Microwave synthesis of vinylphosphonic acid and its derivatives

Article abstract of DOI:10.1134/S1070427212040179

Microwave Pyrolysis of β-substituted ethylphosphonic acid derivatives was studied. The resulting mixture of vinylphosphonic acid derivatives is similar to that obtained by convective heating.

Full text of DOI:10.1134/S1070427212040179

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2012, Vol. 85, No. 4, pp. 639−643. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © S.B. Zotov, M.O. Tuzhikov, O.I. Tuzhikov, T.V. Khokhlova, 2012, published in Zhurnal Prikladnoi Khimii, 2012, Vol. 85, No. 4,
pp. 623−627.
ORGANIC SYNTHESIS
AND INDUSTRIAL ORGANIC CHEMISTRY
Microwave Synthesis of Vinylphosphonic Acid
and Its Derivatives
S. B. Zotov, M. O. Tuzhikov, O. I. Tuzhikov, and T. V. Khokhlova
Volgograd State Technical University, Volgograd, Russia
e-mail: zotov_sb@vstu.ru
Received July 7, 2011
Abstract—Microwave Pyrolysis of β-substituted ethylphosphonic acid derivatives was studied. The resulting
mixture of vinylphosphonic acid derivatives is similar to that obtained by convective heating.
DOI: 10.1134/S1070427212040179
Vinylphosphonic acid and its derivatives have recently
attracted researchers’attention, because conditions were
found for preparing essentially new solid flame retardant
homo- and copolymers of high molecular weight, 105–106
[1], exceeding by 2–3 orders of magnitude the molecular
weight of the previously known products [2]. The most
technologically convenient methods for preparing
vinylphosphonic acid and its derivatives are based on
pyrolysis of such derivatives as β-substituted (mainly
β-acetoxy- or β-chloro-substituted) ethylphosphonic
acid derivatives [3–8]. The pyrolysis is performed at
170–550°С (mainly at 190–270°С) in the presence of
acid or base catalysts. The process occurs within 5–40
h and is accompanied by distillation of low-boiling
products, with the mixture of vinylphosphonic acid
derivatives remaining in the bottoms. The product
composition depends on the reaction conditions. Pyrolysis
of 2-chloroethylphosphonic acid or its aqueous solution
yields a mixture of products containing, along with the
unchanged starting compounds, also the compounds
presented below, including vinylphosphonic acid
polyanhydride [6]:
O
H2
H2
H
C
HC
CH2
OH
HO
O
(
)
C
C
O
HO
P
O
m H,
CH2
HO
P
P
P
CH2
HC
OH
O
OH
O
where m = 1–20.
as bottoms the above-shown product mixture of approxi-
mately the same composition, with relatively complete
release of hydrogen chloride and water [9–11]. The py-
rolysis of dimethyl β-acetoxyethylphosphonate occurs
with higher yield than that of β-chloroethylphosphonic
acid, its aqueous solutions, and bis(β-chloroethyl)
β-chloroethylphosphonate, but even in the presence of
a catalyst the minimal time of pyrolysis of dimethyl
β-acetoxyethylphosphonate is 5.5 h [3], and without
catalyst it is 10–14 h [11]. Other β-substituted ethylphos-
The total content of vinylphosphonic acid and its
derivatives reaches 73%. The distilled-off low-boiling
fraction consists of water and hydrogen chloride.
The pyrolysis of technical-grade 2-chloroethylphos-
phonic acid and of samples of plant growth regulating
agents, Kamposan M (30% aqueous solution of 2-chlo-
roethylphosphonic acid) and 2-CEPA(50% aqueous solu-
tion of 2-chloroethylphosphonic acid), at 220–230°С and
atmospheric pressure without catalyst for 10–15 h yields
639

640
ZOTOV et al.
phonic acid derivatives require considerably longer
time to obtain similar yield of vinylphosphonic acid
derivatives: 12–40 h and more [3–11]. Thus, the most
significant drawback of the existing pyrolysis methods
is long process time.
tion as in thermal pyrolysis of the same substance. The
composition of the distillate of low-boiling products was
determined by elemental analysis and from physico-
chemical constants after the repeated fractionation, and
also by gas chromatography–mass spectrometry. It was
also essentially similar to the composition of the distillate
obtained by thermal pyrolysis. The results of pyrolysis
of β-chloroethylphosphonic acid and β-substituted eth-
ylphosphonates under microwave irradiation are given
in the table.
Microwave irradiation is one of the most promising
ways to intensify the reactions. The major advantage of
using microwave irradiation energy is considerably shorter
reaction time, compared to the reactions performed under
the conditions of traditional convective heating [12–15].
This also concerns organophosphorus synthesis [16].
The data proving the structures of the products
obtained and the compositions of the distillate and
bottoms in microwave pyrolysis of bis(β-chloroethyl)
β-chloroethylphosphonate (1) were reported previously
[9]. Microwave pyrolysis of a melt of technical-grade
β-chloroethylphosphonic acid or of its aqueous solutions
yields bottoms containing a mixture of products similar
in composition to products of common thermal pyrolysis.
Along with vinylphosphonic acid and its polyanhydrides
with various degrees of condensation, it contained the
unchanged β-chloroethylphosphonic acid (up to 5–7%)
and product of its condensation with one of acid groups
of vinylphosphonic acid (β-phosphorylethyl hydrogen
vinylphosphonate).
This study deals with pyrolysis of β-substituted
ethylphosphonic acids and their derivatives under the
conditions of microwave irradiation. Our goal was to
make shorter the time of synthesis of vinylphosphonic
acid derivatives, to compare the composition of the
products with that of thermal pyrolysis products, and
to examine the possibility of using the products ob-
tained for the production of phosphorus-containing fire
retardants and ion-exchange materials. The products
obtained by thermal and microwave pyrolysis were
1
studied by Н and 31Р NMR spectroscopy and by el-
emental analysis, and their bromine and acid numbers
were determined. Pyrolysis was performed by a standard
procedure without catalyst. The starting compounds
were bis(β-chloroethyl) β-chloroethylphosphonate (1),
dimethyl β-acetoxyethylphosphonate (2), technical-
grade β-chloroethylphosphonic acid (3), and commercial
Kamposan (3а) and 2-CEPA (3b). In all the cases, the
process was complete within 4–12 min. A distillate of
low-boiling products and a bottom residue were obtained.
The bottom residue was a mixture of vinylphosphonic
acid derivatives of approximately the same composi-
1
In the Н NMR spectrum of the product, there are
signals from vinyl protons at 6.01, 6.15, and 6.25 ppm.
The signal from the methylene group directly bonded
to the P atom considerably decreases in intensity. The
presence of the vinyl group is confirmed by the presence
in the 1Н NMR spectrum of a typical ABX pattern due
to diastereotopism of НА, НВ, and НХ protons with the
corresponding coupling constants JAB = 2.2, JAX = 17.9,
and JBX = 12.0 Hz. In addition, in the 1Н NMR spectrum
Microwave pyrolysis of β-chloroethylphosphonic acid and its derivatives (220–230°С, air, atmospheric pressure, no catalyst)
Composition of bottom residue, % Composition of low-boiling fractions, %
total vi-
nyl-phos-
phonate
deriva-
tives
dialkyl monoalkyl vinyl-
vinyl- hydrogen phos-
phos- vinyl-phos- phonic atives of vinyl-
phonate phonate acid phos-phonates
pyro
derive-
Starting
compd.
Reaction
time, min
other
HCl water prod-
ucts
methyl
acetate
dimethyl
ether
methanol
1
2
3
–
10
–
54.0
31
–
14.6
10
21.4
43
90
94
77
–
95
–
–
2
–
–
2
–
4
1
1
–
95
–
5–12
4–10
5–10
–
11
66
97
3
50–
30
3а, 3b
–
–
10
63
73
–
–
–
50–70
–
6–10
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 85 No. 4 2012

MICROWAVE SYNTHESIS OF VINYLPHOSPHONIC ACID
641
the triplet of chloromethyl protons (3.65 ppm, J = 5.1 Hz)
considerably decreases in intensity. The 31Р NMR spec-
trum of the products confirms the presence of vinylphos-
phonic acid:Awell-defined signal is observed at 22 ppm.
Potentiometric titration with alkali revealed two jumps
corresponding to neutralization of acid groups of vari-
ous strengths. The shape of the potentiometric titration
curve suggests the presence of a compound containing
dibasic acid fragments. The presence of acid OH groups
In the distillate, within 4–10 min we obtained an aqueous
HCl solution in an amount close to the expected level.
Elimination of HCl with the formation of vinyl groups
was confirmed by the results of double bond determina-
tion in the bottoms (bromide–bromate method) and by
1Н and 31Р NMR spectroscopy.
It should be particularly noted that the total yield of
vinylphosphonates and their derivatives in the micro-
wave synthesis was in all the cases somewhat higher
than in thermal pyrolysis, although we did not examine
the influence of the process temperature on the yield of
the mixture of vinylphosphonic acid derivatives for each
starting compound used. The temperature conditions of
the synthesis were chosen by analogy with those used
most frequently in thermal pyrolysis [3–11]. It should
also be noted that considerable shortening of the pro-
cess time in microwave irradiation could also result in
1
is also confirmed by the Н NMR spectrum (11.55 and
11.96 ppm).
The results of elemental analysis for chlorine (~6–8%)
and phosphorus (~21–30%) and comparison of the bro-
mine number found with the theoretical value suggest
that the pyrolysis products can contain, along with the
above-mentioned compounds, also the following poly-
anhydride structures:
O
O
O
HC
P
CH2
O
CH2
O
HC
OH
O
P
P
P
O
O
P
H
O
O
P
CH2
H2C
CH2
CH2
HC
HC
O
H2C
Cl
CH2
O
Cl
O
P
O
HC CH2
CH2
O
HC
OH
O
P
P
O
O
P
H
O
P
O
CH2
HC
CH2
HC
O
H2C CH2
Cl
O
decreased amount of tars, with a simultaneous increase in
the content of pyrovinylphopshonic acid derivatives. An
increase in the raw material amount loaded (from 10 to
100 g) is accompanied by an increase in the microwave
synthesis time from 4–5 to 10–12 min, but in all the cases
the time required to obtain approximately equal amounts
of a mixture of target products under microwave irradia-
tion is by two and more orders of magnitude shorter than
in common pyrolysis
of the fact that, in this study, experiments were performed
with small amounts of substances (5–100 g), it is hardly
convincing to attribute considerably lower reaction rate at
thermal heating to slow heating of the reactants, because
heating with vigorous stirring on a silicone oil bath at
230–250°С is very fast. We believe that acceleration of
chemical transformations in our process may be due to
the microwave effect, making the time required for the
reaction completion shorter by two orders of magnitude.
By now, there is no common concept of the mechanism
of reaction acceleration under the action of microwave
radiation. Rakhmankulov et al. [14] believe that ac-
celeration of chemical reactions is due to more intense
heat supply to the reaction zone. Based on analysis of
numerous studies, There are substantiations concerning
both the occurrence of microwave catalysis and the pos-
sibility of alternative reaction mechanisms [12]. In view
EXPERIMENTAL
For the pyrolysis, we used samples of technical-grade
2-chloroethylphosphonic acid, technical-grade bis(β-
chloroethyl) β-chloroethylphosphonate, and commercial
2-CEPAproduced by the Khimprom Volgograd Research
Center with the parameters meeting the requirements
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 85 No. 4 2012

642
ZOTOV et al.
the experiments, 220–230°С. Within a few minutes after
switching on the microwave oven, we observed evolution
of vapors of low-boiling products (temperature growth
was monitored with a TP-0198/1 KhA thermocouple 6),
which condensed in a descending water-cooled Liebig
condenser 7. The condensate was collected in a weighed
volumetric test tube 8 and analyzed. After the distillation
completion, the oven was switched off and the bottoms
were cooled, weighed, and analyzed.
of semicommercial production regulations, and also a
sample of Kamposan M produced by Bitterfeld Chemi-
cal Combine (Germany). The starting dimethyl hydrogen
phosphite [TU (Technical Specifications) 6.36.576344-
6–88] was vacuum-distilled with collection of the fraction
boiling at 56–58°С (10 mm Hg), nD20 1.4035. Dimethyl
β-acetoxyethylphosphonate was synthesized as described
in [17]. We obtained the product with bp 116–118°С
(2 mm Hg), nD20 1.4321 {published data: bp 182–194°С
(5–6 mm Hg), nD20 1.4369 [17]; 95–97°С (0.15 mm Hg),
20
nD 1.4316 [18]}. The IR spectra were recorded with
CONCLUSIONS
a Specord M82 spectrometer, and the 1Н NMR spectra,
with a Varian Mercury-300 spectrometer operating at
300 MHz, internal reference HMDS, solvent DMSO;
the 31Р NMR spectra were recorded with a Varian Mer-
cury-300 spectrometer operating at 121.4 MHz, relative
to 85% Н3РО4. The mass spectra were taken with a Varian
МАТ-111 gas chromatograph–mass spectrometer at the
ionizing electron energy of 70 eV.
(1) Microwave irradiation at 220–230°С without
catalyst, compared to convective heating, allows syn-
thesis of vinylphosphonic acid derivatives with the
same product composition but in somewhat higher yield
within 4–12 min instead of 12–40 h required in the case
of convective heating.
(2) The pyrolysis acceleration is attributed to the
microwave effect.
Standard procedure of microwave synthesis. A
weighed two-necked reactor (see figure) was charged with
a sample of the starting organophosphorus compound.
Reactor 1 was attached to Teflon fitting 2 inserted through
the wall of an Elektronika SP01 microwave oven 3 with
М-105-I magnetron (2400 MHz, 630 W). A KIKhА
02.01-С321-I-3-160/3500 thermocouple 4 in glass clad-
ding was inserted through the second neck. The pyrolysis
temperature was controlled with a PID control unit Meta-
kon-523 5. The pyrolysis temperature was constant in all
REFERENCES
1. Wagner, T., Manhart, A., Deniz, N., et al., Macromol.
Chem. Phys., 2009, vol. 210, pp. 1903–1914.
2. Gefter, E.L., Fosfororganicheskie monomery i polimery
(Organophosphorus Monomers and Polymers), Moscow:
Akad. Nauk SSSR, 1960.
3. US Patent 4388252.
4. US Patent 4426336.
5. US Patent 5132444.
6. US Patent 5811575.
7. WO Patent 92/02524.
8. US Patent 6984752 B2.
9. Tuzhikov, O.I., Khokhlova, T.V., and Tuzhikov, M.O., Izv.
Volgogr. Gos. Tekh. Univ.: Mezhvuz. Sb. Nauchn. Tr., no.
1 (39), Ser.: Khim. Tekhnol. Elementoorg. Monom. Polim.
Mater. (Volgograd), 2008, issue 5, pp. 63–67.
10. Tuzhikov, M.O., Zotov, S.B., Tuzhikov, O.I., et al., in
Polimernye materialy ponizhennoi goryuchesti: Trudy
VI Mezhdunarodnoi konferentsii (Polymeric Materials of
Decreased Combustibility: Proc. VI Int. Conf.), Vologda:
Volog. Gos. Tekh. Univ., 2011, pp. 64–67.
11. Zotov, S.B., Tuzhikov, M.O., Tuzhikov, O.I., et al., Izv.
Volgogr. Gos. Tekh. Univ.: Mezhvuz. Sb. Nauchn. Tr., no.
2 (75), Ser.: Khim. Tekhnol. Elementoorg. Monom. Polim.
Mater. (Volgograd), 2011, issue 8, pp. 51–55.
Scheme of laboratory installation for microwave syn-
thesis: (1) reactor, (2) fluoroplastic tube, (3) microwave
oven, (4, 6) thermocouples, (5) PID controller, (7) Liebig
condenser, and (8) receiver.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 85 No. 4 2012

MICROWAVE SYNTHESIS OF VINYLPHOSPHONIC ACID
643
15. Microwave-Assisted Organic Chemistry, Jason, P.., Ed.,
12. Loupy,A., Microwave Synthesis, New York: Wiley–VCH,
Blackwell, 2005.
2002.
16. Kaboudin, B. and Norouzi, H., Abstracts of Papers, XIV
Conf. on the Chemistry of Phosphorus Compounds, Kazan,
2005, p. 68.
13. Kuhnert, N., Angew. Chem. Int. Ed., 2002, vol. 41, no. 11,
pp. 1863–1866.
14. Rakhmankulov, D.L., Bikbulatov, I.Kh., Shulaev, N.S., and
Shavshukova, S.Yu., Mikrovolnovoe izluchenie i intensi-
fikatsiya khimicheskikh protsessov (Microwave Radiation
and Intensification of Chemical Processes), Moscow:
Khimiya, 2003.
17. Brel’,A.K., Rakhimov,A.I., and Mudryi, F.V., Zh. Obshch.
Khim., 1979, vol. 49, no. 3, pp. 714–715.
18. Sasin, R., Olszewski, W.F., Russell, J.R., and Swern, D.,
J. Am. Chem. Soc., 1959, vol. 81, pp. 6275–6277.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 85 No. 4 2012