Solvent-free microwave synthesis of trialkylphosphates

Article abstract of DOI:10.13005/ojc/300131

Trialkylphosphates are synthesized from alcohols by reaction with reactive phosphorus oxychloride under microwave irradiations and classical heating. Effects of these irradiations on the nature of the products and on the yield are studied. The compounds were characterized by their spectroscopic data and elemental analysis. The obtained results show that the products of these reactions are essentially trialkylphosphates and alkylphosphoric acids. They always show as the nature of the formed products and the yields in trialkylphosphates are comparable to those obtained in the classical conditions of heating. The speed of the reaction was increased by a factor from 40 to 120.

Full text of DOI:10.13005/ojc/300131

ISSN: 0970-020 X
CODEN: OJCHEG
2014, Vol. 30, No. (1):
Pg. 255-259
ORIENTAL JOURNAL OF CHEMISTRY
An International Open Free Access, Peer Reviewed Research Journal
www.orientjchem.org
Solvent-Free Microwave Synthesis of Trialkylphosphates
LAALDJA MEDDOUR-BOUKHOBZA1, ABDELHAMID ELIAS2 and MOHAMED A. DIDI3*
1Faculty of Chemistry, University of Sciences and Technology of Alger, Algeria
2Laboratory of Chemical Engineering, Faculty of Sciences, Boukhalfa University,
Tizi-Ouzou, Algeria.
3Laboratory of Separation and Purification Technology, Tlemcen University,
Faculty of Sciences, Department of Chemistry, Box 119, Algeria.
*Corresponding author E-mail: madidi13@yahoo.fr
http://dx.doi.org/10.13005/ojc/300131
(Received: December 13, 2013; Accepted: January 20, 2014)
ABSTRACT
Trialkylphosphates are synthesized from alcohols by reaction with reactive phosphorus
oxychloride under microwave irradiations and classical heating. Effects of these irradiations on the
nature of the products and on the yield are studied. The compounds were characterized by their
spectroscopic data and elemental analysis. The obtained results show that the products of these
reactions are essentially trialkylphosphates and alkylphosphoric acids. They always show as the
nature of the formed products and the yields in trialkylphosphates are comparable to those obtained
in the classical conditions of heating. The speed of the reaction was increased by a factor from 40
to 120.
Key words: Trialkylphosphates, Alkylphosphoric acids, Microwave-irradiation, solvent free.
INTRODUCTION
These reactions are schematized according
to the Scheme 1 and Scheme 2. The hydrochloric
acid formed by this reaction, constitute the major
inconvenience of this synthesis procedure, because
it reacts with the trialkylphosphate to form the
dialkylphosphoric acid.That generates the decrease
of the synthesis yield in trialkylphosphates.
Trialkylphosphates are synthesized from
alcohols by reaction with reactive phosphorus
substances such as phosphorus oxychloride
(POCl3), Phosphorus pentachloride (PCl5), and
phosphorus pentoxide (P2O5)1-3
.
The main product of the reaction of alcohol
with phosphorus pentachloride is trialkylphosphate.
However, small quantities of dialkylphosphoric acid
can be found in the reactional mixture.
Among the different organophos-phorics,
trialkyl phosphates, PO(OR)3, that are produced in
important amount, are noticeably relevant and are
widely employed in different technological areas as

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BOUKHOBZA et al., Orient. J. Chem., Vol. 30(1), 255-259 (2014)
OH
P=O
OR
OR
OR
Cl
P=O
3 HCl
3 R
OH
RO
P=O
Cl
+
+
R
Cl
+
HCl
+
RO
RO
P=O
OR
OR
Cl
Scheme 1:
Scheme 2:
OR
Cl
OR
O
RO
O
OR
O
P=O
+ 3
N
RO
P=O +3 R
OH
N.HCl
3
+
Cl
P=O
H+
+ ROH
P
P
2 RO
+
RO
OR
OR
Cl
OR
Scheme 3:
Scheme 4:
extractors of rare and radioactive elements, inhibitors
of metal corrosion and polymer burning, additives to
combustible and lubricating materials, brake-liquids,
EXPERIMENTAL
The reactions of synthesis under microwave
irradiation were performed in a microwave oven
Delonghi MW 75 working with the frequency of 2450
etc4-6
.
Other secondary reactions are possible, MHz.
know the condensation reaction which from
monophosphates forms pyrophosphates (Scheme
3).
NMR spectrometry analyses were carried
out on a spectrometer with Fourier Bruker AC 250
multinuclear. The infrared spectra were made on a
Also, to avoid the reaction of desalkylation FT IR Perkin Elmer 16 PC spectrometer.
(scheme 2), the synthesis reaction (scheme 1) often
is realized in the presence of pyridine that sets up
HCl following the scheme 4.
Titrations were monitored with a Tacussel
Mini 80 pH-meter with calomel electrode. The gas
chromatography analyses were carried out on a
The pyridinium hydrochloride is easily Perkin-Elmer chromato-graphic apparatus, equipped
separated from reactional mixture by washing to with OV-17.The temperatures were respectively 250,
water. In the classical conditions of heating, the 200 and 300 °C, in the injection port, column and
synthesis of the trialkylphosphates by this method detector (FID). The carrier gas flow of (N2) is fixed
necessitates several hours (3 – 4 hours) of reaction at 30mL/min. The analyzed products were diluted in
and a domain of the temperatures between 80 and the hexane.
95°C.The slowness (some hours) of these reactions,
which constitues the major drawback of this method Reagent
of synthesis, generates interest in the activation of
Reagents used were phosphorus
these reactions by microwave irradiations. We have oxychloride (POCl3, 98%), 3-methyl-1-butanol
already described the use of microwave activation (C4H9OH, 98%), 2-ethyl-1-hexanol (C8H17OH, 98%),
in phosphorus chemistry7.
octanol (C8H17OH, 98%), decanol (C10H21OH, 98%),
4-(1,1’,3,3’,tetramethylbutyl) phenol (4-ter-octyl
In this work, the heating of the reactional phenol, C8H17-C6H4-OH, 90%), pyridine (C5H5N,
mixtures is carried out by microwave irradiations, 97%), heptane (C7H16, 98%), hexane (C6H14, 99%),
of which the effects on the different operating and sodium hydroxide (NaOH, 99%), from Merck
conditions, the nature of the synthetic products products.
and the trialkylphosphate yields will be studied
and compared to the results obtained in the classic General Procedure
heating.
Hydroxycompounds used in these reactions

BOUKHOBZA et al., Orient. J. Chem., Vol. 30(1), 255-259 (2014)
257
were 3-methyl-1-butanol, 2-ethylhexanol, decanol,
and 4(1,1’,3,3'-tetramethylbutyl)phenol.
= 80°C and P = 1 atm, solvent was eliminated, then
under vacuum (P=2mmHg, T=75°C) to evacuate
alcohol and residual waters.The reactional mixtures
thus obtained are analyzed by different analysis
techniques. The separation of the two obtained
products (the di and trialkylphosphates) was
carried out by chromatography, the pure product
(trialkylphosphate) was characterized.
Phosphorus oxychloride (2.5 g, (16.3
mmol.)) and the hydroxycompounds ( molar ratio:
hydroxycompounds/ phosphorus oxychloride = 3)
were stirred in presence of solvent (heptane), and
in presence or in absence of pyridine (molar ratio:
pyridine/ phosphorus oxychloride = 3).
Physical Properties of the trialkylphosphates
Synthesis ofTri 3-Methyl-1-Butyl Phosphate
(T3M1BP) C15H33O4P, nD20 = 1.434.
The first synthesis step consists in to
add successively in the reactor: the alcohol, the
phosphorus oxychloride in the heptane and the
pyridine. The addition operation was realized
between 0 to 10°C and phosphorus oxychloride
wa added slowly. The reactional mixture obtained
was heated by the continuation by the two following
methods:
31P NMR: δ/H3PO4 (ppm) = 0.08.
1H NMR:δ/TMS (ppm) = 0.95 (t, 18H, CH3),
1.52 (m, 6H, CH2), 1.72 (t, 3H, CH), 4.07 (m, 6H,
CH2O-P).
IR: ν (cm-1) = 1014 (P-O-C), 1237 (P=O),
1368 (CH (CH3)), 1430 (CH (CH3 et CH2)), 2872 &
2959 (CH (CH3 et CH2)). tr (GPC)(min): 6.24.
Classic Conditions of Heating
The mixture above was heated and stirred
under dry argon at 80°C for 2 h.
Synthesis of Tri 2-Ethylhexyl Phosphate
(T1EHP)
Microwave Irradiation
The mixture was irradiated in a glass
cylinder reactor (diameter = 3 cm) fitted by a cooler
filled with cooled octane (0-5°C) according to the
conditions (power and time) reported in Table I.
C24H51O4P, nD20 = 1.446.
31P NMR: δ/H3PO4 (ppm) = -0.6.
1H NMR: δ/TMS (ppm) = 0..89 (t, 18H, CH3), 1.28
(m, 24H, CH2), 1.57 (t, 3H, CH2), 3.93 (m, 6H,
CH2O-P).
The reactional mixtures obtained were
washed to water to separate the chlorhydrate of
pyridine. They next were washed by an aqueous
solutionofsoda(1%)(Volumeratio:NaOH/reactional
mixtures = 1) to separate the secondary products
(mono acids and residual dialkylphosphates). At T
IR: ν (cm-1) = 887 et 1002 (P-O-C), 1263 (al P=O),
1380 (CH (CH3)), 1463 (CH (CH3 & CH2)), 2874,
2928 & 2958 (CH ( CH3 et CH2)).
tr (GPC)(min): 8.59.
Table 1: Conditions of Synthesis and yields in trialkylphosphates
Method
Product
With pyridine
Under microwaves
Without pyridine
Under microwaves Classic
Classic
P(W)
t(s)
T(°C) η(%) η (%) P(W)
t(s)
T(°C) η(%) η (%)
3-Methy-l-butanol
2-Ethylhexan-1-ol
1-Decan-ol
4-(1,1’,3,3’)Tetramethyl
butylphenol
450
270
900
270
10
75
35
79
88
72
97
57
53
62
80
55
75
55
89
630
270
900
25
70
45
82
86
78
60
65
75
87
72
67
80
120
270 210 81 83
T, temperature of the reactional mixture; P, irradiation power; t, time of irradiation; η (%), yield

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BOUKHOBZA et al., Orient. J. Chem., Vol. 30(1), 255-259 (2014)
Synthesis of Tri Decyl Phosphate (TDP)
C30H63O4P, nD20 = 1.452.
cm-1; and the PO-H (ν = 2300 – 2400 cm-1)8.
31P NMR: δ/H3PO4 (ppm) = -1.70.
The acid nature of dialkylphosphates
1H NMR: δ/TMS (ppm) = 0.90(t, 9H, CH3), 1.26(m, was confirmed by the titration. The titration curves
48H, CH2), 3.97(m, 6H, CH2O-P)
IR: ν (cm-1) = 894 et 1028 (P-O-C), 1262 (P=O), All the results of these analyses have shown that the
presented one slope shift corresponds to pH= 5.81.
1378 (CH (CH3)), 1466 (CH (CH3 & CH2)), 2854 & products of the reaction of alcohol with phosphorus
2923 (CH ( CH3 & CH2)).
tr (GPC)(min): 10.28.
oxychloride, obtained under micro-wave irradiation,
were identical to those obtained with the classic
method of heating. In fact, the 31P NMR has shown
Synthesis of Tri [4-(1,1,3,3,-Tetra-methylbutyl) the formation of trialkyl-phosphates and the acid
phényl] Phosphate (TOPPA)
C42H63O4P
dialkyl-phosphoric acids according to Schemes 1
and 2.
31P NMR: δ/H3PO4 (ppm) = -16.1.
1H NMR: δ/TMS (ppm) = 0.71 (s, 27H, CH3), 1.33 (s,
Different trialkylphosphates are synthesized
18H, CH2), 1.68 (s, 6H, CH2), 6.76 (d, 6H, CHmeta
,
under microwave irradiations and by classic heating,
J3 H-H = 8.68 Hz), 7.24 (d, 6H, CHortho, J3 H-H = and this in presence or in absence of pyridine. The
9.57Hz).
yields and the different operating conditions are
IR:ν (cm-1) = 825 et 960 (P-O-C), 1165 (PO-C arom.), mentioned in Table 1.
1200 (al. P=O), 1360 (CH (CH3)), 1425 (CH (CH3 &
CH2), C=C & C-C arom.), 2850 et 2945 (CH ( CH3
& CH2)).
The powers and the times of irradiation are
fixed to obtain temperature near of the one applied
*δ, chemical shift; ν, wave number; s, singulet; d, in the classic heating (80-97°C), recommended for
doublet; t, triplet; m, multiplet.
the synthesis of alkylphosphates9-11
.
RESULTS AND DISCUSSION
The comparative study of the results of
synthesis under microwave irradiation has shown
Reactional mixtures obtained under microwave that in presence of pyridine the synthesis yields were
irradiationswereconfirmedbyvarioustechniques weaker than those obtained in absence of pyridine.
of analyses6.
The 31P NMR has shown the presence
In absence of pyridine, the synthesis yields
of two important signals corresponding to tri obtained by classic method of heating were higher
[4-(1,1’,3,3’,-tetramethylbutyl)phenyl] phosphate to those obtained under microwave irradiation.
(TOPPA (δ = -16.10 ppm )) and to di [4-(1,1’,3,3’,-
tetramethylbutyl)phenyl] phosphoric acid (DOPPA (δ
Nevertheless, the yields in the latter cases
= -9.11 ppm )). Small signal corresponding to the were lightly superior, although no precaution was
secondary product (the pyrophosphates (δ = -12 taken to avoid the secondary reactions (scheme
ppm) was also observed.
II). The short times of synthesis under microwave
irradiation have not favored the secondary reactions,
1H NMR confirmed the formation of even to high temperatures.
trialkylphosphates and the acid dialkylphosphoric
acids with characteristic signals corresponding to
Besides, the obtained yields under
the protons CH3(δ = 0.9 ppm), (CH2)n (δ = 1.2 - 1.6 microwave irradiation were lightly inferior to those
ppm), CH2-O-P (δ = 3.8 - 4.1 ppm), P-O-H (δ = 7 - 8 obtained under classic heating. The difference can
ppm) and aromatic C-H from d = 7 ppm.
be explained by the higher reduction of reaction time,
from some seconds to 2h.
The infrared spectra have presented
absorption corresponding to trialkylphosphates and
the acid dialkylphosphoric acids groups: P-OC and
P-OH (ν = 1030-1040 cm-1); P=O (ν = 1260 – 1265
CONCLUSION
The products of the reaction of alcohols

BOUKHOBZA et al., Orient. J. Chem., Vol. 30(1), 255-259 (2014)
259
with the phosphorus oxychloride under microwave
irradiation (with and without pyridine) were identical to
those obtained by classic heating trialkylphosphates
according to reaction 1.The effect of the temperature
was comparable in both methods of heating. The
activation of reaction 1 by microwaves allows also to
obtaining the higher yields.The speed of the reaction
of alcohols with phosphorus oxychloride was 2 h in
the classic conditions of heating, whereas under
microwave irradiation, reaction took place in 25 s to
3 min; the ratio of speed increasing was 40 to 120.
The decrease of the reaction time and the decrease
of degradation products of trialkyl-phosphates are
the main advantages of microwave irradiation.
ACKNOWLEDGEMENTS
We gratefully acknowledge the ATRST
(Agence Thématique de Recherche en Sciences &
Technologie-Algérie) (ex ANDRU) for their financial
support.
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