THEDIPOLEMOMENTOF UREA
Oct., 1960
intensities as the temperature is lowered. Since the other bands in the spectrum apparently are not split in this manner, it does not appear that the doubling of the C=N stretching band is attributable to the presence of tautomers such as the isonitrile form. Fermi resonance between the C=N stretching mode and the combination band of the C-N stretch and the n’Hz deformation, which has been used to account for the similar splitting in CI-13C?j,2*does not seem to be a likely explanation of the doubling here since the doubling occurs in the deutero-compound. A Fermi interaction with the overtone of the C-N stretch is a likely possibility if one assumes that the overtone frequency is slightly above the C=N frequency in cyanamide and slightly below it in deutero-cyanamide This also would require that the observed C-K frequency in D&CN be lower than the unperturbed
1485
G N frequency due to interaction with the XDa bending mode.22 The frequencies of the unperterturbed modes have been estimated from a simple first-order perturbation c a l ~ u l a t i o nand ~ ~the results are included in Table I. Oiie is tempted to attribute unknown splittings of this kind to “crystal effects” since these do often occur, but in this case an indication of the splitting still exists in the liquid and solution spectra so that crystal effects are probably not the cause of the splitting. Apparently many of the cyanamide derivatiTw with at least one hydrogen atom left on the amino nitrogen show this same splittiiig, i e., ?;allSCH, Cn(HSCH)2, etc., while those wthout hydrogens do not appear to show it, Le., G S C S , 1Z2SC,Ry’(11 = methyl, ethyl, allyl, etc.). I t may be, therefore, that the splitting is in some way associated with the H or D atoms pi esent. (22) T. A. Scott, Jr., and E. L. Wagnei, t b d , 30, 165 (19.59).
(21) P. Venkateswarlu, J . Chem. Phus., 19, 293 (1951).
THE DIPOLE MOMENT OF UREA BY W. R. GILKERSON AND K. K. SRIVASTAVA Department of Chemistry of the University of South Carolina, Columbia, South Carolina hceiued April 9,1960
The dielectric constants of solutions of chlorobenzene, nitrobenzene and o-dinitrobenzene in polar solvents have been measured. Using Onsager’s equation, the dipole moments of the solutes have been calculated and compared with those obtained from measurements in non-polar solvents. The method is then applied to the determination of the moment for urea in 20 weight % water-acetone and in pure water. The moment in the former is 6.25 debyes, and in the latter is 4.2 debyes.
There have recently’ been reports of the application of Onsager’s2equation relating the dielectric constant to the dipole moment to solutions of polar solutes in polar solvents. It is of interest to see if one can obtain reliable dipole moment values for solutes in such systems. There are a number of compounds which have high melting points and are difficultly soluble in non-polar solvents, so that their dipole moments remain in doubt. Further a number of polar solutes, mhilc being soluble, indicate dimerization or other complex formation in non-polar solvents. The values of the dipole moment of urea reported ill the l i t e r a t ~ r c ~have - ~ varied from 4.4 to 8.6 dcbyes. Early ineasurenicnts by Furt h6 seemed to place urea in the same class as glycine, as having a “large” dipole moment since both caused a large increase in the dielectric constant of water solutions. However, in the case of urea Furth observed an initial decrease, followed by the increase mentioned above. We report here the results of measurements of the dielectric constants of solutions of chlorobenzene in benzene and in 25 mole yo o-dichlorobenzene-benzene, of nitrobenzene in chlorobenzene, 50, 73 and 100 mole yo o-dichlorobenzene-benzene (1) T. Gaumann. Helu. Ckim. Acto, 41, 1956 (1938). (2) L. OnaaEer, J . A m Chem. Soc., 68, 1486 (1936). (3) C. Beguin and T. Gaumann, Helu. Chim. Acta, 41, 1971 (1958). (4) W. D. Kumler and G. M. Fohlen, J . Am. Chem. SOC.,64, 1944
(1942). (5)
E. Bergmann and A. Weizmann, Trans. Faradau
(1938). (G)
R. Yurth, Ann. I’hysrk, 70, 63 (192:).
SOC.,34, 783
mixtures, of o-dinitrobenzene in benzene, 50 and 100 mole % o-dichlorobenzene-benzene and in nitrobenzene and of urea in 20 weight, % wateracetone and in water, all a t 25’. Experimental Chemicals.-All the organic liquids except acetone were Matheson, Coleman and Bell reagent grade. Acetone was from stock. All were passed through a 35 X 2 cm. column packed with Alcoa activated alumina, grade F-20. Benzene was recrystallized anti distilled from sodium ribbon. Nitrobmzene was recrvstallized twice. Chlorobenzene, o-dichlorobenzene (abbreviated DCB hereafter) and awtone were distilled, middle cuts being taken. o-Dinitrob~nzene(Aldrich Chemical Co.) was used without further purification; m.p. 116.5-116.8’. Urea (J. T. Gaker ana!pzcti, C.P.) was used without further purification.
TABLE I I’IIESICAI,COXSTANTS OF S O L ~ E XATI S25” so.
Solvent
Densits, g./rnl.
niriectrir
constant
Benzene 0.8737“ 2.275’ 25 mole 9% DCB-benzene 0.998 4.16 50 mole % DCB-benzene 1,111 r i 02 4 75 mole yo DCB-benzene 1.311 7.94 5 DCB 1.300c 10.OD“ 6 Chlorobenzene 1.101“ 5.63” 7 Nitrobenzene 1.19s* 34. 8 20 wt. % water-acetone 0.8448 29.Gd 9 Water 0.9971 78. 5qb 0 J. Timmermans, “Physico-chemical Constants of Pure Organic Compounds,” Elsevier Press, Kcw York, PI‘. Y., 1950. J. Wgman, Phys. Rev., 35, 623 (1030). P. H. Flahcrty and K. H. Stern, J . Am. Chem. Soc., 80, 1034 (19.58). G . Akerlof, Ibid , 54, 4125 (1932). H. F:idek arid R.AI. Fuoss, J . Am. Chew. Soc., 76,5005 (1954). 1 2 3
Vol, 64
W. R. GILKERSON AND K. K. SRIVASTAVA
1186
where S,is the number of molecules of type i per cc., E m i is the infinite frequency dielectric constant of the ith component, pi is the gas phase dipole moment of type i, and ai is the radius of the hypothetical spherical molecule containing the point dipole pl. Expressiiig the Ni in terms of the Ci, moles per liter of component i, then we have
'\''..-
01
E
-1=
(A,pLL,2
+ B,)C,
where
+
+ 1)/9000 k T ( 2 e +
A , = 4 5 ~ , 1 ; , ( ~ ~ ~ 2)2s12~
and 1
-
B , = 3T',d€,,
0.4
02
1
c.
Fig. I.-&
(5)
L
vs. molar concentration of urea in water: 0, thic, work at 25'; 0, ref. 0 a t 20".
Physicrtl Constants.-The densities and dielectric constants of the benzene-DCB solvent mixtures mere determined and appear in Table I together with the literature values fo1 the pure solvents and for the water-acetone mixture. The dielectric constants of the benzene-DCB mixtures wwe meamred a t IOOKc using a General Radio Type 716-C capacitance bridge, equipped with a Type 716-P4 Guard Circuit. The dielectric cell s a s similar in design to that of Sadek and FLIOSS .7 Solution Measurements.-These were carried out using a Sargent 3IodelI- Oscillometer (5Mc) and accompanying cell and rang(=extender. The cell was thermostated at 25". The cell was eqlivalent to two capacitors in series; a glass section having a capacity C, 133.5 pf., and a solution section having an air capacity CxO of 5.94 pf. These values were determined by separate measurements on the General Radio bridge. The calculations will be discussed below. The solvent mixtiires and solutions were all made up by weight.
E,!)'
- 1)/(2€ + Em,)
is Avogadro's number, and VI is the molar volume of pure i. We deal a t most with three components. Subscript 1 will designate the polar solute, while 2 and 3 designate the two possible solvent components. We assume the solutions to be ideal. Then in terms of the solute concentration Cz = CzO/l
- VICl) and Ca =
Caoll - VIC1)
where the superscript zero designates the concentrations of the solvent components in pure solvent. Taking the derivative of E with respect to C1 and solviiig for p12,we obtain p12
= Fld€/dC,)/A,
+ ~Azp22cz'v-l + A3p?"C?OV, + B2Cz'Vi + B1Ca'V'l;l Bi)/A,
(6)
where
Results The oscillometer yielded only scale readings, S, directly. This is related to the capacity of the cell sections by the equation
Since the slope is taken in the limit as C1 approaches zero, then the A,, B, and F are calculated using the (1) dielectric coiistant of pure solvent for e. The values of t m lare taken to be the square of the refractive where c', = e is the dielectric constant of the solution aiid K is the proportionality constant. nidex (Ka-u line) of the pure liquids at 25'. The This equation can be arranged to give the dielectric value for o-dinitrobenzene was calculated from atomic refractions.8 The value for urea was calcoiistant culated from the molar refraction of 13 cc. given E = S/(LYC,O - SC,O/C,) (2) by Gauman1i.l The values of the dipole moments The value OF KCxowas determined for each solu- of the solvent molecules used in equation 6 were calculated by applying equation 5 to the pure tion by the Pquation liquids. The value of Vi, 0.061 I., for urea was KCXO = sori + Cx~€0/Cg)/€0 (3) determined froin density d a m 9 The value of where So is the scale reading with pure solvent V1 for o-dinitrobeiizene mas calculatrd from density data in benzene so1ution.l' present alone. The concentratioii ranges used, the slopes del dCi, The dielectric constants calculated from equation 2 were plotted 11s. molar concentration of solute, and the dipole moments calculated using equation 6 C1, and the slopes of the resulting straight lines are are shown in Table 11. given in Table 11. The plot for urea in water did Discussion not give a straight line and is shown in Fig. 1. The moments of the substituted benzenes are The dipole moments of the solutes were calculated cwmparable with those obtained previously in using a modificiation of Onsager's equation The (81 Auwera and Eiaenlohr, Ber., 48, 806 (1910) latter may be written as ( 9 ) F. T.Gucker. P R Gage and C F . M r w r I 4 rrr Chew6 60, 9. =
k'c,cx/re,+ C),
(IxUt,
( 7 ) H. Sadek and
R. hl Fuoss, J . A m Chew
Sac.,
76,5905 (1954).
60, 2582 (1938).
THECONDUCTAWE OF HEXAFLUOROARSENIC ACID
Oct., 1960
1487
TABLE 11 effects aside, the calculation oi monierits of polar DIELECTRIC INCREMENTS AND DIPOLE MOMENTS OF CHLORO- solutes in polar solvents is as feasible as the calculation of the moments from dielectric constants of BENZENE, NITROBENZENE, 0-DINITROBENZENE AND UREAIN pure liquids.’* POLAR SOLVENTS AT 25”
benzene solution.10 The decrease of the nitrobenzene moment with increasing dielectric constant of solvent is to be compared to the increase in the case of o-dinitrobenzene. The similarity in size and shape of the two molecules would seem to rule out any “shape effect” as an explanation for this behavior. Further, a 10% error in t mfor o-dinitrobenzene could only give rise to a 6% error in the dipole moment in benzene, the most extreme case. One might also expect that a failure in the assumption of ideal solutions would operate in the same direction for these two compounds. The moment for o-dinitrobenzene in benzene is slightly higher than the previously reported” value of 6.05 debyes. Smythghas calculated on t,he basis of the solution moment. for nit,robenzene, that if there is no L ‘ ~ ~ t effect, l ~ ~ ’ operating ’ in the case of o-dinitrobenzene, its moment should be 6.9 debyes. From the results for chlorobenzene, nitrobenzene and o-dinitrobenzene we conclude that, association
Proceeding to the case of urea, the moment found in water compares favorably with those observed by Gaumann’ in acetone (-1.48 debyes) and ethanol (4.51 debyes). The latter author gives a value of 5.68 debyes for the moment in water, but does not give the experimental data from which he made the calculation, nor does he refer to another source. It can be seen in Fig. 1 that the slope one obtains depends greatly on the concentration range one is working in. This is probably the source of the discrepancy. The higher value found in the case of the water-acetone solvent we believe to be due to failure of the assumption of a continuum in such a mixed solvent. If association with water were going to occur it would be favored by a lower dielectric constant. Let us now estimate the moment for urea assuming the group moments of Smythl3 to be applicable. The group moment for ketone or aldehyde (aliphatic) is given as 2.7 debyes. That for the -NI& group is 1.2. Further, the angle the -XHz moment makes with the C-N bond is 100’. It is assumed that the hydrogens are pointing away from the oxygen. Using the 0-C-K bond angle is 120°, we calculate that the resulting moment is 4.5 debyes. Gaumann3 estimated a value of 3.4 using bond moments. If one assumes the amino moment to make an angle of 140’ with the C-K bond, which is characteristic of aromatic amines, then one obtains a moment of 3.1 debye.. . in closer agreement with the bond-moment value. It might be concluded from this that there is 1 1 0 apparent aromaticity in urea, with respect to it, dipole moment. R e wish to ackhowledge support of this work in part by contract mith the Office of Ordnance Rrsearch, U. S. -1rmy.
(IO) C. P. Smyth, “Dielectric Behavior a n d Structure,” MoGraTvHill Book Co.. New York. N. Y.. 1955, pp. 314 and 332. (11) .J. W. Williams and C. H. Schwinnel, J . A m . Chem. Snc.. 6 0 , 362 (1928).
(12) C J F Boettcher Phywca, 6, 39 (1939) (13) C P Smyth, “Phqsical Methods of Organic Chemistrv,” 4 Weissberger, editor Intersrlenoe Publishers, Inc U e x York U Y 2nd edition 1949 Vol I. Pt TI. Chap X X I I
601-
Compound
vento
Chlorobenzene
1 2
Concn. range X 10, 31‘
0.5-2.0 2.3-4.3
Slope dt/dCl
Dipole nioinent, debyes
0.292 0.195
1.51 1.50 Kitrobenzene lh 3.98 6 0.8-5.3 2.29 3.82 3 0.5-4.7 2.19 3.77 4 1.1-8.2 1.74 3.59 5 1.2-7.1 1.26 3.39 o-Dinitro1 0.22-0.88 5.38 6.31 benzene 3 .61- .99 8.37 6.52 5 .28- .91 8.00 6.51 m .47-1,2 7.05 6.87 {,-rea 8 .23-0.85 5.60 6.25 9 1.3-2.5 -1.12 4.20 a See rorresponding number in Table I. Reference 10.
THE COSDUCTAKCE OF HEXAFLUOAKSENIC ACID AKD ITS I,ITHIUM, SODIUM ASD POTASSIUM SALTS IS WATER AT 2.5’ BYGORDON ATKINSON AND CALVINJ. HALLADA DepartinPat of Chemistry, the University of Michigan, Ann d4rbor, Michigan Received A p m 1 i i , 1060 7 .
1he conduetarires of IIXfiFe, IiAsFs, Sa:1sFb and KAsFe have been measured in water a t 25”. The concelltratiorl raiigc covered was to 10 -z M ; and the data were analyzed using the extended Fuoss-Onsager theory. Three of the salts have anabatic phoreograms only the K.4sF6 data approaching the limiting tangent from the bottom. The KAoc.alculated for the KAsFs is 1.76. The solution “a” values are larger than the crystallographic ones bv approximatelv 1.5 A. A romparisoil is made with the literature detil on NaC104 and KSO,.
Introduction In recent years, there has been a continuous search for ion-interfering anions, that is, anions usefiil in elevtrolytea both for stability constant,
and solution thermodynamics studies. In the first case, the electrolyte is such that it can be added to change the ionic strength of the solution without, c,omplexing the cation or anion under investigation.
