been
studied and the decomposition products have been
identi®ed. Both pyrethroids are established as active
principles with high insecticidal activity against T
infestans, the principal vector of Chagas' disease in
Latin America, and they were studied as components
of smoke-generating formulations for vector control.
The factors favouring isomerization during combus-
tion of the smoke-generating mixtures was studied,
particularly the catalytic effect of the inorganic salt
#potassium chlorate) in the mixture.
The recoveries of different pyrethroids from the
smoke of smoke-generating formulations has been
measured5 as well as the in¯uence on their thermal
decomposition of foaming agents and antioxidants
incorporated with the mixture. In addition, the
isomerization process during pyrethroid delivery has
been evaluated.
2
MATERIALS AND METHODS
2.1 Insecticides
cis-Permethrin, [3-phenoxybenzyl #1RS)-cis-3-#2,2-
dichlorovinyl)-2,2-dimethylcyclopropanecarboxylate]
#cis:trans 99:1), was crystallized in our laboratory
according to a patented procedure,12 starting from
commercial permethrin #97.4%; cis:trans 60:40;
Chemotecnica #Argentine). b-Cypermethrin [#RS)-a-
Once conditions for the best recovery and least
isomerization of pyrethroids in the smoke had been
established, it was necessary to know the decomposi-
* Correspondence to: Eduardo Zerba, Centro de Investigaciones de Plagas e Insecticidas (CIPEIN-CITEFA/CONICET), Zufriategui 4380,
Villa Martelli (1603) Buenos Aires, Argentina
E-mail: ezerba@citefa.gov.ar
(Received 12 December 2000; revised version received 28 September 2001; accepted 23 October 2001)
# 2001 Society of Chemical Industry. Pest Manag Sci 1526±498X/2002/$30.00
183

P GonzaÂlez Audino et al
cyano-3-phenoxybenzyl #1RS)-cis-3-#2,2-dibromovi-
nyl)-2,2-dimethylcyclopropanecarboxylate] #95.1%)
was also provided by Chemotecnica #Argentine).
Tetramethrin [#cyclohex-1-ene-1,2-dicarboxymido-
methyl #1RS)-cis-trans-2,2-dimethyl-3-#2-methylprop-
1-enyl)-cyclopropanecarboxylate] #94%), phenothrin
[3-phenoxybenzyl #1RS)-cis-trans-2,2-dimethyl-3-#2-
methylprop-1-enyl)cyclopropanecarboxylate] #cis:trans
15:85) and allethrin [#RS)-3-allyl-2-methyl-4-oxo-
cyclopent-2-enyl #1RS)-cis-trans-2,2-dimethyl-3-#2-
methylprop-1-enyl)cyclopropanecarboxylate]#92.2%)
were all provided by Sumitomo #Japan). Deltamethrin
[#S)-a-cyano-3-phenoxybenzyl #1R, 3R)-cis-3-#2,2-
dibromovinyl)-2,2-dimethylcyclopropanecarboxylate]
#98%), l-cyhalothrin [#S)-a-cyano-3-phenoxybenzyl
#Z)-#1R)-cis-3-#2-chloro-3,3,3-tri¯uoropropenyl)-
2,2-dimethylcyclopropanecarboxylate‡#R)-a-cyano-
3-phenoxybenzyl #Z)-#1S)-cis-3-#2-chloro-3,3,3-tri-
¯uoro-propenyl)-2,2-dimethylcyclopropanecarboxy-
late, 1‡ 1] #99.7%) and b-cy¯uthrin [#SR)-a-cyano-4-
¯uoro-3-phenoxybenzyl #1RS)-cis-3-#2,2-dichlorovi-
nyl)-2,2-dimethylcyclopropanecarboxylate and #SR)-
a-cyano-4-¯uoro-3-phenoxybenzyl #1RS)-trans-3-#2,2-
dichlorovinyl)-2,2-dimethylcyclopropanecarboxylate,
1‡2] #94.5%) were from AgrEvo, ICI and Bayer,
respectively.
alcohol #5g; 0.025mol) in dichloromethane with
thionyl chloride #0.025mol) added dropwise with
vigorous agitation over 4h. The organic layer was
separated and dried over calcium chloride, evaporated
and characterized by GC±MS #see Table 4).
3-Phenoxybenzyl cyanide was prepared14 by treat-
ing 3-phenoxybenzyl chloride #0.02mol) obtained
previously with a solution of potassium cyanide
#0.04mol) in N-N-dimethylformamide‡acetonitrile
#1‡1 by volume) and re¯uxing overnight. The salt
was ®ltered, the precipitate washed and solvent
evaporated. The resulting oil was distilled #bp
137°C/13mmHg) and characterized by GC±MS #see
Table 4).
2.6 Thermal decomposition in solid
phase-potassium chlorate catalysis
Amber glass ampoules #2ml) containing insecticide
#cis-permethrin or b-cypermethrin; 20mg) alone, or
insecticide‡potassium chlorate #80‡20), or insecti-
cide‡potassium chlorate‡dextrin #80‡10‡10), or
other inorganic salts replacing potassium chlorate were
heated in a furnace tube #Thermolyne Type F21100,
USA), held at 210 #Æ0.5)°C. At intervals, samples
were removed, dissolved in dichloromethane and
analyzed by GC±MS.
2.2 Chemicals
2.7 Potassium chlorate-catalyzed isomerization of
other pyrethroids
Other pyrethroids #Fig 1) and related compounds
were heated under the conditions of Section 2.6 in the
presence of potassium chlorate and analyzed by GLC
to study whether these were also isomerized.
All solvents were analytical grade from Sintorgan
#Argentina) and distilled when necessary. The inor-
ganic salts were analytical reagent grade #Aldrich,
USA). Permethrinic acid, 3-phenoxybenzaldehyde
and 3-phenoxybenzyl alcohol were provided by
Â
Chemotecnica #Argentina) and 3-phenoxybenzoic
acid was from Aldrich #USA).
2.8 Isomerization in solution
Solutions of cis-permethrin #1.0g litreÀ1) in solvents of
different polarities #xylene, toluene, acetone, metha-
nol, N,N-dimethylformamide, hexane‡water, chloro-
form‡water, ethanol‡water and dioxan‡water #all
1‡1 by volume) containing potassium chlorate
#0.2g litreÀ1) were re¯uxed for 20min and then
analyzed by GLC as described. A phase-transfer
catalyzer #Aliquat1 336) was also added in two-phase
systems.
2.3 Smoke-generating mixtures
These mixtures where prepared with potassium
chlorate, kaolin, dextrin and a foaming agent as pre-
viously described.5 Dextrin is an important compo-
nent of smoke-generating mixtures.
2.4 Chromatographic analysis
Decomposition and isomerization products were
analyzed by GC against standards in a Varian
Chromatograph 3400 CX, using a 0.53-mm DB-1
column, with temperature programming starting from
70°C for 1min, then for 10°C minÀ1 up to 280°C and
®nally 5min at 280°C, and by GC±MS using a
Finnigan Mat GCQ Trio-2 VG Mass Lab apparatus
#USA) with a 50-m DB-5 column having a 0.25mm
®lm. The column temperature was as before, the
injector temperature 240°C and the detector tem-
perature 280°C. Carrier gas was helium, and electro-
nic impact #EI) of positive ions at 70eV and chemical
ionization #CI) with methane at 0.08mmHg.
3
RESULTS AND DISCUSSION
3.1 Decomposition and isomerization of
cis-permethrin
To avoid photochemical reactions, the sample was
heated at 210°C in the dark. This temperature was
chosen as the combustion temperature of smoke-
generating mixtures as measured by differential scan-
ning calorimetry.5 The percentage thermal decom-
position and isomerization of cis-permethrin in the
presence and absence of potassium chlorate was
plotted against the reaction time. Without potassium
chlorate, cis-permethrin was the major compound
recovered #see Section 3.5). When 200g kgÀ1 potas-
sium chlorate was added, other important peaks
2.5 Preparation of authentic samples
3-Phenoxybenzyl chloride was prepared according to a
published procedure13 by treating 3-phenoxybenzyl
184
Pest Manag Sci 58:183±189 #online: 2001)

Thermal decomposition of pyrethroids
appeared in the GC trace and the decomposition
increased.
permethrin plus potassium chlorate and dextrin and
by cis-permethrin alone #0.70 and 0.56minÀ1 respec-
tively). The rates for appearance of trans-permethrin
followed the same relative order #0.4, 0.3 and
0minÀ1).
No trans-permethrin was formed #by isomerization)
in the absence of the salt at 210°C, and cis-permethrin
was decomposed to other products #Fig 2A). How-
ever, in the presence of potassium chlorate there was
signi®cant isomerization and the amount of trans-
permethrin rose to a maximum and then fell #Fig 2B),
probably due to other decomposition processes. From
the curves of Fig 2A±C, the initial reaction rates for
loss of cis-permethrin and appearance of trans-perme-
thrin were calculated for cis-permethrin alone and in
the presence of potassium chlorate and of potassium
chlorate plus dextrin. The highest rate of cis-perme-
thrin decomposition was observed for cis-permethrin
plus potassium chlorate #1.18minÀ1), followed by cis-
The isomerization process only occurred in the
presence of potassium chlorate and higher amounts of
the latter increasing the percentage of isomerization
Â
#Paola Gonzalez Audino, unpublished results); thus, it
is not exclusively of thermal origin. In the presence of
dextrin, the percentage of isomerization and decom-
position were lower, probably because dextrin reacted
with potassium chlorate during the thermolysis pro-
cess at 210°C.
3.2 Solid phase reaction: catalysis by inorganic
salts
Of the other inorganic salts tested for their in¯uence
on the thermal isomerization of cis-permethrin, only
structures of the type KXO3 or NaXO3, where X is
halogen or nitrogen, gave substantial cis- to trans-
permethrin isomerization #Table 1).
3.3 Potassium chlorate-catalyzed isomerization of
other pyrethroids
Pyrethroids whose acid moiety is permethrinic or
deltamethrinic acid #chlorine and bromine respectively
as substituents on vinyl carbons) and whose alcohol
moiety is 3-phenoxybenzyl alcohol or its cyano
derivative were susceptible to thermal isomerization
by potassium chlorate. This susceptibility was retained
when ¯uorine is introduced as a substituent on the
alcohol moiety #b-cy¯uthrin) #Table 2). Chrysanthe-
mic acid derivatives #methyl substituent on vinyl
carbons) were not susceptible to isomerization.
Neither free permethrinic acid nor its methyl ester
were isomerized.
3.4 Isomerization catalyzed by potassium chlorate
in solution
Xylene, toluene, acetone, methanol, N, N-dimethyl-
formamide and two-phase systems such as hexane±
water, chloroform±water, ethanol±water and dioxan±
water were used to determine whether isomerization
occurred only in the solid state or in solution as well. In
none of the cases studied, including those including a
phase-transfer catalyzer, was any degree of isomeriza-
tion obtained.
3.5 Identification of decomposition products
3.5.1 cis-Permethrin
An oily brown liquid was obtained as a product of
heating. Analysis showed that decomposition was
greater in the presence of potassium chlorate and the
degree of oxidation of products formed was also
increased.
The main reactions observed were isomerization,
ester cleavage and, in the presence of the salt, oxida-
tion of the products formed.
Figure 1. Pyrethroids and precursors submitted to thermal isomerization
with potassium chlorate. Stereochemistry is given in Section 2.1.
Permethrinic acid, 3-phenoxybenzyl chloride, 3-
Pest Manag Sci 58:183±189 #online: 2001)
185

P GonzaÂlez Audino et al
Figure 2. Thermal decomposition and isomerization of cis-permethrin vs time: (A) Cis-permethrin alone; (B) Cis-permethrin‡potassium chlorate (80‡20); (C)
cis-permethrin‡potassium chlorate‡dextrin (80‡10‡10).
phenoxybenzyl alcohol, 3-phenoxybenzoic acid, 3-
phenoxybenzaldehyde and trans-permethrin were
identi®ed by EI and CI GC±MS #Tables 3 and 4)
and co-chromatography with authentic samples.
A peak present in very small amounts, at Rt =20.09,
Figure 4shows a possible thermal-decomposition
scheme for cis-permethrin in the presence and absence
of potassium chlorate. The formation of permethrinic
acid, 3-phenoxybenzaldehyde and 3-phenoxybenzyl
alcohol has also been described for the photochemical
decomposition of permethrin.16 The loss of hydrogen
chloride from permethrin could be the origin of the
chloride derivative formed during thermolysis. 3-
Phenoxybenzyl chloride must be formed prior to
rupture, since 3-phenoxybenzyl alcohol heated to
210°C in the presence of hydrochloric acid did not
produce 3-phenoxybenzyl chloride.
‡
presented an M ion of 318 and a mass spectrum
#m/z =119, m/z =91) that could be compatible with
the o-toluic derivative shown in Fig 3. This compound
might be formed in an aromatization process pre-
viously described for the thermolysis of some esters of
permethrinic acid.15
Table 1. Thermal isomerization of cis-perme-
thrin at 210°C for 30min in the presence of
inorganic salts
Table 2. Thermal isomerization of pyrethroids at 210°C in
the presence of potassium chlorate
Salt ꢀ200g kgÀ1
)
Isomerization ꢀ%)
KClO3
47 ꢀÆ5)
Compound
Isomerization ꢀ%)
a
KNO3
NaIO3
23 ꢀÆ4)
15 ꢀÆ3)
Deltamethrin
30 ꢀÆ4)a
KBrO3
Na NO2
KCl
11 ꢀÆ4)
l-Cyhalothrin
Allethrin
0
0
0
0
0
0
0
0
0
b-Cy¯uthrin
Fenothrin
7 ꢀÆ3)b
NaCr2O7
AlCl3
0
0
0
0
0
Tetramethrin
NaOCH3
KMnO4
NH4ClO4
Furamethrin
Permethrinic acid
Permethrinic acid methyl ester
a
a
Heating was interrupted at 5min owing to
the high rate of decomposition of the pyre-
throid.
Measured as percentage of cis 1‡trans 1‡trans
2
ꢀReference 17).
b Measured as percentage of cis 1‡trans 1.
Product
m/z ꢀrelative abundance)
‡
209 ꢀM 1,35), 173 ꢀMÀ 35, 32), 113 ꢀ100), 112 ꢀ62), 93 ꢀ61)
Permethrinic acid
3-Phenoxybenzyl aldehyde 199 ꢀ100, M 1), 171 ꢀM-CO, 84), 227 ꢀM 29)
‡
‡
‡
‡
3-Phenoxybenzoic acid
3-Phenoxybenzyl alcohol
b-Cypermethrin
215 ꢀ100, M 1), 243 ꢀM 29), 197 ꢀMÀ 17, 12), 153 ꢀ18)
183 ꢀ100, MÀ 17), 201 ꢀ22, M 1)
181 ꢀ100), 183 ꢀ68), 208 ꢀ52), 210 ꢀ82), 191 ꢀ95)a
‡
a
Relative abundance of the ions for the major isomer.
Table 3. CI-MS of identified products
186
Pest Manag Sci 58:183±189 #online: 2001)

Thermal decomposition of pyrethroids
Table 4. EI-MS of identified products
Product
m/z ꢀRelative abundance)
Permethrinic acid
173 ꢀ100, M-Cl), 163 ꢀM-COOH, 35), 127 ꢀ163-HCl, 95), 91 ꢀ127-HCl, 78)
‡
3-Phenoxybenzyl alcohol
3-Phenoxybenzyl chloride
3-Phenoxybenzyl cyanide
3-Phenoxybenzyl aldehyde
3-Phenoxybenzoic acid
Permethrin
200 ꢀM , 100), 171 ꢀ24)
‡
218 ꢀM , 69), 183 ꢀM-Cl, 100)
209 ꢀ100), 210 ꢀ12), 181 ꢀ15.34), 141 ꢀ21.21), 77 ꢀ21.59)
‡
197 ꢀ100), 198 ꢀ11.6, M ), 169 ꢀ5.74) 141 ꢀ21.21), 77 ꢀ21.59)
‡
214 ꢀ100, M ), 196 ꢀ68 M-H2O), 169 ꢀ84 M-COOH)
183 ꢀ100), 184 ꢀ12), 163 ꢀ25), 390 ꢀ<1, IM), 77 ꢀ9)
163 ꢀ100), 165 ꢀ70), 167 ꢀ13), 181 ꢀ61.), 209 ꢀ21), 208 ꢀ12.35), 415 ꢀ2 M )
‡ a
b-Cypermethrin
a Relative abundance of the ions for the major isomer.
The solutions obtained after combustion of the
smoke-generating formulations were analyzed in the
same way.5 In Table 5 the retention times and
percentage areas of thermal decomposition products
formed in the presence and absence of potassium
chlorate are given and compared with the values
obtained in the combustion of smoke-generating
mixtures for cis-permethrin. As can be seen, in
smoke-generating mixtures a similar decomposition
pattern is obtained.
Figure 3. Postulated structure for the unknown decomposition product
arising from cis-permethrin or b-cypermethrin.
Figure 4. Proposed thermal
decomposition pathways for cis-
permethrin at 210°C in the presence of
potassium chlorate.
Areas ꢀ%) a
Product
R ꢀmin)
Without KClO3
With KClO3
After combustion b
Permethrinic acid
10.09
13.19
13.82
14.09
15.12
21.92
0.3
0.3
0.7
0.7
0
0.9
0.7
1.1
0.9
0.3
6.6
0
3-Phenoxybenzyl aldehyde
3-Phenoxybenzyl chloride
3-Phenoxybenzyl alcohol
3-Phenoxybenzoic acid
Permethrinc
3.7
3.3
2.4
98.0
88.5
91.1
a
From cis-permethrin at 210°C during 15min.
In the smoke generating formulation prepared according to Reference 5.
Table 5. Percentage distribution of
decomposition products formed from
cis-permethrin
b
c
cis‡trans.
Pest Manag Sci 58:183±189 #online: 2001)
187

P GonzaÂlez Audino et al
Areas ꢀ%) a
Compound
R ꢀmin)
Without KClO3 With KClO3 After combustion b
Permethrinic acid
10.09
0.4
0.5
Ð
3.1
22.1
2.5
0.1
1.9
0.09
0.5
0.2
Ð
3-Phenoxybenzyl aldehyde 13.19
3-Phenoxybenzoyl cyanidec 13.29
3-Phenoxybenzyl alcohol
3-Phenoxybenzyl cyanide
Unknownd
14.09
0.3
0.1
Ð
Ð
14.88
4.8
20.96
2.9
Total cypermethrin
22.76/22.88
98.7
57.0
97.2
a
From b-cypermethrin at 210°C during 15min.
From the smoke generating formulation.
Structure assigned according to MS:
b
c
Table 6. Percentage distribution of
decomposition products formed from
b-cypermethrin
d
Based on CI and EI-MS, an aromatic structure is postulated ꢀsee Fig 3).
3.5.2 b-Cypermethrin
possible thermal-decomposition scheme for b-cyper-
methrin in the presence and absence of potassium
chlorate.
An oily brown liquid was obtained on heating, analysis
showing that decomposition patterns of b-cyperme-
thrin were similar for those of cis-permethrin.
Permethrinic acid, 3-phenoxybenzyl alcohol, 3-
phenoxybenzoic acid, 3-phenoxybenzaldehyde, 3-
phenoxybenzyl cyanide and cypermethrin isomers
were identi®ed by EI and CI GC±MS #Tables 3 and
4) and co-chromatography with authentic samples. 3-
Phenoxybenzoyl cyanide as characterized by EI and CI
MS #Table 6).
This improved understanding of the thermal beha-
viour of pyrethroid compounds aids the development
of smoke-generating formulations. These formulations
are part of a programme whose objective is to reduce
the ®nancial costs of vector-control programmes and
so protect communities at risk from Chagas' disease.
As for permethrin, a possible aromatization process
was observed #Table 6). Comparison of thermal
decomposition products with those following combus-
tion of the fumigant formulation revealed a similar
decomposition pattern #Table 6). Figure 5 shows a
ACKNOWLEDGEMENTS
This investigation received ®nancial support from the
UNDP/World Bank/WHO Special Programme for
Research and Training in Tropical Diseases and from
Â
Chemotecnica SA #Argentina).
Figure 5. Proposed thermal
decomposition pathways for b-
cypermethrin at 210°C in the presence
of potassium chlorate.
188
Pest Manag Sci 58:183±189 #online: 2001)

Thermal decomposition of pyrethroids
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