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Alkaline hydrolysis of linear tetra-, hexa- and octaphosphate
Polyhedron Vol. 9, No. 17, pp. Printed in Great Britain
2177-2179.
1990 0
0277-5387/90 $3.00 + .OO 1990 Pergamon Press plc
COMMUNICATION ALKALINE
HYDROLYSIS OF LINEAR TETRA-, HEXA- AND OCTAPHOSPHATE GENICHIRO
KURA
Department of Chemistry, Fukuoka University of Education, Akama, Munakata, Fukuoka 8 1l-41, Japan (Received 5 April 1990 ; accepted 7 June 1990) hydrolysis rates for the linear tetra-, hexa- and octaphosphate in several alkaline solutions have been determined. The hydrolysis reaction takes place most rapidly in LiOH solution. Activation energies of the linear polyphosphates in 1 M (CH3)4NOH were higher than those for the cyclic oligomers with a similar polymerization degree. Abstract-The
Inorganic condensed phosphate polymers are one of the most commonly used materials in solution and solid-state chemistry. ’ These polymers consist mainly of the linear and cyclic phosphates. The structures of cycle-octaphosphate and linear octaphosphate are shown in Fig. 1. We have studied the chemistry of oligomers with a polymerization degree smaller than about eight.2-4 In this paper, the hydrolysis reaction of linear tetra-, hexa- and octaphosphates in alkaline solutions were investigated. These linear oligomers are very interesting materials for solution chemistry and their stability in aqueous solutions against hydrolysis must be known. Since the preparative methods of the hexa- and octaphosphate in macro quantity have not been reported, the sample solutions to be hydrolysed were prepared by the partial hydrolysis of cyclo-
-9 v- O 0=P-+p__0-~‘__0:, d
-0. 0 HP ’
+0
)
p:,
-o-,,A-,-,;-o-,:-, 0
-0
‘0’24
0
hexa- and cycle-octaphosphate. To prepare sample solutions which contain adequate quantities of linear hexamer or octamer, cycle-hexa- or cyclooctaphosphate were hydrolysed in 0.5 M LiOH at 40°C for about 7-9 hand the reactions were stopped by neutralization with HCl. The Li+ ions contained in the sample solutions in the quantities prepared by the above procedure have been known to affect strongly the hydrolysis rates. 5 Thus, the Li+ ions in the hydrolysis samples were converted to (CH,),N+ ions by the use of ion-exchange resins. The linear tetraphosphate was prepared as the ammonium salt by the method of Griffith.6 Analyses of the parent phosphates and the hydrolysis products were performed using automated liquid chromatography. 7 In the analysis of the hydrolysis rates of hexa- and octaphosphates, the concentration of the corresponding cycle-polyphosphates and the linear
9
P
I:
P
‘:
‘:
‘:
t
b-
0-
o-
o-
o-
o-
o-
o-
-o-P-o-p-o-p-o-~-o-p-o-~-o-~-o-~-o-
0-
8- ( Pam
lopao25
)
Fig. I. The structures of cycle-octaphosphate 2177
(
Pg
1
and linear octaphosphate.
Communication
2178
31
Oh
\
.
p5
\
27 s
p2
.\. ii? z u 0
22
Experimental \
-Calculated
I 20
I
IO
I
30
I 40
I 50
Time(h)
Fig. 3. Variation of the concentration of octaphosphate with time in 0.5 M KOH at 50°C.
50h
0
20
40
Effluent
60
(cm31
Fig. 2. Chromatograms of the hydrolysis samples in 0.5 M KOH at 50°C.
polyphosphates as the first hydrolysis product at time t are defined as x and y, respectively, and also their first-order rate constants are designated as k, and k,, respectively. The hydrolysis rate constant of hexa- or octaphosphates, k,, can be obtained by solving the following differential equations : 7 -dx/dt -dy/dt
= k,,,x
= k,y-k,,,x.
(1) (2)
The chromatograms of the hydrolysis samples of octaphosphate in 0.5 M KOH at 50°C are shown in Fig. 2. In this paper, cyclic and linear oligomers are abbreviated as P, and P,,, in which n is the polymerization degree. The rate constant, k,,, was estimated by successive approximation involving substitution of the initial value of x and y, and km which had been already determined by the separate experiments in eq. (2). In Fig. 3, the time dependence of the concentration of octaphosphate is shown, where the curved line was drawn by substituting k, and k, into eq. (2) and the experimental points are indicated by black circles. The coincidence of both values is satisfactory. The hydrolysis rates in alkaline solutions have been known to be markedly affected not only by OH- concentration
but by the counter ion of OH-.” The effect of the counter ions, Li+, Na+ and K+, on the hydrolysis rates of linear tetra-, hexa- and octaphosphates at [OH-] = 0.5 M were also examined. In Table 1, rate constants in each alkaline solution at 50°C are shown. For every linear phosphate, the hydrolysis reaction takes place most rapidly in 0.5 M LiOH and the reaction rate decreases in the order, LiOH > NaOH > KOH. In every alkaline solution, the hydrolysis rate decreases with decreasing polymerization degree, i.e. Ps > P6 > P4. The Li+ ions accelerate the hydrolysis rates the most and thus interact most strongly with linear phosphate anions. Next, the hydrolysis rates in (CH3)4NOH were measured, where the (CH3)4N+ ion is known to be an inert cation which does not interact with many anions. In this solution, the hydrolysis reactions of linear oligomers were very slow, thus the rates in 1.OOM (CH3)4NOH and at relatively higher temperatures, i.e. 70,80 and 90°C were determined. However, since the rates for tetraphosphate were very slow, taking into account the experimental error, only the rate at 90°C was measured. The rate constants obtained are shown in Table 2. Tetra-
Table 1. Hydrolysis rate constants of linear phosphates at 50°C
P4 0.5 M LiOH 0.5 M NaOH 0.5 M KOH
k (h- ‘) P6
6.19 x lop3 4.92 x lo-’ 3.01 x 10m3 1.42 x lo-* 2.02 x lo- 3 9.34 x lo- 3
P* 0.128 2.19x lo-* 1.38 x lo- *
Communication Table 2. Hydrolysis
rate constants of linear phosphates in 1 M (CH3)4NOH
P4 70°C 80°C 90°C
1.33x lo-’
kn (h- ‘) P6 4.25 x lo- 3 1.31 x 10m2 7.07x lo-’
5.55 x lo- 3 1.72x 1O-2 5.72x lop2
34.7
28.8
E, (kcal mall ‘)
2179
shown in Fig. 4 ; linear relations were obtained. The rate constants of hexa- and octaphosphates at 50°C were estimated by the linear extrapolation of the Arrhenius plots as 1.64 x 1O-4 and 4.04 x 10M4, respectively. Although the concentration of OH- is twice that of other alkalis, the hydrolysis reaction is very slow. The hydrolysis rates of linear phosphates in alkaline solution are markedly influenced by the cations present. Arrhenius activation energy of cycle-hexa- and cycle-octaphosphate in 1.00 M (CHJ4NOH solution were 19.4 and 21.7 kcal mol- ‘, respectively. The activation energies of linear hexa- and octaphosphates in Table 2 are about 10-15 kcal mol- ’ larger. For the cleavage of the P-O-P bond due to the attack of OH- ion on the P atoms, the electrostatic repulsion between OH- and the anionic atmosphere near the P atoms is enhanced for the linear oligomer structure, and thus the activation energies are larger for the hydrolysis of the linear oligomers. When the hydrolysis rates, at 50°C of the cyclic and linear oligomers are compared with each other, linear tetraphosphate is extremely stable relative to cyclotetraphosphate but the rate constants for the hexamer or octamer are comparable. REFERENCES
T-'/UK-' Fig. 4. Arrhenius
plots of hexa- and octaphosphate 1 M (CH,),NOH.
in
phosphate is most stable and the half-life even at 90°C is 52 h. The large difference between the rates of hexa- and octaphosphates was not observed. The Arrhenius plots for hexa- and octaphosphates are
1. J. R. Van Wazer, Phosphorus and its Compounds. Interscience, New York (1958). 2. G. Kura and S. Ohashi, J. Znorg. Nucl. Chem. 1976, 38,1151. 3. S. Ohashi, G. Kura, Y. Shimada and M. Hara, J. Znorg. Nucl. Chem. 1977,39, 1513. 4. G. Kura, Polyhedron 1986,5,2097. 5. G. Kura, Polyhedron 1987,6, 1863. 6. E. J. Griffith, J. Znorg. Nucl. Chem. 1964, 26, 1381. 7. G. Kura, J. Chromatogr. 1988,447, 91. 8. G. Kura, BUN. Chem. Sot. Jpn 1987,60,2857.
2177-2179.
1990 0
0277-5387/90 $3.00 + .OO 1990 Pergamon Press plc
COMMUNICATION ALKALINE
HYDROLYSIS OF LINEAR TETRA-, HEXA- AND OCTAPHOSPHATE GENICHIRO
KURA
Department of Chemistry, Fukuoka University of Education, Akama, Munakata, Fukuoka 8 1l-41, Japan (Received 5 April 1990 ; accepted 7 June 1990) hydrolysis rates for the linear tetra-, hexa- and octaphosphate in several alkaline solutions have been determined. The hydrolysis reaction takes place most rapidly in LiOH solution. Activation energies of the linear polyphosphates in 1 M (CH3)4NOH were higher than those for the cyclic oligomers with a similar polymerization degree. Abstract-The
Inorganic condensed phosphate polymers are one of the most commonly used materials in solution and solid-state chemistry. ’ These polymers consist mainly of the linear and cyclic phosphates. The structures of cycle-octaphosphate and linear octaphosphate are shown in Fig. 1. We have studied the chemistry of oligomers with a polymerization degree smaller than about eight.2-4 In this paper, the hydrolysis reaction of linear tetra-, hexa- and octaphosphates in alkaline solutions were investigated. These linear oligomers are very interesting materials for solution chemistry and their stability in aqueous solutions against hydrolysis must be known. Since the preparative methods of the hexa- and octaphosphate in macro quantity have not been reported, the sample solutions to be hydrolysed were prepared by the partial hydrolysis of cyclo-
-9 v- O 0=P-+p__0-~‘__0:, d
-0. 0 HP ’
+0
)
p:,
-o-,,A-,-,;-o-,:-, 0
-0
‘0’24
0
hexa- and cycle-octaphosphate. To prepare sample solutions which contain adequate quantities of linear hexamer or octamer, cycle-hexa- or cyclooctaphosphate were hydrolysed in 0.5 M LiOH at 40°C for about 7-9 hand the reactions were stopped by neutralization with HCl. The Li+ ions contained in the sample solutions in the quantities prepared by the above procedure have been known to affect strongly the hydrolysis rates. 5 Thus, the Li+ ions in the hydrolysis samples were converted to (CH,),N+ ions by the use of ion-exchange resins. The linear tetraphosphate was prepared as the ammonium salt by the method of Griffith.6 Analyses of the parent phosphates and the hydrolysis products were performed using automated liquid chromatography. 7 In the analysis of the hydrolysis rates of hexa- and octaphosphates, the concentration of the corresponding cycle-polyphosphates and the linear
9
P
I:
P
‘:
‘:
‘:
t
b-
0-
o-
o-
o-
o-
o-
o-
-o-P-o-p-o-p-o-~-o-p-o-~-o-~-o-~-o-
0-
8- ( Pam
lopao25
)
Fig. I. The structures of cycle-octaphosphate 2177
(
Pg
1
and linear octaphosphate.
Communication
2178
31
Oh
\
.
p5
\
27 s
p2
.\. ii? z u 0
22
Experimental \
-Calculated
I 20
I
IO
I
30
I 40
I 50
Time(h)
Fig. 3. Variation of the concentration of octaphosphate with time in 0.5 M KOH at 50°C.
50h
0
20
40
Effluent
60
(cm31
Fig. 2. Chromatograms of the hydrolysis samples in 0.5 M KOH at 50°C.
polyphosphates as the first hydrolysis product at time t are defined as x and y, respectively, and also their first-order rate constants are designated as k, and k,, respectively. The hydrolysis rate constant of hexa- or octaphosphates, k,, can be obtained by solving the following differential equations : 7 -dx/dt -dy/dt
= k,,,x
= k,y-k,,,x.
(1) (2)
The chromatograms of the hydrolysis samples of octaphosphate in 0.5 M KOH at 50°C are shown in Fig. 2. In this paper, cyclic and linear oligomers are abbreviated as P, and P,,, in which n is the polymerization degree. The rate constant, k,,, was estimated by successive approximation involving substitution of the initial value of x and y, and km which had been already determined by the separate experiments in eq. (2). In Fig. 3, the time dependence of the concentration of octaphosphate is shown, where the curved line was drawn by substituting k, and k, into eq. (2) and the experimental points are indicated by black circles. The coincidence of both values is satisfactory. The hydrolysis rates in alkaline solutions have been known to be markedly affected not only by OH- concentration
but by the counter ion of OH-.” The effect of the counter ions, Li+, Na+ and K+, on the hydrolysis rates of linear tetra-, hexa- and octaphosphates at [OH-] = 0.5 M were also examined. In Table 1, rate constants in each alkaline solution at 50°C are shown. For every linear phosphate, the hydrolysis reaction takes place most rapidly in 0.5 M LiOH and the reaction rate decreases in the order, LiOH > NaOH > KOH. In every alkaline solution, the hydrolysis rate decreases with decreasing polymerization degree, i.e. Ps > P6 > P4. The Li+ ions accelerate the hydrolysis rates the most and thus interact most strongly with linear phosphate anions. Next, the hydrolysis rates in (CH3)4NOH were measured, where the (CH3)4N+ ion is known to be an inert cation which does not interact with many anions. In this solution, the hydrolysis reactions of linear oligomers were very slow, thus the rates in 1.OOM (CH3)4NOH and at relatively higher temperatures, i.e. 70,80 and 90°C were determined. However, since the rates for tetraphosphate were very slow, taking into account the experimental error, only the rate at 90°C was measured. The rate constants obtained are shown in Table 2. Tetra-
Table 1. Hydrolysis rate constants of linear phosphates at 50°C
P4 0.5 M LiOH 0.5 M NaOH 0.5 M KOH
k (h- ‘) P6
6.19 x lop3 4.92 x lo-’ 3.01 x 10m3 1.42 x lo-* 2.02 x lo- 3 9.34 x lo- 3
P* 0.128 2.19x lo-* 1.38 x lo- *
Communication Table 2. Hydrolysis
rate constants of linear phosphates in 1 M (CH3)4NOH
P4 70°C 80°C 90°C
1.33x lo-’
kn (h- ‘) P6 4.25 x lo- 3 1.31 x 10m2 7.07x lo-’
5.55 x lo- 3 1.72x 1O-2 5.72x lop2
34.7
28.8
E, (kcal mall ‘)
2179
shown in Fig. 4 ; linear relations were obtained. The rate constants of hexa- and octaphosphates at 50°C were estimated by the linear extrapolation of the Arrhenius plots as 1.64 x 1O-4 and 4.04 x 10M4, respectively. Although the concentration of OH- is twice that of other alkalis, the hydrolysis reaction is very slow. The hydrolysis rates of linear phosphates in alkaline solution are markedly influenced by the cations present. Arrhenius activation energy of cycle-hexa- and cycle-octaphosphate in 1.00 M (CHJ4NOH solution were 19.4 and 21.7 kcal mol- ‘, respectively. The activation energies of linear hexa- and octaphosphates in Table 2 are about 10-15 kcal mol- ’ larger. For the cleavage of the P-O-P bond due to the attack of OH- ion on the P atoms, the electrostatic repulsion between OH- and the anionic atmosphere near the P atoms is enhanced for the linear oligomer structure, and thus the activation energies are larger for the hydrolysis of the linear oligomers. When the hydrolysis rates, at 50°C of the cyclic and linear oligomers are compared with each other, linear tetraphosphate is extremely stable relative to cyclotetraphosphate but the rate constants for the hexamer or octamer are comparable. REFERENCES
T-'/UK-' Fig. 4. Arrhenius
plots of hexa- and octaphosphate 1 M (CH,),NOH.
in
phosphate is most stable and the half-life even at 90°C is 52 h. The large difference between the rates of hexa- and octaphosphates was not observed. The Arrhenius plots for hexa- and octaphosphates are
1. J. R. Van Wazer, Phosphorus and its Compounds. Interscience, New York (1958). 2. G. Kura and S. Ohashi, J. Znorg. Nucl. Chem. 1976, 38,1151. 3. S. Ohashi, G. Kura, Y. Shimada and M. Hara, J. Znorg. Nucl. Chem. 1977,39, 1513. 4. G. Kura, Polyhedron 1986,5,2097. 5. G. Kura, Polyhedron 1987,6, 1863. 6. E. J. Griffith, J. Znorg. Nucl. Chem. 1964, 26, 1381. 7. G. Kura, J. Chromatogr. 1988,447, 91. 8. G. Kura, BUN. Chem. Sot. Jpn 1987,60,2857.