3596
JOSEPH
EPSTEINA N D DAVID H. ROSENBLATT
VOl.
\o
u. S.ARMYCHEMICAL TARFA FARE LAUORAIORIESJ Kinetics of Some Metal Ion-catalyzed Hydrolyses of Isopropyl Methylphosphonofluoridate (GB) at 25' [ C o S T R I U U T I O N FROX THE S A X I T A R Y CHEXISTRY B R A S C H O F TIIE
BY JOSEPH
EPSTEINiIXD DA\TDH.
ROSENULATT
RECEIVED J A S U A R V 7, 1958 HI-drolysis of isopropyl tiietli).lplio~:pliono~u~ridate (GB) in water is catalytically accelerated b y cerous, cupric tnclnganous ions. T h e active forms of these ions are most probably of the type M(OH)(H2O);Ii.
The introduction in recent years of toxic phosphorus-containing chemical warfare agents (nerve gases) has produced a need for rapid and c o n \ w k n t decontaminating methods for removing such substances from water and surfaces. Because of the potency of these compounds even in very dilute aqueous solution,l decontaminating reactions possessing high rate constants have been sought. -1 study of the effect of aquometallic ions on the hydrolysis rate of isopropyl methylphosphonofluoridate (GB) was indicated, both by the abnormally high rates of decomposition of GB iii brackish waters' (which could be explained by the presence of trace quantities of heavy metal cations) and by extrapolation of the extraordinary enhaiicing effects of positively charged hydroxoiiietallic ions3 on the rates of other generally acid--base catalyzed reactiomi Furthermore, hydrolysis of this compound is strongly accelerated by certain copper complexes of heterocyclic nitrogen bases.; A number of other instances of metal ion-catalyzed hydrolysis of phosphorus esters also have been observed.G T h e purpose of the present series of experiments was to study the efTect of hydrated cupric, cerous and manganous ions 011 the hydrolysis of GB, and to attempt to correlate the results with known acidbase effects. I t was established t h a t the hydrolysis is catalyzed by metal ions. -4lthough the reaction a t constant PH is first order with respect to each of the reactants (see data in Table I, cupric ion a t PH 3.3), first-order kinetics were observed a t approximately equal concentrations of reactants (Fig. 1). In bimolecular reactions in which the reactants are present in approximately equal concentrations, first-order kinetics generally are observed only if the concentration of one of the reactants remains constant. In one experiment the rate of decomposition of GB in the presence of an equimolar concentration of cupric ion was determined at constant PH. After approximately SOYGof the phos(1) J. Epstein, P i i b l i ~I l d u l l h &.b
1" 2 1' 1"
3"
2 1
1 I 1 ( I GI3 concentration 0.001 11, e\cept tiotttl ' (,11 concentration 0 0005 &lf GB ci)iiceiitr,ltioii (1 002 II
In the PH region studied, the doininant iiietdl ioLi species are the undissociated ions Cu(HK))I17 , Ce(H20)6+++ and l I n ( H 2 0 ) 6 T +(in general AI(HzO);) with small amounts of the rnonohydroxo species Cu(0H) (H20)~+,C e ( 0 H ) ( H L O ) ~ + +and l l n ( O H ) ( H 2 0 ) 6 +(in general Al(OH)(HsO):::), respectively. Their concentrations are related' according to the general equation
July 20, 1938
HYDROLYSES OF ISOPROPYL METHYLPHOSPHONOFLUORIDXTE
where K A is the acid dissociation constant of the hydrated metal ion. Thus, the concentration of the monohydroxo species is inversely proportional to the fraction [H30+]/[hI(H~O)i] or K w / [OH-][M(H20)%1where K , equals the ion product of water, and, from the kinetic data above, no decision can be made as to whether the decomposition is facilitated by an attack of the monohydroxo species or of both the hydrated metal ion and hydroxyl ion. On the basis of other reactions of GB and D F P , for which mechanisms have been postulated, we are inclined to favor a mechanism involving the monohydroxo species. The following discussion sets forth the pertinent background. \\'here sufficient investigation has been made, series of related reagents have given typical Bronsted relationships.4 i-lo It has further been observed that although basic catalysis is considerably more elCfective than acid catalysis,i the latter may be considered to play a role of importance in reactions best explained by push-pull (concerted attack) mechanisms."-'" Such mechanisms require reagents characterized by centers of both high and low electron density. X dissociated aquometallic ion, of the type M(OH) (HLO)z-:,which has a basic region in the hydroxyl group and an acidic region in the metal might qualify as such a reagent and, indeed, 11-agner-Jauregg and co-workers have considered copper chelates of heterocyclic nitrogen bases to act in this fashi0n.j In examining old data3 on the general base catalysis oE the mutarotation of glucose, an inexplicably high activity was noted in the case of two metal complexes. These were the only catalysts in the published series for which centers of high and low electron density can be postulated, and which might, therefore, be expected to function through a push-pull mechanism. T h e plausibility of such an explanation is increased by other, better substantiated, instances of this type of mechanism in the mutarotation reaction." Finally, the role of metal ions in other examples of catalysis of organic reactions has likewise been recognized and catalysis mechanisms have been postulated.l? I t is therefore proposed t h a t the metal ion-catalyzed hydrolysis of GB actually takes place through positively charged hydroxometallic bases in a bimolecular mechanism. T h e second-order rate constant ( k 2 ) equals ki/[iLI(OH)(H?O)t-:]
KI[H,O-]/KA[M(H~O):]
h W3COI / K t PI1
3597
100 90
80
70
60
50
40
.-.--
M
.."
j e
r 3u
c;
€i?
'0
10
50
100 150 Time in minutes plot of GB concentration Rith t i m e ;
200
Fig 1 -First-order [GB] 1 x 10-3 M ; [ c ~ c i=~1]x 10-3 M; PH = 5 0 ; T = 250: 0 , H101-a-tolidine method; 0, S a O H titration. =
(8) J . Epstein, n H. Rmenhlatt and M . M . Demek, THISJ O U R X A L , 78, 341 (195(j), (9) B . E . Hackley, J r , P h D . Dissertation, University c,f Delaware, 1950. (10) R . Swidler, R 12 Plapinger and C . 11.Steinberg. manuscript in preparation. (11) C. G. Swain. THISJ O V R N A I . , 70, 1119 (1048); 72, 4578 (1950); C:G. Swain and J. F. Brown, Jr , itid.. 7 4 , 2534, 2538 (lY.52).
From average Values Of kl [H,fO] [ A I ] in Table 1 and values of K A from the literature for cupric ionI3 112) F H. Westheimer, T v a n s . S . Y.A c o d . S c i , 18, 13 (1,955). (13) K. J . Pedersen, K g i . D a ~ s k e Videjiskab. Selskab. ,Mafh.-Fys. Lbfedd.,20, X o . 7 ( l Y 4 3 ) ; C . A , 38, 48.54 1194-11. In the p H range and concentration concerned, the polymerization reactions of hydroxucupric ion are unimportant.
and cerous ionL4of 1.07 X and 3.0 x 10-lo, second-order rate constants of 1.2 X lo4 and l.iX 10.' 1. mole.-' min.-', respectively, are obtained for these ions. ,Ilthough these hydroxometallic ions are considerably weaker bases than hydroxyl ion (KAfor they are catalytically more hydroxyl ion = active than the latter (for which k? = 2 x 10").!5 This would tend to lend support to the view t h a t a push-pull mechanism is involved. Similar effects have also been observed in these laboratories with magnesium, calcium and uranyl ions. inight be expected, both calcium and magnesium, whose hydrated species have very srnall dissociation constants, shoxed this behavior c.)nly in alkaline niedia where the large hydrolytic effect of hydroxyl ions prevented quantitatiL-e measurement of the part played by the hydroxoiiietallic ions. Uranyl ion, on the other hand, cxhibited a marked effect in extremely dilute solutioii e\.en a t low pH levels. Experimental Reagents.---C .P. grade chcniicals were used as purcliaserl, without further purification. Cupric chloride vas obtained :is the dihydrate, manganous chloride as t h e tetrahydrate and ccroiis iiitrate as the hexah\-drate. GB of high purity w:is obtained from the Chernical Research Division of tlie Chernic:il \\:irfare Laboratories. Apparatus.---In earl>- experiinants the Beckman model G pH meter \\':is used. 1,nter the p H x i s hcltl constant with ;t 13cckin:tn niotlel E; automatic titrator set to deliver the srnallest pciisiblc increnients of titrant. m t l provided with a -filled iriicrol)uret gratlu:~tcd to 0.02 1111. 1Ce;ictioii teniper,tture i i i :L jacketed 2,X-iiil. beaker w:is m t i i i -
taincti a t 25.0" by it Precision Scientific Co. constant temperature circulating bath. Reaction mixtures were constatitl>-stirred with a 1-inch Teflon-covered magnetic stirring bar. .I E;lett--Surxiinerson photoelectric colorimeter provided rt-ith a So. It' filter \vas used in the atialysis for GI1. Kinetic Measurements.-Solutions of G B a r i d the InetLiI sult (on the acid side) were mixed in the reaction vessel to give the required concentration of each, antl the pH ~t-xs adjusted with base from the microburet. The amount i i f , T',, required to keep the PH constant duriiig tlic ,e of reaction (hydrolysis liberates one inclle e:ich ( i f i s i ~ p r i ~ ~ ~ y I t r i e t h ~ I p l i o s p lacid i o n i canti hydrofluoric acid) \\:is rcctircletl against time. Basic hydrolysis ant1 h c k titration of an nliquiit of tlie GB stock gave the \-olunne of basc ( 1-a i' required a t infinite time. Suitnble corrections were nplilietl to t h e d a t a to :tceount for initial acidity, \vitlidr:iw:il o f iamiples anid vvlunie changes, and tlie corrected \values [ i f ( 1" - I'L)/ I.=, which represeuts t h e fr;tetion o f GI3 remaitling, irere plotted 011 semi-l~igaritliiriic p'tper. T h e plc>ts \\sere virtuall\- linear; thus, thc reactions cshibite(l first-order kinetics. 111 three espcri~tictitstliese resiilt.~\ZC'I-C confirtried 111- direct aiial\-sis for C B b y t h e perositle~0 tolitiiiie mctllotl.'fi LImt of t h e esperiinenti Xt-ere eontlttctcd in the pH ruiigc 5-ii.S, iii irliicli the s;lx~iit:~ne~ius hydrolysis rate of GB,d u e to catalysis by !\-:iter a i i t l by h!-tlrnxyl anti hydronium ions is negligible. From tlie slopes r ~ fthc sctni-logarithinic plots, t h e first-order rate coiist:tiits, h , \vcre calculated in t h e usual manner . I i These v:irietl ivitli metal ion concentrations and PH. I n experiments \\-itti iiiangsnous a i i d cerous ions, nitrogen had ti) lie liubhletl through the reaction mixtures t o prevent oxitlation, which would h:ive changed the cimcentr;ttivxi of the dcsired metal iiin m d caused an annnialous increase in the uptake (if b:tse.
Acknowledgment.-The authors wish to acknowledge the technical assistance of Peter Hlinka, Richard Ess, Edward Costigan, Mary AI. Demck and Ra?-i~ioiidFeinlantl. 111;) 11. G e h a u l , J . ICpstein, C;, U . \ViIs
