PHOSPHORYLATION OF MODEL LIGNIN COMPOUNDS.

COMMUNICATION 6. RATE OF HYDROLYSIS

OF CERTAIN THIOPHOSPHATES IN AN ALKALINE MEDIUM

I. G. Lykhina, L. A. Pershina

(Presented by the scientific seminar of the Faculty of Chemical Technology)

Hydrolysis in an alkaline medium of phosphates, thiophosphates, and phosphonates has been studied by many investigators [1—6]. It was established that this reaction proceeds by a bimolecular mechanism and, most probably, is catalyzed by OH⁻ ions, although, strictly speaking, alkaline hydrolysis is not a truly catalytic process, since hydroxide ions are consumed in the course of the reaction. During the hydrolysis of triesters of phosphoric and thiophosphoric acids in an alkaline medium, cleavage of the P—OAr bond occurs [7, 8], whereas in neutral medium the O—R bond is preferentially cleaved [8]. As a result of alkaline hydrolysis of O,O-dialkyl-O-aryl thiophosphates, the corresponding phenolate and the sodium salt of dialkylthiophosphoric acid are formed, the latter being fairly resistant to further hydrolysis [1, 5].

\[ \mathrm{ (RO)_2P(=S)-OAr + 2NaOH \longrightarrow NaOAr + (RO)_2P(=S)-ONa } \]

Fig. 1.

Deeper hydrolysis in an alkaline medium, as emphasized in a number of studies [9], does not proceed to any significant extent.

The literature contains no information on the hydrolysis of triesters of phosphoric and thiophosphoric acids and model lignin compounds of phenolic character. Study of this question is important both for establishing the practical applicability of these thiophosphates as insecticides and for establishing the relationship between the resistance of the P—OAr bond to hydrolysis and the degree of acidity of the phenolic hydroxyl groups of model lignin compounds.

In the present work, results are given for the study of alkaline hydrolysis of O-methyl-O-ethyl thiophosphates of model lignin compounds, not described in the literature, with differing acidity of the phenolic hydroxyl groups (Table 1).

Table 1

Characteristics of O-methyl-O-ethyl thiophosphates of model lignin compounds

For the study of hydrolysis kinetics, the following thiophosphates were selected: 1) O-methyl-O-ethyl-O-(2-methoxy-4-formylphenyl) thiophosphate, 2) O-methyl-O-ethyl-O-(2-methoxy-4-propan-4¹-ol-phenyl) thiophosphate, 3) O-methyl-O-ethyl-O-(2-methoxy-4-propen-4¹-ylphenyl) thiophosphate, 4) O-methyl-O-ethyl-O-(2-methoxy-4-propan-4¹-ol-phenyl) thiophosphate, 5) O-methyl-O-ethyl-O-(2-methoxy-4-propen-4²-ylphenyl) thiophosphate.

Experimental Part

Starting Reagents

In all experiments, bidistilled water freed from carbon dioxide was used.

96% ethyl alcohol was distilled over alkali and calcium oxide.

Sodium hydroxide was of “chemically pure” grade.

O-methyl-O-ethyl thiophosphates of model lignin compounds were obtained by the procedure described by us earlier [10]. Their characteristics are given in Table 1.

Experimental Procedure

Into a two-necked flask with a capacity of 250 ml, equipped with a reflux condenser and placed in a U8 thermostat, 100 ml of an alcoholic solution of caustic sodium was added, prepared on the basis of 0.5 M NaOH per 1 l of 50% ethyl alcohol. After heating the alcoholic alkali solution to the temperature at which the hydrolysis was carried out, 0.002 M thiophosphate dissolved in 1 ml of 96% ethanol was rapidly added to the flask. At specified time intervals, a sample was taken and rapidly cooled in ice water. The amount of the sodium salt of the corresponding phenol formed was determined by a photocolorimetric method on an FEK-1M instrument, at a wavelength with an absorption maximum of 405 mµ.

The rate constant of the second-order reaction was calculated by the formula [16]:

\[ K=\frac{2.3}{t(C_{O_2}-C_{O_1})}\cdot \lg \frac{C_{O_1}(C_{O_2}-2x)}{C_{O_2}(C_{O_1}-x)}\ \text{l}\cdot\text{mol}^{-1}\cdot\text{min}^{-1}, \tag{1} \]

where \(C_{O_1}\) and \(C_{O_2}\) are the initial concentrations of thiophosphate and alkali, respectively, mol/l,

\(x\) is the amount of the sodium salt of the phenol formed (or thiophosphate decomposed) at time \(t\), mol/l.

Hydrolysis of vanillin O-methyl-O-ethyl thiophosphate was carried out in the temperature range 30–60°C (Fig. 2). The hydrolysis rate constants in alkaline medium are equal to:

\[ \begin{aligned} 60^\circ\mathrm{C}&\ —\ 8.91\times10^{-2}\ \text{l}/\text{mol}\cdot\text{min},\\ 50^\circ\mathrm{C}&\ —\ 3.80\times10^{-2}\ \text{l}/\text{mol}\cdot\text{min},\\ 40^\circ\mathrm{C}&\ —\ 1.65\times10^{-2}\ \text{l}/\text{mol}\cdot\text{min},\\ 30^\circ\mathrm{C}&\ —\ 0.698\times10^{-2}\ \text{l}/\text{mol}\cdot\text{min}. \end{aligned} \]

On the basis of the data obtained, a plot of the dependence of the rate constant on temperature in Arrhenius coordinates was constructed (Fig. 3). As can be seen from the plot, the dependence \(\lg K - 1/T^\circ\) is linear.

The activation energy, calculated by the formula [16]:

\[ E=4.57\,[\operatorname{tg}\alpha]\,e, \tag{2} \]

is 17,000 kcal/mol.

Fig. 2. Kinetic curves for the formation of the sodium salt of vanillin and their anamorphoses in the coordinates \(\lg \frac{C_{O_1}(C_{O_2}-2x)}{C_{O_2}(C_{O_1}-x)}\)—\(t\) in the reaction of alkaline hydrolysis of vanillin O-methyl-O-ethyl thiophosphate at various temperatures: 1—60°C; 2—50°C; 3—40°C; 4—30°C.

Fig. 3. Dependence of \(\lg K\) on \(1/T^\circ\) in the hydrolysis reaction of vanillin O-methyl-O-ethyl thiophosphate in an alkaline medium.

Under the same conditions, at a temperature of 50°C, hydrolysis was carried out of the O-methyl-O-ethyl thiophosphates of: 1-guaiacyl-1-propanone, isoeugenol, 1-guaiacyl-1-propanol, and eugenol. The hydrolysis rate constants are given in Table 1.

From the experimental data it is evident that the increase in the rate constants of thiophosphate hydrolysis is in agreement with the increasing acidic properties of the phenolic hydroxyl groups of the model lignin compounds. Figure 4 presents the correlation of the logarithm of the constants with the \(pK_a\) of the initial phenol. These data show that an increase in the acidity of the phenolic hydroxyl group of the model lignin compound decreases the stability of the P—OAr bond with respect to hydrolysis in an alkaline medium.

Fig. 4. Dependence of \(\lg K\) on the \(pK_a\) of the initial phenol for thiophosphates of the type \((\mathrm{CH_3O})(\mathrm{C_2H_5O})\mathrm{P}(S)\mathrm{OAr}\) in the alkaline hydrolysis reaction.

Conclusions

1. The hydrolysis reaction in an alkaline medium of vanillin \(O\)-methyl-\(O\)-ethyl thiophosphate was studied in the temperature range 30–60°C, and the rate constants at these temperatures were determined. The activation energy was calculated, and a linear dependence of the rate constant on temperature in Arrhenius coordinates was shown.

2. The hydrolysis rate constants in an alkaline medium at 50°C were determined for \(O\)-methyl-\(O\)-ethyl thiophosphates of vanillin, 1-guaiacyl-1-propanone, isoeugenol, 1-guaiacyl-1-propanol, and eugenol.

3. It was shown that the dependence of the logarithm of the reaction rate constant on the degree of acidity of the phenolic hydroxyl groups of model lignin compounds is linear.

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