The Mechanism of the Hydrolysis of Condensed Phosphates. IV. The Hydrolysis of Pyro-, Tripoly-, Trimeta-, and Tetrametaphosphates in Aqueous Organic Solvents

Abstract The hydrolysis of sodium pyro-, tripoly-, trimeta-, and tetrametaphosphates was run in water, aqueous ethanol, aqueous glycerol, aqueous acetonitrile, aqueous dimethyl sulfoxide, and aqueous tetrahydrofuran at various pH values and temperatures. The initial concentration of the phosphates was 0.025 mol/l. The hydrolysis of the poly- and metaphosphates followed first-order kinetics with respect to the concentration of the phosphate, under all conditions studied. The hydrolysis of sodium pyro- and tripolyphosphates was an acid-catalyzed reaction and that of sodium trimeta- and tetrametaphosphates was an acid- and base-catalyzed reaction. In acidic solutions, the rate of hydrolysis of the poly- and metaphosphates in water was slower than that in any aqueous organic solution. It is concluded that the nucleophilicity of the water molecule in the aqueous organic solutions may increase on account of the interaction of the water molecule with the organic molecules. The hydrolysis of sodium pyro-, tripoly-, trimeta-, and tetrametaphosphates in acidic solutions is considered to be of the SN2 type, because the overall-reaction rate is highly dependent on the nucleophilicity of the water molecule. The solvent effect on the rate of hydrolysis of sodium tripolyphosphate in basic solutions was the same as that in acidic solutions and hence the mechanism of the hydrolysis of sodium tripolyphosphate in basic solutions may be the same as that in acidic solutions. The rate of hydrolysis of sodium trimeta- and tetrametaphosphates in a basic water solvent was much faster than that in basic aqueous ethanol and basic aqueous dimethyl sulfoxide and was largely slower than that in basic aqueous glycerol. Consequently, the mechanism of the hydrolysis of sodium trimeta- and tetrametaphosphates in basic solutions seems to differ from that in acidic solutions. The acceleration of the rate of hydrolysis of the metaphosphates by the organic molecules depends significantly upon the activation energy. The activation energy of the hydrolysis of sodium pyro-, tripoly-, trimeta-, and tetrametaphosphates was 17–34 kcal/mol in the pH range of 1.0–12.2 and increased with an increase in the pH value of the phosphate solution in water and aqueous ethyl alcohol solventss.