Synthesis of mono-and dialkylphosphates by the reactions of hydroxycompounds with the phosphorus pentaoxide under microwave irradiation

Article abstract of DOI:10.1080/10426500490494705

The reactions of phosphorus pentoxide with two alcohols and one phenol were performed in different conditions under microwave irradiation. The products (alkylphosphates and dialkylphosphates) were identical to those formed by classic heating and were obtained with better yields. The speed of the reaction was increased by a factor from 100 to 4000.

Full text of DOI:10.1080/10426500490494705

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SYNTHESIS OF MONO-
AND DIALKYLPHOSPHATES
BY THE REACTIONS OF
HYDROXYCOMPOUNDS WITH
THE PHOSPHORUS PENTAOXIDE
UNDER MICROWAVE
IRRADIATION
b
c
Abdelhamid Elias , M. Amine Didi , Didier Villemin
a
a
b
b
b
,
T. Semaoune & S. Ouattas
LCMT, ENSICAEN, Caen University , Caen, France
Organic Synthesis Laboratory, CRND, Draria ,
Algers, Algeria
c
Catalysis Laboratory, Chemistry Department,
Tlemcen University , Algeria
Published online: 16 Aug 2010.
To cite this article: Abdelhamid Elias , M. Amine Didi , Didier Villemin , T. Semaoune
&
S. Ouattas (2004) SYNTHESIS OF MONO-AND DIALKYLPHOSPHATES BY THE REACTIONS
OF HYDROXYCOMPOUNDS WITH THE PHOSPHORUS PENTAOXIDE UNDER MICROWAVE
IRRADIATION, Phosphorus, Sulfur, and Silicon and the Related Elements, 179:12,
2599-2607, DOI: 10.1080/10426500490494705
To link to this article: http://dx.doi.org/10.1080/10426500490494705
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Phosphorus, Sulfur, and Silicon, 179:2599–2607, 2004
Copyright ꢀC Taylor & Francis Inc.
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500490494705
SYNTHESIS OF MONO-AND DIALKYLPHOSPHATES
BY THE REACTIONS OF HYDROXYCOMPOUNDS
WITH THE PHOSPHORUS PENTAOXIDE UNDER
MICROWAVE IRRADIATION
b
c
a
Abdelhamid Elias, M. Amine Didi, Didier Villemin,
b
b
T. Semaoune, and S. Ouattas
a
LCMT, ENSICAEN, Caen University, Caen, France; Organic
b
Synthesis Laboratory, CRND, Draria, Algers, Algeria; and
Catalysis Laboratory, Chemistry Department, Tlemcen
University, Algeriac
(
Received May 13, 2004; accepted May 26, 2004)
The reactions of phosphorus pentoxide with two alcohols and one phenol
were performed in different conditions under microwave irradiation.
The products (alkylphosphates and dialkylphosphates) were identical
to those formed by classic heating and were obtained with better yields.
The speed of the reaction was increased by a factor from 100 to 4000.
Keywords: Alkylphosphates; dialkylphosphate; microwave irradiation;
solvent free
Alkylphosphates are synthesized from alcohols by reaction with reac-
tive phosphorus substances such as phosphorus pentoxide (P2O5), phos-
phorus oxychloride (POCl3), phosphorus trichloride (PCl3), and phos-
phorus pentachloride (PCl5).1
The reaction of alcohols with the phosphorus pentoxide (P2O5)
presents several advantages, such as the simplicity of the procedure,
the availability of the reagent and the absence of volatile and poten-
2
tially toxic products. These reactions are schematized according to
the Scheme 1.
The main products of this reaction of alcohols with phosphorus pen-
toxide are generally mono and dialkylphosphoric acids. However, small
quantities of secondary products, such as orthophosphoric acid and tri-
alkylphosphate, as well as intermediary products—essentially some
dialkylpyrophosphoric acids [(RO) (OH) (O) P] 2(O)—can be found in
the reactional mixture.
Address correspondence to Didier Villemin, LCMT, UMR CNRS 6507, ENSICAEN,
Caen University, F-14050, Caen, France. E-mail: didier.villemin@ismra.fr
2
599

2
600
A. Elias et al.
SCHEME 1
The action of the alcohol according to Scheme 1 consists of gradually
breaking the bonds P O P in the polymeric structure of phosphorus
2
anhydride according to the reaction in Scheme 2.
SCHEME 2
This process can go on until all the bonds P O P of the anhydride
are broken to form monophosphorus products.
The reactions of alcohols with the phosphorus pentoxide in the usual
2
,3
conditions of synthesis were already studied. The slowness (some
hours) of these reactions, which constitutes the major drawback of this
method of synthesis, generates interest in the activation of these reac-
tions by microwave irradiation. We have already described the use of
4
microwave activation in phosphorus chemistry.
The reaction was studied in the synthesis of important indus-
trial mono- and diesters of phosphoric acid used as metal ex-
tractant, such as MBP (monobutyl phosphoric acid), DBP (dibutyl
phosphoric acid), M2EHPA (mono-2-ethylhexyl phosphoric acid),
ꢁ
ꢁ
ꢁ
ꢁ
D2EHPA (di-2-ethylhexyl phosphoric acid), MOPPA (mono(4-(1,1 ,3,3 -
tetramethyl)butyl) phenyl phosphoric acid), and DOPPA(di(4-(1,1 ,3,3 -
5
,6
tetramethyl)butyl)phenyl phosphoric acid).
RESULTS AND DISCUSSION
In addition to its thermic effect, the effect of microwave irradiation on
the proportions of mono- and dialkylphosphates formed were studied
in different operating conditions (Table I).
The growth of microwave energy (increase of the power and the
duration of microwave irradiation) increases the temperature of reac-
tion mixtures. The temperature of mixtures of light alkylphosphates

Alkylphosphates
2601
TABLE I Temperatures and Proportions of Alkylphosphates
Obtained under Microwave Irradiation
Hydroxy compounds
Irradiation conditions and ratio of alkylphosphates
R
3
4
P (W)
270
450
270
450
Butan-1-ol
t (S)
T ( C)
ND
NM
5
10
95
1.0
1.0
5
10
116
1.0
1.0
5
10
82
1.0
1.0
5
10
◦
55
1.2
0.7
67
1.2
0.8
50
1.2
0.7
60
1.2
0.7
110
1.1
0.8
2
4
-Ethylhexan-1-ol
t (min)
T ( C)
2
4
0.83
93
0.9
1.1
1.5
111
0.5
1.2
2
4
0.83
83
1.4
0.5
1.5
91
1.1
0.7
◦
80
1.1
0.9
108
0.7
1.1
57
1.4
0.5
97
1.1
0.9
ND
NM
ꢁ
ꢁ
(1,1 ,3,3 ) Tetramethyl Butylphenol
t (min)
T ( C)
ND
NM
3
4
2
3
3
4
2
3
◦
92
0.9
1.1
110
1.5
0.5
84
0.8
1.2
110
1.3
0.7
81
0.8
1.2
102
1.1
0.9
73
0.2
1.8
107
1.3
0.7
R, molar ratio: hydroxy compound/P2O5; P and t, power and time of
microwave irradiation; T, temperature of the reactional mixture at the end of
irradiation; ND and NM, number of moles of dialkylphosphates and
monoalkylphosphates, obtained per mole of pentoxide used.
increases more quickly than those of the heavy homologues, probably
because of their weak calorific capacities, Cp (Table II).
◦
A temperature variation, about 50 C, was observed after 5 s of irra-
diation in the case of MBP–DBP mixture, but for the other mixtures
of product such a temperature variation required irradiations of some
minutes. The temperatures observed with mixtures of alkylphosphates
synthesized with an excess of alcohol (R = 4) were relatively lower,
compared with those obtained with a stoichiometric quantity (R = 3).
The nature of synthetic products under microwave irradiation
8
,9
31
was confirmed by various techniques of analyses.
The P NMR
TABLE II Calorific Capacities of Alkylphosphates
Products Cp (cal/mole.K)
Products
Cp (cal/mole.K) Products Cp (cal/mole.K)
n-butanol
MBP
DBP
27
40
62
2-ethylhexanol
M2EHPA
D2EHPA
49
62
106
R-Ar-OH
MOPPA
DOPPA
70
83
147
◦
Cp, calorific capacities at 25 C calculated by the Benson’s method of contribution;
ꢁ
ꢁ
7
R-Ar-OH, 4-(1,1 ,3,3 ,-tetramethyl butyl) phenol.

2
602
A. Elias et al.
has shown the presence of two important signals corresponding to
monoalkylphosphates and to dialkylphosphates (MBP (δ = 1.19 ppm)
and the DBP (δ = 0.02 ppm), the M2EHPA (δ = 0.86 ppm) and the
D2EHPA (δ = 0.02 ppm), the MOPPA (δ = −3.7 ppm), and the DOPPA
(
(
δ = −9.4 ppm)). Small signals corresponding to the secondary products
the orthophosphoric acid and trialkylphosphates (δ = 1.6 ppm)) were
also observed. On the other band, the signal of intermediary products
(
di- and polypyrophosphates) were not noticed.
1
H NMR confirmed the formation of alkylphosphates with character-
istic signals corresponding to the protons CH2 O P (δ = 3.9–4 ppm)
10
and P O H from δ = 7 ppm.
The infrared spectra has presented absorption corresponding to
−
1
alkylphosphates groups: P O (ν = 1200–1240 cm ), P OC and P OH
−1
−1 11
(
ν = 950–1045 cm ), and the PO H (ν = 2250–2370 cm ).
The acid nature of monoalkylphosphates and dialkylphosphates was
confirmed by the titration of these products. The titration curves pre-
sented two slope shifts. The first one corresponds to the neutralization
of the dialkylphosphate and one of the protons of the monoalkylphos-
phate (pH = 6). The second one (pH = 10.5) marks the neutralization
1
2
of the second proton of the monoalkylphosphate.
All the results of these analyses have shown that the products of the
reaction of alcohol with the phosphorus pentoxide, obtained under mi-
crowave irradiation, were identical to those obtained with the classic
method of heating. These products were essentially monoalkylphos-
phates and dialkylphosphates. Dialkylphosphates (DBP, D2EHPA,
DOPPA) were generally the main products. However, the proportions
of monoalkylphosphates (MOPPA, M2EHPA) can surpass those of the
◦
dialkylphosphates. When the temperature was higher than 90 C un-
der microwave irradiation or under usual conditions, the side products
were attributed to the well-known thermal elimination of alkene during
◦
the pyrolysis of phosphate esters at a temperature superior to 100 C
according to Schemes 3 and 4.1
3,14
SCHEME 3
ꢁ
In the case of 4-(1,1,3,3 -tetramethylbutyl)phenylphosphates
(
MOPPA and DOPPA), this reaction is not possible. MOPPA and
DOPPA are thermically more stable, so the quantity of the diester

Alkylphosphates
2603
SCHEME 4
(
DOPPA) was found to be higher than with other alcohols after longer
irradiation time. The comparative study of the results of syntheses
under classic heating condition and under microwave irradiation has
shown that in the case of the usual heating, monoalkylphosphates
always dominated (NM = 1.0–1.6 moles/mole of pentoxide), and that
the quantities of dialkylphosphate do not exceed, in the best of cases,
0.8 moles per mole of pentoxide. Under microwave irradiation, the
amounts of monoalkylphosphates and dialkylphosphates can respec-
tively reach the ratio of 1.8 and 1.5 per mole of P2O5. The difference
between the two activations can be explained by the higher speed of
heating under microwave condition and consequently the reduction of
reaction time and degradation products (Table III).
In the classic conditions and under microwave irradiation, the tem-
perature was the most important parameter that influences the ratio
1
5
and the speed of formation of alkylphosphates. The domain of the
◦
temperatures between 70 and 95 C was recommended for the synthe-
sis of alkylphosphates. Under microwave irradiation, a control of the
◦
temperature in this domain of temperatures (70–95 C) during the re-
action allows the best formation of alkylphosphates (N > 1 mole of
product/mole of pentaoxide).
The molar ratio of alcohol/pentaoxide fixed between 3–4 as adequate
for the synthesis of mono- and dialkylphosphates. The excess of alcohol
does not considerably influence the proportions of products in both
methods of heating.
TABLE III Proportions of Alkylphosphates in the Classic Conditions
of Synthesis
Produit
MBP-DBP
M2EHPA-D2EHPA
80 95
MOPPA-DOPPA
80 93
◦
T ( C)
80
95
R
3
4
3
4
3
4
3
4
3
4
3
4
ND
NM
0.7
1.3
0.8
1.1
0.3
1.6
0.4
1.4
0.7
1.0
0.7
1.1
0.6
1.0
0.6
1.3
0.6
1.1
0.6
1.4
0.6
1.4
0.5
1.5
R, molar ratio alcohol/P2O5; T, temperature of the reactional mixture; ND and NM,
number of moles of di- and of monoalkylphosphates obtained per mole of pentoxide
used.

2
604
A. Elias et al.
The speed of the reaction of alcohols with the phosphorus pentoxide
was 6 h in the classic conditions of heating, whereas under microwave
irradiation, reaction took place in 4 min; the ratio of speed increasing
was 100 to 4000.
CONCLUSION
The products of the reaction of alcohols with the phosphorus pentoxide
under microwave irradiation were identical to those obtained by classic
heating—monoalkylphosphates and dialkylphosphates according to re-
action 1. The effects of the temperature and the excess of alcohol were
comparable in both methods of heating. The activation of reaction 1 by
microwaves allows better yields and a higher amount of dialkylphos-
phate. The decrease of the reaction time and the decrease of degradation
products of dialkylphosphates are the main advantages of microwave
irradiation.
EXPERIMENTAL
The infrared spectra were made on a FT IR Perkin Elmer 16 PC spec-
trometer. Samples were analyzed in the form of films deposited between
two windows in KBr. NMR spectrometry analyses were carried out on
a spectrometer with Fourier Bruker AC 250 multinuclear. Samples of
the products were diluted in CDCl3, with tetramethylsilane (TMS) as
the internal reference.
◦
Densities and refractive indexes were measured at 25 C using a
Supelco pycnometer and a Euromax Honland refractometer.
The titrations of mono- and dialkylphosphates were made on a
Tacussel Mini 80 pH-meter provided with an electrode (calomel). The
procedure of titration by NaOH 0.1N is classic after dissolution of
0.1 g of the reaction mixture in water–acetone (volumic ratio: acetone/
water = 3). The reactions of synthesis under microwave irradiation
were performed in a microwave oven Delonghi MW 715 working with
the frequency of 2450 MHz.
Reagent
Reagents used—phosphorus pentaoxide (P2O5, 98%), n-butanol
(
C4H9OH, 98%), 2-ethyl-1-hexanol (C8H17OH, 98%) and 4-(1,1,3,3,-
tetramethylbutyl) phenol (C8H17-C6H4-OH, 90%), sodium hydrox-
ide 99%, hydrochloric acid 32%, and acetone 99.5%—are Merck
(
Darmstadt) products.

Alkylphosphates
2605
General Procedure
Hydroxycompounds used in these reactions were n-butanol, 2-ethyl-1-
ꢁ
ꢁ
hexanol, and 4-(1,1 ,3, 3 , -tetramethylbutyl)phenol.
Solid phosphorus pentoxide (2 g (0.014 mole)) and the hydroxycom-
pounds (molar ratio: hydroxyl compounds/pentaoxide: R = 3–4) were
◦
stirred and heated at 40 C under dry nitrogen.
Microwave Irradiation
The mixture above was irradiated in a glass cylinder reactor (diame-
◦
ter = 3 cm) fitted by a cooler filled with cooled octane (0–5 C) according
to the conditions (power and time) reported in Table I.
Classic Conditions of Heating
◦
The mixture was heated and stirred under dry nitrogen at 80–95 C
for 6 h according to the conditions reported in Table III.
The separation of monoalkylphosphates and dialkylphosphates was
performed by liquid–liquid extraction with an aqueous solution of soda
1
6
according to the classic procedure described in the literature.
∗
Physical Properties of the Alkylphosphates
Monobutylphosphate (MBP)
25
25
(
n-C4H9O)(O)P(OH)2, d = 1.198, ηD = 1.447.
4
−
1
IR: ν (cm ) = 1025 (vS, L), 1200 (S, L), 1370 (w), 1460 (m), 1620 (w, L),
2
330 (w, L), 2850 (S), 2940 (S, L).
1
H NMR: δ/TMS (ppm) = 0.94 (t, 3H, CH3), 1.43 (sext, 2H, CH2), 1.64
(
quint, 2H, CH2), 4.03 (m, 2H, CH2O P), 8.73 (s, 2H, P(OH)2).
3
1
P NMR: δ/H3PO4 (ppm) = 1.19.
Dibutylphosphate (DBP)
25
25
(
n-C4H9O)2(O)P(OH), d = 1.058, ηD = 1.426.
4
−
1
IR: ν (cm ) = 895 (w), 1010 (vS, L), 1210 (vS,L), 1365 (w), 1440 (m),
1
630 (w, L), 2250 (w, L), 2875 (vS), 2960 (vS, L).
1
H NMR: δ/TMS (ppm) = 0.96 (t, 6H, CH3), 1.39 (sext, 4H, CH2), 1.63
(
quint, 4H, CH2), 4.03 (m, 4H, CH2O P), 8.87 (s, 1 H, PO H).
3
1
P NMR: δ/H3PO4 (ppm) = −0.02.
∗
δ, chemical shift; ν, wave number; s, singulet; d, doublet; t, triplet; q, quadruplet;
quint, quintuplet; sext, sextuplet; m, multiplet; J, coupling constante; S, strong; L, large;
w, weak.

2
606
A. Elias et al.
Mono 2-Ethylhexyl Phosphate (M EHPA)
2
2
5
25
(
C8H17O)(O)P(OH)2, d = 0.987, ηD = 1.435.
4
−
1
IR: ν (cm ) = 1005 (vS, L), 1235 (vS, L), 1375 (f), 1475 (m), 1615 (w),
2
370 (w, L), 2880, 2970 (vS).
1
H NMR: δ/TMS (ppm) = 0.89 (t, 6H, CH3), 1.28 (m, 8H, CH2), 1.57 (m,
1
H, CH), 3.92 (m, 2H, CH2O P), 8.13 (s, 2 H, P(OH)2).
3
1
P NMR: δ/H3PO4 (ppm) = 0.86.
Di 2-Ethylhexyl Phosphate (D2EHPA)
2
5
25
(
C8H17O)2(O)P(OH), d = 0.977, ηD = 1.442.
4
−
1
IR: ν (cm ) = 895 (w), 1045 (vS, L), 1240 (S, L), 1380 (w), 1470 (m),
1
685 (w, L), 2275 (w, L), 2855 (S), 2955 (vS).
1
H NMR: δ/TMS (ppm) = 0.89 (t, 12H, CH3), 1.28 (m, 16H, CH2), 1.57
(
m, 2H, CH), 3.93 (m, 4H, CH2O P), 7.11 (s, 1 H, P(OH)).
3
1
P NMR: δ/H3PO4 (ppm) = 0.02 ppm.
Mono[4-(1,1,3,3,-Tetramethylbutyl)phenyl]
Phosphate (MOPPA)
[
C8H17(C6H4)O](O)P(OH)2.
−1
IR: ν(cm ) = 825 (S), 960 (vS, L), 1005 (S, L), 1090 (w), 1165 (S), 1200
vS, L), 1353 (m), 1375 (w), 1450 (S, L), 1490, 1585 (m), 1870 (w, L),
300 (w, L), 2880, 2930 (vS).
(
2
1
H NMR: δ/TMS (ppm) = 0.64 (s, 9H, CH3), 1.26 (s, 6H, CH3), 1.62 (s,
3
2
H, CH2), 6.68 (d, 2H, CHmeta, J H H = 8.65 Hz), 7.14 (d, 2H, CHortho,
3
J H H = 8.65 Hz).
3
1
P NMR: δ/H3PO4 (ppm) = −3.7.
Di [4-(1,1,3,3,-Tetramethylbutyl)phenyl]
Phosphate (DOPPA)
[
C8H17(C6H4)O]2(O)P(OH).
−1
IR: ν (cm ) = 820 (S), 950 (vS, L), 1000 (S, L), 1080 (w, L), 1155 (S),
210 (vS, L), 1350 (m), 1370 (w), 1440 (S, L), 1580 (m), 1860 (w, L), 2300
w, L), 2850, 2880, 2940 (vS).
1
(
1
H NMR: δ/TMS (ppm) = 0.71 (s, 18H, CH3), 1.33 (s, 12H, CH3), 1.68 (s,
3
4
H, CH2), 6.76 (d, 4H, CHmeta, J H H = 8.64 Hz), 7.2 (d, 4H, CHortho,
3
J H H = 8.65 Hz).
3
1
P NMR: δ/H3PO4 (ppm) = −9.4.
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1] G. M. Kosolapoff and L. Maier, Organic Phosphorus Compounds (Wiley Interscience,
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Alkylphosphates
2607
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[
[
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