On the kinetics of the hydrolysis of organophosphorus compounds

Hidroliza organofosfornih spojeva sarina, tabuna i diizopropilfluorofosfata (DFP) istraživana je kinetički u razrijeđenim vodenim otopinama, u različnim pokusnim uvjetima. Za određivanje koncentracije aktivne nehidrolizirane tvarl služila su mjerenja intenzlteta fuorescencije oksidacionih produkata indola, koji se stvaraju pri oksidaciji »katalitičkim« utjecajem organofosfornog spoja, u prisutnosti alkalične otopine natrijeva perborata. Kinetička mjerenja su vršena kod različnih temperatura, u vodenim otopinama, u prisutnosti pufera, raznih alkohola i inhibitora. Određivani su temperaturni kvocijenti, energije aktivacija i vremena poluraspada radi provjeravanja reda reakcija tih hidrolitičkih procesa. Rezultati izvršenih eksperimenata mogu poslužiti za praktično prosuđivanje stabilnosti navedenih spojeva u vodenim otopinama.Chemical kinetics of the hydrolytic breakdown of sarin, tabun and DFP in waiter solutions under various experimental conditions was studied. The concentration of the compounds in solutions during hydrolysis was estimated by the photoelectric measurement of the fluorescence intensity which occurs as the result of the effectoric action of the active substance on the indol oxydation reaction. The experimental data led to the following conclusions: At all temperatures studied (25°, 35° and 45° C) the highest was the rate of the spontaneous hydrolysis of tabun, the lower that of DFP, and the lowest that of sarin. It follows that sarin is the most stabile and tabun the most labile compound among them. The specific rates of the spontaneous hydrolysis of sarin, tabun and DFP increase with the temperature and decrease in the presence of alcohols. The inhibitory effect of alcohols increases with ,the concentration and molecular weight which was established in the experiments with methanol, ethanol and isopropanol. The same concentrations of the same alcohol have the strongest effect on the hydrolysis of DFP, less effect on the hydrolysis of sarin and the least on that of tabun. Citrate buffer pH 3 and phosphate buffer pH 5 exert a promotory effect on the hydrolysis of sarin; the rate of reaction increases with the acidity of the solution and with the ionic strength of the buffer used. The effect of these buffers on tabun and DFP was not studied. Aniline, potassium iodide and bromide in concentrations studied also Increase the specific rate of sarin hydrolysis. Potassium iodide acts as a strong quenching agent on the fluoresence reaction so that the experiments had to be carried out with the concentrations ranging between 1.10-6 and 1.10-4 moles of KJ in the reaction solution. Neither phenol nor potassium chloride have a significant influence on the change of the specific rate of the hydrolysis of sarin at 35° C. The highest temperature quotients (Q10) and energies of activation (Δ H), expressed in cal/mol were found in the hydrolysis of DFP and the least in the hydrolysis of tabun. In spiste of the fact that the specific rate of the hydrolysis of DFP was found to be higher than that of sarin under ,the same experimental conditions, the energy of activation of the former was found higher too. It can be consequently derived that the steric factor affects the specific rate of the spontaneous hydrolysis of DFP. The Iowering of pH as well as the increase of the ionic strength of the buffer decrease the temperature quotient and the energy of activation of the hydrolysis of sarin at 25°, 35° and 45° C respectively. This is in accordance with the corresponding increase of the specific rate of the hydrolysis of this compound under above mentioned experimental conditions. AU processes studied are first order reactions because 1) experimental data satisfy the equation for the determination of the specific rate of the first order reaction, 2) the time of half-decay ( t1/2) IB independent of the concentration of the compound at the beginning of the reaction, and 3) there exists a linearity between the time which elapses from the beginning of the reaction and the logarithm of the concentration of the active substance (intact part of the organophosphorus compound) at the time of measurement