The decomposition of 2-chloroethylphosphonic acid in aqueous solution has been studied at pH values from 6 to 9 and at temperatures in the 30 to 55 C range. The rate of decomposition is estimated from the rate of formation of ethylene. The rate is proportional to the concentration of the phosphonate dianion and is independent of the hydroxyl ion concentration. The rate constant at 40 C is 1.9 x 10-4 sec-I and the activation energy is 29.8 kcal mol-'. The rate of reaction is not affected significantly by the presence of potassium iodide or urea (substances which increase the rate of leaf abscission in trees sprayed by 2-chloroethylphosphonic add). The rate decreases sghtdy in the presence of low concentrations of magnesium and caldum ions.2-Chloroethylphosphonic acid is widely used in regulating plant development. Its plant growth activity is primarily associated with its ability to release ethylene to the plant tissues (2). A number of groups have studied aspects of the breakdown of the acid (1, 4-6). Maynard and Swan (4) showed that the products are chloride ion, ethylene, and phosphate. The yield of ethylene was found to be quantitative (1, 5), and according to Yang (6), the yield of phosphate is also quantitative. Maynard and Swan noted that the decomposition proceeds only at pH values above 4.5 and came to the tentative conclusion that a doubly ionized phosphonate group is required before fragmentation can occur. Edgerton and Blanpied (3) showed that the rate of formation of ethylene in buffered solutions increases rapidly with pH in the S to 7 region, and Warner and Leopold (5) claimed that ethylene evolution proceeds linearly with time for the first 7 hr, the rate increasing with increasing pH. The decomposition has been described as a base-catalyzed elimination reaction (1), and a second order reaction (5). Yang (6) suggests that the mechanism involves the nucleophilic attack of water on the phosphonate dianion and adds that OH-may also serve as a nucleophile in the reaction. Kinetic data for the decomposition under controlled conditions of both pH and temperature do not appear to be available. This note reports such data, together with some information on the effects of certain additives on the rate of decomposition. MATERIALS AND METHODSThe 2-chloroethylphosphonic acid, provided by Amchem Products, Amber, Pa., was of purity >99%; titration with NaOH indicated that it contained 99.8 + 0.4% acid. The material is hygroscopic and was exposed only to dry air. Buffer solutions were prepared by addition of NaOH solution to solutions of reagent grade sodium dihydrogen phosphate, boric acid, or sodium bicarbonate. Urea and KI were of reagent grade.The reaction vessel for the kinetic experiments consisted of a modified 125-ml conical flask with inner and outer compartments. Ten ml 0.1 M 2-chloroethylphosphonic acid solution, whose pH had been adjusted to 4.1 by addition of NaOH, was placed in the central compartment; 25 ml buffer solution was placed in the outer compartment, and the flask was immersed in ...