CODEN (USA): IJCRGG, ISSN: 0974-4290, ISSN(Online):2455-9555 Vol.10 No.6, pp 653-660, 2017
Abstract : In present investigation, kinetic study of hydrolysis of di-2,3-dichloroaniline phosphate has been carried out in pH range from 0.00 to 7.49 in 30% dioxane-water medium at 80oC (±0.5). The pH log rate profile shows rate maximum at pH 4.11. Neutral and mononegative species have been found to be reactive in the range of pH 0.00 to 2.21 and only mononegative species in the range of pH 2.21 to 7.49. The theoretical rates determined from Debye Huckel equation have been found in close agreement with the experimental rates.Molecularity and Bond fission have been discussed in terms of isokinetic relationship. Probable reaction mechanism has been proposed for the hydrolysis of di-ester via its neutral and mononegative species. Keywords: Hydrolysis, Di-2,3-dichloroaniline phosphate, Neutral and Mononegative species, P-N bond fission, Bimolecularity.
Phosphate esters are fundamental molecules in cellular chemistry and have a wide range of applications in the industrial, agricultural and academic field owing to their unique biological and physicochemical properties as well as their utility as synthetic intermediates1.Other significant applications are as environmentallydegradable pesticides due to their toxicity to a wide range of insects and their relatively short residence time in the environment2.The phosphate esters have also found application as surfactants and as brighteners in detergents3.The cyclic and acyclic phosphate esters are normally considered as important pharmacological compounds4-7 . Nucleoside derivatives of phosphate esters have found to be extremely important agents for anticancer and antiviral therapy8.Prodrugs of phosphate esters have been successfully utilized to overcome a variety of drug delivery problems9.Phosphate ester hydrolysis is a crucially important process for several biological systems, as it is involved in energy and signal transduction processes, the control
10, 11
of cellular metabolism, and the regulation of protein function.Phosphate diesters play an essential role in biology, particularly as the backbone of DNA and RNA and therefore its hydrolysis represents a reaction of
12, 13
fundamental importance in living systems.
Di-2,3-dichloroaniline phosphate was prepared from the reaction of 2,3-dichloroaniline with phosphorylating agent phosphorus oxychloride (POCl3) by the procedure described earlier14. Characterization of the compound was done by melting point determination, elemental and IR spectral analysis techniques.Hydrolysis of di-2,3-dichloroaniline phosphate were carried out in the pH range 0.00-7.49 in 30%
oC (±0.5) employing 5 × 10-4mol dm-3 solution of diester. Buffer solutions 6H4.COOK, NaOH and H3BO315.
Inorganic phosphate produced during the hydrolysis was determined by Allen’s modified 16spectrophotometrically. Phosphomolybdate complex[(NH4)3PO4.12MoO3] is formed by inorganic This complex is reduced to molybdenum blue, a soluble complex, by
Kinetics of hydrolysis of di-2,3-dichloroaniline phosphate was carried out in pH range from 0.00-7.49 oC (±0.5)in 30% dioxane-water (v/v) medium. In this pH range reaction was found to be governed by two
Fig.1: pH log rate profile for the hydrolysis of di-2,3-dichloroaniline phosphate
The rise in rate in higher pH range (more than pH 1.00) is due to the incursion of more reactive mononegative species while the rise in rate in lower pH range (less than pH 1.00) is attributed to the incursion of neutral and conjugate acid species. Similar nature of hydrolysis has been observed in the case of m-toluidine phosphate17.Neutral and mononegative rates may be represented as:
Where kNo is specific neutral rate, kMo is the experimental rate (specific mononegative rate) at pH 4.11 and N/(N+M), M/(M+N) are the fraction of neutral and mononegative species, respectively. The value of specific neutral rates (kNo) was determined from the following equation:
Where,k is experimental rate. There is a good agreement between values of specific neutral rate kNo determined
-3 -1
by equation (3) and the ionic strength data. The value of kNo determined byequation (3) is 1.55 × 10minat
-3 -1
different pH from 0.00 to 1.00 and the value of kNo obtained from ionic strength data is 1.59 × 10min.
Hydrolysis via mononegative species:
Mono-and diesters are known to dissociate almost completely at pH 4.00, thus in pH range 4.00 to 7.00 mononegative species of diester is reactive. Since they are not further converted into dinegative species as mononegative species of monoester do, their concentration remain constant. As the mononegative rates are directly proportional to the concentration of their ions, the experimental rate of hydrolysis in this range is expected to be constant. The experimental rates (pH log rate profile fig.1) also show consistency of the rate of hydrolysis of the diester. A slight elevation in rates at 5.42 may be attributed to incursion of more nucleophilic hydroxide ion than water molecule. Similar observation has also been made for the hydrolysis of di-phenyl phosphate and di-p-chlorophenyl phosphate18.
It is clear from the results (Table 1) that in the pH range 0.00 to 1.00, hydrolysis governs by neutral, conjugate acid and mononegative species. In the pH range 1.00 to 1.22, the reaction moves forward via neutral and mononegative species. In the pH range 1.22 to 7.49, only mononegative species are reactive.
Kinetic rate laws for the hydrolysis of di-2,3-dichloroaniline phosphate may be represented as:
I. In the region pH 0.00 to 1.00
II. In the region pH 1.00 to 1.22
III. In the region pH 1.22 to 7.49
pH
0.00
0.30
0.70
1.00
1.22
2.21
3.29
4.11
5.42
6.21
7.49
N/N+M | M/M+N | kM×103 (min -1) | kN×103 (min 1) | kH +.CH + × 103 (min -1) | k×103 (min -1) (Estd.) | k×103 (min -1) (Expt.) | 3+logk(Estd.) | 3+logk(Expt.) |
---|---|---|---|---|---|---|---|---|
0.937 | 0.063 | 0.42 | 1.487 | 6.22 | 8.13 | 9.38 | 0.91 | 0.97 |
0.881 | 0.119 | 0.80 | 1.399 | 2.83 | 5.03 | 5.30 | 0.70 | 0.72 |
0.747 | 0.253 | 1.67 | 1.186 | 1.07 | 3.93 | 3.25 | 0.59 | 0.51 |
0.597 | 0.403 | 2.68 | 0.948 | 0.52 | 4.15 | 3.63 | 0.62 | 0.56 |
0.450 | 0.540 | 3.61 | 0.714 | - | 4.33 | 4.12 | 0.64 | 0.61 |
0.085 | 0.915 | 6.12 | 0.135 | - | 6.26 | 5.22 | 0.79 | 0.71 |
0.007 | 0.993 | 6.64 | 0.011 | - | 6.65 | 5.80 | 0.82 | 0.76 |
0.001 | 0.999 | 6.68 | 0.001 | - | 6.68 | 6.69 | 0.82 | 0.83 |
0.999 | 6.68 | 6.68 | 5.56 | 0.82 | 0.75 | |||
0.999 | 6.68 | 6.68 | 5.21 | 0.82 | 0.72 | |||
0.999 | 6.68 | 6.68 | 4.76 | 0.82 | 0.68 |
Prediction of molecularity of the reaction may be made by determining Arrhenius parameters. The plot temperature at pH 1.22 and 4.11 is described (Fig. 2). oC.The
19 (Table 2).
Fig. 2. Arrhenius plot for the hydrolysis of di-2,3-dichloroaniline phosphate
Table 2. Arrhenius parameters for the hydrolysis of di-2,3-dichloroaniline phosphate via neutral and mononegative species
pH | Slope | Ea (Kcal/mol) | A (Sec -1) | -∆S≠ (e.u.) |
---|---|---|---|---|
1.22 | -0.0258 | 11.82 | 8.66 × 104 | 38.29 |
4.11 | -0.0264 | 12.09 | 2.06 × 105 | 36.56 |
Solvent effect has been studied by taking different compositions of 1,4-dioxane-water mixture which
20
.
pH
1.22
1,4-dioxane (v/v) | ke × 103 (min -1) | pH | 1,4-dioxane (v/v) | ke × 103 (min -1) |
---|---|---|---|---|
0.00 | 4.12 | 0.00 | 6.69 | |
5.00 | 4.35 | 5.00 | 6.72 | |
10.0 | 4.75 | 4.11 | 10.0 | 7.10 |
15.0 | 4.98 | 15.0 | 7.43 | |
20.0 | 5.48 | 20.0 | 7.97 |
Di-2,3-dichloroaniline phosphate may undergo hydrolysis either by P-N or C-N bond fission. 21for the hydrolysis of diesters studied kinetically shows isokinetic relationship. of di-2,3-dichloroaniline phosphate is collinear with those diesters which are known to undergo
Fig. 3. Isokinetic relationship plot for the hydrolysis of some phosphate diesters via their neutral species
Chart-1: The Mechanism of hydrolysis of di-2,3-dichloroaniline phosphate via its neutral species: Bimolecular attack of water on phosphorus of the neutral species SN2 (P)
(a) Formaton of mononegative species
(b) Bimolecular nucleophilic attack of water on phosphorus of mononegative species SN2 (P). Chart-2: The Mechanism of hydrolysis of di-2,3-dichloroaniline phosphate via its mononegative species:
Di-2,3-dichloroaniline phosphate in pH 1.00 to 7.49 was found to hydrolyze via neutral and mononegative species. The maximum value at pH 4.11 is due to hydrolysis via mononegative and neutral species. The pH log rate profile shows that the estimated rates closely agreed well with those experimental rates. Bimolecular nature of hydrolytic reactions has been supported by Arrhenius parameters. The diester involves P-N bond fission, which strengthened by comparative kinetic rate data.
The authors are thankful tothe Pt. Ravishankar Shukla University, Raipur for providing University Fellowship to one of the authors (Nisha Chhetri) and also grateful to Head of School of Studies in Chemistry, Pt. Ravishankar Shukla University, Raipur, C.G., India, for providing research facilities.
*****