Jan. 20, 1950
341
REACTION OF ISOPROPYL METHYLPHOSPHONOFLUORIDATE WITH CATECHOLS
zene a t 50°, and in tetralin a t 92O.S All polymerization experiments were carried out in sealed and degassed ampoules, the polymer isolated by precipitation with methanol and compared with a control experiment. Polymerization of acrylonitrile is also induced by dianisoyl diimide in ethanol-carbon tetrachloride a t 50' and in benzene a t 75' and by p-methoxy-p'-nitrodibenzoyl diimide in ethanol-carbon tetrachloride a t 50°, in ethanol-benzene a t 50°, and in benzene a t 75'. Dibenzoyl diimide failed to initiate polymeri-
zation of acrylonitrile in benzonitrile containing aniline at 75'. However, amines have been reported t o inhibit the polymerization of acrylonitrile.*Q In none of our qualitative polymerization experiments has there been any noticeable effect of the acrylonitrile on the rate. This was also the case in a kinetic experiment in which p-methoxy-p'-nitrodibenzoyl diimide was decomposed in 50% ethanol-benzene containing 1% of acrylonitrile. Acrylonitrile does not inhibit the decomposition.
(28) L. Horner and W. Naumann, Ann., 687, 93 (1954), reported no polymerization in refluxing acrylonitrile or in refluxing acrylonitriletetralin under carbon dioxide.
TALLAHASSEE,
(29) C. J. Stehman, U. S. Patent 2,607,795, August 19, 1952.
FLORIDA
[CONTRIBUTION FROM THE SANITARY CHEMISTRY BRANCH, CHEMICAL CORPS MEDICAL LABORATORIES]
Kinetics of the Reaction of Isopropyl Methylphosphonofluoridate with Catechols at 25' BY JOSEPH EPSTEIN,DAVIDH. ROSENBLATT AND MARYM. DEMEK RECEIVED SEPTEMBER 6, 1955 The reaction of Sarin (isopropyl methylphosphonofluoridate) with catechol or one of its nuclearly substituted derivatives is of second order; the rate is proportional to the first power of both the Sarin concentration and singly dissociated catecholate ion. The second-order rate constant (1. mole-' min.-l) for the reaction of a particular catechol derivative may be expressed as a function of the basic dissociation constant of the catecholate ion, according to the equation kz = 2.57 X 10- Kk8*. The enhanced reactivity of the catechols toward Sarin as compared to phenols, and the relationship of k f t o KB are in agreement with the proposed reaction mechanism.
Sarin (isopropyl methylphosphonofluoridate) is among the more potent of the German nerve gases. It resembles diisopropylphosphorofluoridate in chemical behavior but reacts more rapidly with most reagents. Our purpose, in the present study was to obtain an understanding of the behavior of catechols,2 which react more rapidly than phenol^,^^^ with nerve gases, using Sarin as a model compound, in order to permit prediction of reactive catechol structures. It has been shown3 that the reaction rates of phosphorofluoridate esters with catechols, which, in aqueous solution, increase with rising pH, are consistent with the assumption that the rate is dependent upon the concentrations of the dissociated catecholate ions; but the effect of substituents on the aromatic ring has not, up to now, been clarified. At constant pH the rate of disappearance of Sarin when a catechol is present in sufficient excess concentration is of first order. The first-order rate constant thus obtained, corrected for spontaneous hydrolysis, (Le., kl - k,) where k1 is the observed first-order rate constant, k, is the rate constant for hydrolysis of Sarin in catechol-free aqueous solution a t the pH and buffer strength of the experiment, is proportional t o the catechol concentration. Thus, an apparent second-order rate constant k2' = (kl - k,)/[C] may be calculated, where [C] is the molar concentration of catechol plus catecholate ion, and k1 and k, are as previously defined. The apparent second-order rate constant, kz', for the reaction of unsubstituted catechol with Sarin over the PH range 6-9 varies directly with hy( 1 ) Anon., Chcnt. E W E Nsws, . 31, 4676 (1953). (2) "Catechol" will refer to the class of compounds that Includes the parent compound (unsubstituted catechol) and its nudearly substituted derivatives. (a) B. J. Jandorf, T. Wagner-Jauregg, J. J . O'Neill and M. Stolberg, THIS JOURNAL, 14, 1521 (1952). (4) K.B. Augustinsson, Acta Ckcm. Scand., 6, 949 (1952).
droxyl ion concentrations, but the corresponding constants for the more acidic mononitrocatechols approach constancy in the regions of the greatest monodissociation (Fig. 1). While k2' varies with pH, lot
-
CATECHOL
- 7 - 3-NITROCATECHOL ---&---
5
-
4-NITROCATECHOL
05
10
05 LOG OK;.
Fig. 1.-Plot of the log kn' against PH for reactions between Sarin and catechol, 3-nitrocatechol and 4-nitrocatechol.
k~ = '(kl - ks)/[C-] (where [C-] is the concentration of monocatecholate ion) remains constant over a range of pH values (Table I). EFFECTOF pH
TABLE I RATECONSTANTS FOR REACTION OF 3NITROCATECHOL WITH SARIN ON
ka'
pH
6 6
7
(I. mole-' rnin.-I)
0.27 .24 .81
When [H+]
ks mole-1 (Lin.-I)
1.48 1.33 1.18
kr'
pH
7 8 8
(1. mole-I mm.-1)
ka
(I. mole-' min.-l)
0.80 1.31 1.25
1.16 1.37 1.31
= hydronium ion concentration, and [C-] = KA[CI/([H+I KA).
K A is known,
+
J.
342
EPSTEIN,
D. H. ROSENBLATT AND M. ihf. DEMEK
Vol. 78
solutions, adjusted to the desired PH were added simultaneously t o the reaction vessel. BIMOLECULAR RATE CONSTANTS FOR REACTIONOF CATEIn general, the rate of disappearance of Sarin was followed CHOLS WITH SARIN colorimetrically. Aliquots were withdrawn from time t o kr (1. mole-1 time for analysis. As checks, the rate constants obtained Compound KB min. - 8 ) from the base uptake rates for three experiments with unsubstituted catechol a t pH 7 (outside the natural buffer Unsubstituted catechol 3.03 X lo0 589 range of the catechol) were compared with the constants from Protocatechualdoxime 4.76 X lo* 158 colorimetric measurements; the two methods were in exProtccatechunitrile 5.26 X lo7 22.6 cellent agreement. 2,3,4-Trihydroxyacetophenone 4.17 X 107 17.7 In all experiments initial catechol concentrations were a t least four times the Sarin concentrations, with [C] varying Protocatechualdehyde 2.27 X loT 8.5 between 2 X and 6 X 10-2 M and Sarin concentrations 4-Nitrocatechol 7.69 X lo6 4.6 between 5 X and 10-3 M . 3-Nitrocatechol 4.54 x 10s 1.5 Colorimetric analyses were performed by two modifications of the o-tolidine perborate method.8 In experiments A plot of average values (Table 11, showing results with unsubstituted catechol, including all those with bufof a t least four runs per compound) of log kz against fered media, a suitable aliquot of reaction mixture containing log K B ( K B = l/K# shows a linear relationship approximately 0.4 mg. of Sarin was added to a mixture of 10 ml. of 0.05 M phosphate buffer (pH 8.8), 10 ml. of alde(Fig. 2) ; the line of best fit gives the equation kz = hyde-frees acetone, 2 ml. of o-tolidine reagent and 50 ml. 2.57 X 10-6K$sg. water in a 100-ml. volumetric flask; this aliquot was immediately followed by 3 ml. of perborate reagent and the resulting mixture diluted to volume with water; the colorimeI ter reading was made 20 minutes after addition of the last reagent. In the remaining experiments with unsubstituted catechol, as well as with all other catechols, the procedure was altered, in that the volume of the aliquot was such as to contain approximately 0.04 mg. of Sarin, and 10 ml. of xylene was added before bringing the mixture to the volume. During the 20-minute color development period the flask was inverted a sufficient number of times to effect complete I /' extraction of the yellow dye formed; the xylene volume was then separated and its color measured. The spontaneous hydrolysis rate constants for Sarin in /
