The Mechanism of the Hydrolysis of Condensed Phosphates. III. The Mechanism of the Hydrolysis of Trimeta- and Tetrametaphosphates

Abstract

Abstract The hydrolysis of sodium trimeta- and tetrametaphosphates was run in water, dioxane–water, and formic acid–water solvents with an initial concentration of 0.025 mol/l at various pH values and temperatures. The hydrolysis of both the metaphosphates followed first-order kinetics with respect to the concentration of the phosphate under all the conditions studied and was an acid- and base-catalyzed reaction. In acidic solutions, the rate of hydrolysis of both the metaphosphates in water is faster than that in formic acid–water and slower than that in dioxane-water. Therefore, it is concluded that, in the formic acid–water solvent, the nucleophilicity of the water molecule may decrease on account of the solvation of the water molecule with the acid, while, in the dioxane–water solvent, since the scission of the hydrogen bond between water molecules may occur by the interaction of dioxane and water molecules, the nucleophilicity of the water molecule may increase. The hydrolysis of both metaphosphates in acidic solutions is considered to be an SN2 reaction, because the overall-reaction rate is highly dependent upon the nucleophilicity of the water molecule. In basic solutions, the rate of hydrolysis of sodium trimeta- and tetrametaphosphates in water is faster than or almost the same as that in dioxane-water. Consequently, the mechanism of the hydrolysis of both the metaphosphates in basic solutions seems to differ from that in acidic solutions. The increase in the rate of hydrolysis of small-ring phosphates in acidic solutions depends upon the activation energy and frequency factor, while that in basic solutions depends significantly upon the frequency factor. This may cause the difference in the solvent effect on the rate of hydrolysis of small-ring phosphates between acidic and basic solutions. The activation energy of the hydrolysis of both ring phosphates was 20–40 kcal/mol in the pH range of 1.0–12.5. The rate of hydrolysis of the metaphosphates in an aqueous sodium hydroxide solution is faster than that in an aqueous tetramethyl ammonium hydroxide solution and this can be attributed to catalysis by a sodium ion. The hydrolysis of sodium tripolyphosphate was also carried out in basic water and dioxane–water solvents. The solvent effect on the rate of hydrolysis of tripolyphosphate in basic solutions was the same as that in acidic solutions.