Nov., 1961
DIPOLEMOMENTS OF PHOSPHITE ESTERS AND THEIR DERIVATIVES
as the C t,o B transformations in his statement. The almost vertical boundary between the Aand B-type polymorphs also was postulated by Roth and Schneider,* while both Goldschmidt, et a1.,2 and Shafer and Roy7 show the boundary to have a moderate slope. These moderate slopes are due to the fact that Goldschmidt, et al., obtained A-type Smz03and B-type NdzOa while Shafer and Roy also found B-type KdZO3. I t is believed that the availability of purer samples accounts for some of the differences between the present and previous studies. It is especially likely that the sesquioxides which Goldschmidt, et al., used were impure since B ~ m m e rwho , ~ claimed that he used the purest rare earth oxides which were available in 1939, points out that his samarium oxide contained 0.8% europium and 0.3% gadolinium. The differences between this work and that of Roth and Schneider8 are mostly due to their not using hydrothermal techniques to attain equilibrium a t the lower temperatures and to the fact that, with only three exceptions, their study was limited to a maximum temperature of 1500". In one instance they used an arc image furnace to melt DyZO3, but they were not able to quench the sample rapidly and did not observe the B-type polymorph. The sesquioxides of the largest ions (La, Ce, Pr and Kd) most commonly exhibit the A-type structure. Laiithana has been reported4 to exist in the C form, but it has not been possible to repeat this preparation of C-type LazOa. This polymorph has been prepared hydrothermally from the appropriate A- or B-type oxides of S d , Sm and Eu. It should be emphasized here that, in the method used, the oxides were in contact with water only at the highest temperature of the run. It is not possible to obtain C-type oxides for the still larger rare earths hydrothermally since the A-type polymorph is stable down to the upper temperature limit for the oxyhydrosides. The inversion of A to C by dry heating has not been ef-fectedexcept in the instance
2051
of neodymia, which indicates either that C is not a stable form of Laz03,Cez03 or Prz03or that the inversion temperature is too lorn for it to occur in a reasonable length of time. C transition of NdzO,, is about 600" The A whereas Roth and Schneider8stated that a temperature of 650" was required to change C to A. While Shafer and Roy7 found that Nd203 could exist as the B-type polymorph as well as in the X and C forms, it was not possible to reproduce their results even though the same conditions and hydrothermal equipment were used in this study. This is probably due to a sample of higher purity being used in the present study. The intermediate rare earth oxides exist in both the B- and C-type polymorphs and these transfoimations are reversible, as shown in Table I. These include Sm203, Eus03, Gd203. Tb2O3 and Dy203. While B-type Dy,03 was reported preneither they nor viously by Goldschmidt, et any other previous investigators mentioned B-type Tb208 even though this oxide transforms at a lower temperature than Dyz03. As for Dy203,it should be noted that it has not been possible to obtain pure B-type. As stated previously, the transformations are very sluggish and thus a considerable length of time is required to transform completely a particular sample. With our equipment it was not possible to maintain approximately 2400' for more than 15 to 30 minutes and thus the samples of Dy,Oa consist of both the B- and C-type polymorphs. The interpretation of such data js based on the fact that one polymorph will not grow a t the expense of a second unless the first is the stable form under the conditions of the experiment. The sesquioxides of the smallest ions, 1'. Ho, Er, Tm, Yb and Lu, exist only as the C-type polymorph. All previous studies agree on this point. Acknowledgment.-This work forms part of a research program in crystal chemistry supported by the Chemical Physics Branch of the U. S. Army Signal Corps.
THE DIPOLE MOMENTS OF SOME PHOSPHITE ESTERS AND THEIR DERIVATIVES BY THEODORE L. BROWS,J. G. VERKADE llXD T. S. PIPER Soyes Cheirizcal Laboratory, University of Illinois, rrbana, Ill. Rcceized June 6, 1951
The dipole moments of the constrainrd phosphite esters l-methyl-4-phospha-3,5,8- trioxabicyclo [2.2.2]octane (I) and l-phospha-2,8,9-trioxa-adamantine (VI) have been determined in dioxane solution. In addition the moments of the phosphate and thiophosphate of I have been determined. The constraints imposed by the bonding in I preclude free rotation about the P-0-R bonds, thus permitting a better estimate of the apparent P=O and P=S group moments. These are 2.95 and 2.62 D, respectively.
Introduction
lated comDounds have been determined. Rotation of the alkdxy groups about the P-0 bond is not possi-
This paper reports the measurements of the di- ble in either 1or 1 7 1 . The interpretation of the dipole pole moments of the constrained phosphite esters, moments of these COmpOUndS, and more particul-methyl-4-phospha-3,5,8-trioxab~cyclo[2.~.~l-~clarly of the difference in moment between phosphite tane (I) and 1- phospha - 2,8,9 - trioxa - adamantine and phosphate or thiophosphate, is more straight(VI). In addition the moments of a number of re- forward 6han in ordinary phosphite compounds.
T. L. BROWN, J. G. VERKADE AND T. S. PIPER
2052
Vol. 65
TABLE I DIPOLE MOMENT DATAFOR COMPOUNDS IN DIOXANE AT 25.0" Values in parentheses refer to cyclohexane solution a t 25.0'
I
Compound
Mol. wt.
CH&( CH20)aP
148.10
P
a
Pz (cm.8)
M R (cm.3)
p
(Debyes)
14.60 -0.179 388 33.6" 4.15 (8.89) ( - .45) (348) (3.91) I1 CHsC(CHz0)rPO 164.10 38.6 - .311 1068 33b 7.10 I11 CH&( CHz0)aPS 180.17 32.0 - .237 984 41" 6.77 110 42d 1.82 ,062 IV (.C&O)aP 166.16 2.22 (1.71) (1.44) ( - .22) (102) 2.36 - .340 152 37e V CHsC( C H ~ O ) ~ A S 192.03 3.72 VI CeHsOaP 160.11 17.2 - ,290 485 36" 4.7 From the value for triethyl phosphite using acceuted values of C-C and C-H bond refractivities: A. I. Vogel, W. T. Cresswell, C. H. Jeffrey and J. Leicester, J . Chem. SOC.,514 (1952). b From I, and comparison of triethyl phosphate (41.3 cm.3) with triet,hyl phosphite, (42.3 cm.3). From I, and comparison of M R values of Pocl3 (25.1 ~ m . and ~ ) PSC18 (32.8 cm.3). d From TPD1.4079 and d**, 0.9687. 8 From I, by adding 3 cm.3; based on comparison of triphenylarsine with triphenylphosphine (E. Bergmann and W.Shut,z, 2. physik. Chem., B19, 401 (1932)),and of arsenic trichloride with phosphorus trichloride (J. W.Smith, Proc. Roy. SOC.(London),136,256 (1932)).
+
Q
I
VI
Experimental
estimated as indicated by the footnotes to Table I. Since all of the dipole moments are quite large, an uncertainty of one or two C M . in ~ MR results in a very small uncertainty in the calculated dipole moment. No allowance was made in the calculations for atom polarization. The estimated uncertainty in the values of dipole moments is 0.05 D or less for all compounds except VI. This compound appeared to be more susceptible to oxidation than any of the others, and the quality of the data indicate an uncertainty in the moment of perhaps 0.2 Debye.
Dipole Moment Measurements.-The apparatus and procedure and purification of solvents have been described previously.' For some of the measurements a new heteroDiscussion dyne be:tt apparatus of conventional design waa employed . 2 Weight fractions were in the range 0.002 to 0.015. The dipole moment values listed in Table I afPreparation of Compounds.-The derivatives of pentaglycerol CH,C( CH20H)! were prepared by a method pre- ford a number of interesting comparisons with the viously d e ~ c r i b e d . ~Triethyl phosphite (Eastman-yellow values for other phosphorus-containing compounds, label) was fractionally distilled. The fraction boiling at 48' Table 11. (12.5mm.)4was taken. The synthesis previously reported5 for P O ~ C ~ H (VI) S affords a relatively low yield (20%). A TABLE I1 method6 for the preparation of phosphites similar to comA COMPARISON OF SOME PHOSPHITE, PHOSPHATE A N D THIOpound I was extended. Phloroglucinol (1,3,5-trihydroxyPHOSPHATE COhlPOUNDS benzene) was hydrogenated to cis-1,3,5-trihydroxycyclohexane by a previously described method6 with the p(n) aU(n) modification that high pressure (10atm.) was used. Hydro(CzHbO)s(IV) 1.82 gen absorption essentially crased after 6 hr. A mixture of 1.3 ( CzHs0)sPO" 3.07 cis-l,3,5-trihydroxycyclohexaneand a 5 mole excese of trimethylphosphite was allowed to stand (with occasional CHaC(CHz0)aVI) 4.15 2.95 shaking) under nitrogen for 10 days a t 50'. A t the end of CHaC(CHzO)aPO(11) 7.10 this timc. the reaction mixture was a clear colorless solution. 4.15 CHBC(CH,O)~P(I) When the methanol formed in the reaction and the 2.62 excess trimethylphosphite had been removed in vacuo, a 6.57 CH&( CH20)3PS(111) white crystalline residue remained. The white solid was (CsH5)oPb 1'45 3.86 sublimed a t 100' (0.05 mm.), the sublimate recrystallized (CsH5)sPOb 4.31 from a minimum of pentane a t -80" and the product reRublimed a t 100' (0.05 mm.), yield, 70%, m.p. (uncor.), ( CbH5)zP 1.45 3.29 208-209 " . (C,H5)aPSb 4.74 Anal. Calcd.: C, 45.00; H, 5.67. Found: C, 45.05; H, 5.77. ( ct&O),Pc 2'03 0.78 (c~60)3Poc 2.81 Results (CsH60)aP The method of Halverstadt and Kumler,' with 2'03 0.55 ( CeHaO )3PSc 2.58 slight modification, was employed in calculations a W. J. Svirbely and J. J. Lander, J . Am. Chem. SOC., 70, of the dipole moments. The data and results are 4121 (1948). Value for benzene solution. * K. A . Jensen, shown in Table I. Values for molar refractions were 2. anorg. u. allgem. Chem., 250, 268 (1943). Values for benzene solution. e G. L. Lewis and C. P. Smyth, J. A m . (1) T. I.. Brown, J . A m . Chem. Sac., 81, 3232 (1959). Chem. Soc., 6 2 , 1529 (1940). Values for benzene solution.
(2) A. W.Cordes, Ph.D. Thesis. University of Illinois, 1960. (3) J. 0. Verksde and L. T. Reynolds, J . Org. Chem.. 36, 663, (1960). ( 4 ) A. Ford-Moore and J. Williams, J . Cham. Soc.. 1465 (1947). (5) H. Stetter and K. Steinacker, Bsr., 86. 451 (1952). (6) W. S. Wadsworth and W. D. Emmons, Abatracts, 138th Meeting A.C.S., New York, p. 97P; M. S. Newman, private communication. (7) I. F. Ihherstadt and W. D. Kumler, J . A m . Cham. Soe., 64, 2988 (1942).
The moment of I is considerably greater than that of IV; this may be due in part to a greater lone pair moment in the former as a result of the tying back of the alkyl groups, but the major part of the increase probably arises from the presence of rotation about the P-0 bonds in IV. Spectral evi-
Nov., 1961
A COMPLETE IONIZATION SCHEME FOR CITRICACID
dences indicates rotational isomerism in triethylphosphate, and it is very likely also present in the phosphite IV.9 Presuming that rotational isomerism exists in both triethyl phosphate and triethyl phosphite, the difference in moments between these two does not afford a good measure of the P = 0 group moment, because the conformations of the isomers, and the distributions between them, may be different in the two compounds. This objection is absent from the comparison of I with 11, or of I with I11 in the case of the P = S group. The difference in moments between triphenyl phosphine and triphenyl phosphine oxide (Table 11),is close to the difference between I and 11. It is interesting in the light of this that the difference between triphenyl phosphine and triphenyl phosphine sulfide is significantly larger than that between I and 111. A similar situation is encountered in the series triphenyl phosphite, phosphate and thiophosphate, but here the variation in moment may include contributions from rotational (8) F. S. Mortimer, Spectrochim. Acta, 9, 270 (1957). (9) Measurement of the dipole moments of both I and I V a t 35.0' in dioxane gave values of 4.17 and 1.60 D,respectively. The moment of I is therefore essentially constant, whereas the moment of I V decreases by about 0.2 D. The existence of a temperature-dependent distribution of rotational forms in I V is indicated.
2053
isomerism. The present results, therefore, appear to be the first which show unambiguously that the P =S group moment in thiophosphates is lower than the P = O group moment in the analogous phosphates, as distinct from the phosphine analogs in which the opposite is true. It is widely recognized that the relatively low values for the P = 0 and P = S group moments are the result of a-bonding between oxygen or sulfur and phosphorus.lOpll Undoubtedly a-bonding also occurs in phosphites, and the difference in the moments of I and I1 or I and I11 includes changes in the bonding between phosphorus and the alkoxy oxygens in going from the phosphite to the phosphate or thiophosphate. It is clear that the group moments obtained by the comparisons made above are only apparent values, and include a number of contributions which cannot, on the basis of the dipole moment data alone, be separately evaluated. Acknowledgments.-We thank the National Science Foundation for a grant and a fellowship
(J.G.V.). (10) J. W. Smith, "Electric Dipole Moments." Butterworths, London, 1955,p. 230. (11) G. M. Phillips, J. 6. Hunter and L. E. Sutton, J . Chem. Soc., 146 (1945).
A COMPLETE IONIZATION SCHEME FOR CITRIC ACID BY R. BRUCEMARTIN Cobb Chemical Laboratory, University of Virginia, Charlottesville, Vu. Received May 39, 1961
Titration analysis of citric acid and selected methyl esters indicates that about 60% of the monoionized and 45% of the diionized species are symmetrical.
Recently two independent determinations were made of the relative ionizing tendency of the two kinds of carboxylic acid groups in citric acid. A nuclear magnetic resonance (n.m.r.) study indicated that ionization from the terminal carboxylic acid groups is predominant.' By measuring the relative chemical shifts of the methylene doublet as citric acid underwent progressive ionization and comparing these values with a scale for the chemical shift established by similar measurements on selected methyl esters, the mole fractions of the several ionic species could be estimated. In a second study, analysis of X-ray diffraction data demonstrates that the central carboxylic acid group is ionized in solid sodium dihydrogen citrate.2 These two studies are not necessarily contradictory because the ionization favored in the solid state may not be favored in solution. For many purposes it is the equilibria in solution that are of interest. A more direct method than n.m.r. exists for estimating the favored ionizations in citric acid with the aid of selected methyl esters.
Titration of the esters and comparison with the accurately known acid ionization constants of citric acids permits quantitative determination of the favored ionizations. This paper presents a titration analysis of the same esters used in the n.m.r. study and arrives a t the opposite conclusion ; the ionization from the central carboxylic acid group is predominant. Experimental
(1) A. Loewenatein and J. D. Roberts, J . A m . Chem. Soc., 82, 2705 (1960). (2) J. P. Glusker, D. van der Helm, W. E. Love, M. L. Dornberg and A. L. Petterwn, ibid., 83, 2964 (1960).
(3) R. G. Bates and G. D. Pinching, ibid., 71. 1274 (1949). (4) W. E. Donaldson, R. F. McCleary and E. F. Degering, ibid., 66, 459 (1934). (5) G. Bahroeter, Bm., 88, 3190 (1906).
Citric acid trimethyl ester4had m.p. 75-76', lit. 76"* and 73-73.5" .1 Citric acid symmetrical dimethyl ester6 had m.p. 116-118', lit 125-126'6 and 115-117'.' Anal. Calcd. for monohydrate: C, 40.3; H, 5.9; eq: wt., 238. Found: C, 40.9; H, 6.0; eq. wt., 257. Titrations were performed at 25" on a Beckman Model G pH meter by addition of standard base from an ultramicroburet. KOsalt was added to keep the ionic strength low so the thermodynamic ionization constants of citric acids could be used. Because of the problem of ester purity and the selectivity of the hydrolyses described later, the results are rounded to 0.05 log units, but should be considered reliable to only &0.1 log units.
Results The complete ionization scheme for citric acid is
