NOTES
1809
molecules have been examined. Actually, the best fitting model consists of two chains standing up and connected to one another by a water molecule. It must be assumed that the terminal CH, groups are keying into the opposite methyl surfaces of adjacent layers. The observed basal spacings are then almost exactly equal to thos? computed by assuming a C-C bond length of 1.542 A, a tetrahedral C-C-C angle of 109” 28’) a C-N bond length of 1.47 A, a C-C-K angle of l l O o , all these parameters being taken from Pauling6 and S u t t ~ n . The ~ 0-H. . . N hydrogen bond length is 2.80 A, in agreement with Pimentel and hIcClellan,’O while the usual H-0-H angle of 104” 45’ was maintained. The absence of heavy atoms in the chain as well as the weakness of the hkO reflections precludes any defined conclusions about the complete structure of these amine-water liquid complexes. (8) L. Pauling, “The Nature of the Chemical Bond,” Cornel1 University Press, Ithaca, N. Y.,1948,p 167. (9) L. E.Sutton, “Tables of Interatomic Distances and Configuration in Molecules and Ions, Special Publication No. 11, The Chemical Society, London, 1358,p M174. (10) G.C.Pimentel and A. L. McClellan, ”Hydrogen Bond,” Freeman and Co., London, 1960,p 289.
water to form the phosphite form (11) from which the deuterated phosphonate is produced by accepting a proton. If the two deuteroxyl groups of (I) are equivalent (although this is not necessarily so), then, since there will be an equal change of either group being lost, the above mechanism could also serve as a path for the isotopic exchange of oxygen between dialkylphosphonates and solvent water. It is therefore of interest to ascertain whether isotopic oxygen exchange occurs at a comparable rate with isotopic hydrogen exchange. The rate of oxygen exchange is also of importance in considering the acid- and base-catalyzed hydrolysis of dialkylphosphonate~,~since in principle hydrolysis may also involve the addition of water to form a pentacovalent intermediate which then breaks down to give the hydrolysis products
RO
0
\/ P /\
RO
H
RO
+ HzO
OH
\/ P-OH /\
---j
H
RO
(111)
RO
\/ P /\
Isotopic Oxygen Exchange and Hydrolysis
H
in Dialkylphosphonates
by David Samuel and Brian L. Silver Isotope Department, W e i z m a n n Institute of Science, Rehovoth, Israel Accepted and Transmitted by T h e Faradau Society
( J u n e $1, 1967)
The phosphorus-bonded hydrogen in dialkylphosphonates has been to undergo exchange with the deuterium in solvent DzO. This isotopic exchange is both acid and base catalyzed. A possible mechanism for the acid-catalyzed reaction is the addition of water to form a pentacovalent intermediate (I) containing two equivalent lhydroxyl groups which could then lose
RO
\/ P
/\
RO
RO
0
H
+DzOG
\/
/\
RO
-HDO
c
H
(1) RO
RO
OD
\/ P: /
RO
(11)
---f
OR
+ ROH 0
If the first step is reversible and the two hydroxyl groups of (111) are equivalent, a pathway is available for the isotopic exchange of oxygen between dialkyl phosphonates and water which is analogous to the exchange found by Bendere in the hydrolysis of carboxylic acid esters. We have, therefore, determined whether oxygen exchange between substrate and solvent occurs (a) during the acid- and base-catalyzed hydrolysis of dialkylphosphonates and (b) during the isotopic exchange of hydrogen in dialkylphosphonates. We have also determined the position of bond fission during the hydrolysis of these compounds using l8O as tracer.
Experimental Section Materials. Water enriched in l8O was obtained from the Separation Plant of this Institute. Dialkylphosphonates were prepared from phosphorus trichloride and the appropriate alcohol7 and were fractionally distilled twice before use. Monoalkylphosphonates were made by adding 0.1 mole of sodium hydroxide in water to an aqueous solution of 0.1 mole of dialkyl-
OD
P-OD-
OH
0
\/ P /\
D
(1) P. R. Hammond, J. Chem. Soc., 1365 (1962). (2) Z.Lus and B. L. Silver, J . A m e r . Chem. Soc., 83, 4518 (1961). (3) B. L. Silver and Z. Lus, ibid., 84, 1091 (1962). (4) Z.Lu5 and B. L. Silver, ibid., 84, 1095 (1962). (5) P. Nylen, Svenisk K e m . Tidsskr., 49, 79 (1937). (6) M. Bender, J . A m e r . Chem. Soc., 73, 1626 (1951). (7) H.McCombie, B. C. Saunders, and G. J. Stacie, J . Chem. Soc., 380 (1945). V o l u m e 72, Number 6
M a y 1068
1810 phosphonate and removing the solvent by lyophilization. Isotopic Oxygen Analysis. All analyses were by the meihod of Anbar and Guttmann.6 Oxygen Exchange. Di-n-propylphosphonate was chosen for stady since it has the advantage of being readily saltcd out of aqueous solution, thus facilitating its separation froin the reaction mixture. Di-n-prqpylphosphonate (1 ml) was dissolved in aqueous solutions of HC1 or NaOH (6 ml) in '80-enriched water. The hydrolysis of dialkylphosphonates in strong base i s very rapid. Consequently, only enough NaOH solution was added to hydrolyze 50% of the phosphonate. The remaining unhydrolyzed substrate was recovered from the reaction solution immediately. I n acid media, samples of the reaction solution were removed at measured times and the phosphonate was salted out by the addition of sodium chloride. The upper layer (phosphonate) was removed with R pipet, dissolved in normal water ( 5 ml), and immediately salted out again. This procedure was repeated and the phosphonate was then dissolved in dry dioxane (20 ml) and dried over Iinde molecular sieve 4A for 16-18 hr. The residual phosphonate was left on the high-vacuum system for a further 4-5 hr and analyzed for its oxygen-18. Samples of phosphonate were recovered from the reaction mixture until a time corresponding to at least the halflife for the hydrogen exchange. To check that no exchange occurred during the isolation procedure, a sample of diethylphosphonate-'*O (1.0 ml) in wet ether (25 ml) was dried overnight with Iinde molecular seive (5 g) and then isolated and analyzed for l80as described above, There was no significant difference in 1 8 0 content of the phosphonate before and after the isolation procedure, the excess atomic percentage of l a 0 being, respectively, 74.8 and 73.9%. Bond Fission. Alkyl phosphonates (5-10 g) were dissolved in aqueous solutions of HC1 or S a O H (120 ml) in IsO-enriched water. After refluxing for 1 hr, the alcohol was isolated by distilling the reaction mixtures through a vacuum-jacketed Vigreux column (30 X 0.5 cm). I n the case of dimetliylphosphonate, the first fraction was methanol which was refractionated in a microcolumn (10 x 0.5 cm), the middle fraction (bp 64.5') being taken for isotopic analysis. I n the case of diethylphosphonate, the azeotrope of ethanol and water was collected (bp 78-81'), dried with two SUCcessive batches of Linde molecular seive, and distilled in a microcolumn.
Results Exchange. The results of the oxygen exchange experiments with di-n-propylphosphonate are given in Table
T h e Journal of Phyaical Chemistry
XOTES Table I : Oxygen Exchange in Di-n-propylphosphonate (Temp 22 i l o )
Time, hr
Solvent
HC1, 1 N
0.02
1 HC1, 0 . 5 N HC1, 5 . 0 N NaOH, 1.O N
2 3 1 0.15
..
Excess atom 7% 1 8 0 in solvent
Excess atom % 1 8 0 in recovered substrate
8.81 8.81 8.81 8.81 3.45 3.28 2.7
0.004 0.002 0,003 0.007 Q.011 0,010 0.007
Table I1 : Position of Bond Fission in the Hydrolysis of Alkylphosphonates
Phosphonate
Diethyl Dimethyi
Solvent
Excess atom % 1 8 0 in solvent Ha0
Excess atom % 1 8 0 in recovered alcohol
% P-0 bond fission
HC1, 1 N NaOH, 2 . 5 N HC1, 1 N NaOH, 2 . 5 iV
1.90 1.93 5.08 3.55
0.030 0.020 0,050 0.007
98 99 99 100
I. From Table I it may be seen that no oxygen exchange occurs between phosphonates and solvent water for the stated reaction times (which correspond to at least one half-life of the acid-catalyzed hydrolysis5 and 5-10 half-lives of the acid-catalyzed hydrogen exBond Fission. The results are given in Table 11. The results show that in the hydrolysis of dimethyl- and diethylphosphonates in both acid and basic solution, the bond between phosphorus and oxygen is broken in every case (P 0-R). The isotopic results confirmthe observation by Cerrardgthat in the hydrolysis of optical active di-sec-octylphosphonate the alcohol retains its optical activity. Since hydrolysis is complete under the experimerltal conditions, the results imply Y -0 bond fission in the hydrolysis of the monoalkyl phosphonates. The lack of observable oxygen exchange is very strong evidence against a mechanism involving addition of water in the hydrolysis and hydrogen exchange of the compounds studied. (8) M. Anbar and 8.Guttmann, J. A p p l . Radiation Isotopes, 3, 233 (1969). (9) W.Gerrard, W. J. Green, andR. A . Nutkina, J . Chem. Soc., 4076 (1962).
