1300
FRANCES WRIGHT BALFOUR AND THOMAS F. FAGLEY
interpret the frequently observed solvent effects in solvent-extraction studies. Due to hydrogen-bonding of chloroform to organophosphorus compounds and long-chain tertiary alkylamines, extraction of solutes by these compounds as extractants is appreciably de-
pressed in chloroform medium, and this has been observed. l4l16 (14) (15)
T. Sato, J . Inorg. Nucl. Chem., 29, 555 (1967). T.Sekine and D. Dyrssen, ibid., 26, 1727 (1964).
The Kinetics of the Rearrangement of 2’,4’=Dinitro=2=aminodiphenyl Ether in Methyl Alcohol-Carbon Tetrachloride Solutions by Frances Wright Balfour and Thomas F. Fagley Richardson Chemistry Laboratory, Tulane Unizersity, N e w Orleans, Louisiana 70118 (Receiaed October 8 , 1967)
The kinetics of the rearrangement of 2’,4’-dinitro-2-aminodiphenyl ether to the diphenylamine in methyl alcohol-carbon tetrachloride solutions over a wide composition range at 25 and 35” has been investigated. Corrections of the activation parameters for the cosolvent effect reveal that the enhanced rate with increase in methyl alcohol is probably due to the trimer of the methanol.
I. Introduction Studies of the isomerization of the o-aminodiphenyl ethers were undertaken by Roberts and De Worms’ and Roberts, De Worms, and ClarkJ2who reported that such reactions are irreversible and, at the temperatures used in the kinetic studies reported here, produce only the diphenylamine as a product. This observation has been confirmed by a comparison of the melting point of the diphenylamine produced by the standard method3 with that of the product of the rearrangement of the ether. Both products melted at 198-199”. Roberts, De Worms, and Clark2 observed that hydroxylic solvents aid the rearrangement of the ether to the amine. We have followed the kinetics of this rearrangement in methyl alcohol-carbon tetrachloride solutions as part of a program of study of the “cosolvent effect” in solution kinetics. Since the methanol does not react with the ether chemically but does play a role in the activation process, such a mechanism should be a severe test of a model of the cosolvent e f f e ~ t . ~
11. Experimental Section Materials. Baker Analyzed reagent grade methanol was purified further to remove water by treatment with magnesium? Distillation from the methoxide was carried out using a Todd still. The pure methanol ( n Z 51.3265, ~ lit. n Z 61.3264) ~ was stored under dry nitrogen in a tightly capped brown bottle, placed within a desiccator, for periods no longer than 2 weeks. T h e Journal of Physical Chemistry
Baker Analyzed reagent grade carbon tetrachloride was purified by the method of Scatchard,‘j distilled, and similarly stored under dry nitrogen. The product had a ~ (lit,, 1.4573). The 2’,4’refractive index n Z 51.4573 dinitro-2-aminodiphenyl ether was prepared by adding to 0.05 mol of sodium ethoxide in 50 ml of dry ethanol, 0.05 mol (5.5 g) of K & K Chemical Laboratories o-amino phenol, with shaking. Then 10 g of 2,4-dinitrochlorobenzene (K & K Chemicals reagent grade) was ground to a fine powder in a mortar and added in small portions to the first solution with shaking after each addition until the dinitrochlorobenzene went into solution. The mixture was immediately placed in an ice bath and within a few minutes crystallization of the ether began. After 3 hr, the brown crystals were collected on a Buchner funnel and washed with small portions of ethanol until they turned yellow. The washed precipitate was extracted with warm benzene and filtered. This extraction was repeated on an average of five times. Ether recrystallized from these extrac(1) K. C. Roberts and C. G. De Worms, J . Chem. Soc., 727 (1934). (2) K. C. Roberts, C. G. De Worms, and M. B. Clark, ibid., 196 (1 934). (3) Chem. Ber., 53, 2265 (1920). (4) T. F. Fagley, G. A. Von Bodungen, J. J. Rathmell, and J. D . Hutchison, J. Phy8. Chem., 71, 1374 (1967). (5) J. F. Fieser, “Experiments in Organic Chemistry,” D. C. Heath and Company, Boston, Mass., 1965. (6) G. Scatchard, 9. E. Wood, and J. M. Moohel, J. Amer. Chem. SOC.,61, 3206 (1939).
REARRANGEMENT OF 2',4'-DINITRO-2-AMINODIPHENYL ETHER tions was subsequently recrystallized four more times before use in kinetic meaurements. The pure compound melted at 123" (lit. 123°).2 Inadequate washing of the crystals, impurities in the benzene, or failure to crystallize the ether early in its preparation proved to decrease substantially ether yield; in some cases, ether recovery was impossible. The 2',4'-dinitro-2-hydroxydiphenylamine was prepared by the standard methoda and also by the rearrangement of thLeether in methanol. The products of both these preparations melted at 189-199°.3 Apparatus. A constant temperature bath, A. H. Thomas infrared research &Iode19926-D, fitted with a water-levelling device, maintained a constant temperature within 0,001". Round-bottomed flasks of 125-ml capacity, fitted Kith Teflon stopcocks, were used as reaction flasks, They were so designed to permit attachment to a source of dry nitrogen and to permit sampling by a no. 18 needle, 6 in. long, which was fitted to a dried, nitrogen-filled syringe. The flasks were held in a rocking rack which allowed constant stirring of the reaction mixture. A Beckman DU spectrophotometer with a power supply was used for all absorption measurements. An A. H. Thomas calibrated thermometer was used to measure the bath temperature. A General Electric clock with a second hand which had been calibrated against Xational Bureau of Standards time was used to time all runs. Procedure. Into a weighed 100-ml volumetric flask, filled with dry nitrogen and capped with a standard medical serum cap under which was a polyethylene liner, methanol was added from a dry hypodermic syringe in the a,mount needed to produce the desired mole fraction. The flask was reweighed and carbon tetrachloride added to the mark and the flask again weighed. After buoyancy corrections, mole fractions could be calculated. The solid ether was added to the solution in the following manner: within a drybox, the serum cap on the solution flask was removed, leaving the polyethylene liner in place, and a small hole was punctured in the liner and a weighed amount (between 0.0035 and 0.0045 g) of the ether quickly added through the hole with the aid of a small funnel and a brush. The serum cap was immediately replaced and the flask shaken. The flask was then placed in a dry bag under dry nitrogen, and transfer of the solution to the reaction bulb (which had been previously filled with dry nitrogen) was made by means of a syringe. All dry nitrogen was obtained by passing the gas successively through concentrated sulfuric acid, phosphorus pentoxide, and a scrubber consisting of a cylinder filled with glass wool. Sampling was accomplished with a 5-ml hypodermic syringe fitted with a 6-in. no. 18 needle. This syringe was flushed with dry nitrogen prior to use. The needle was then inserted into the open stopcock of the reaction vessel under a positive pressure of dry nitrogen, the solution was withdrawn and was placed in a cuvette.
1301
A maximum time of 1min elapsed between the sampling and the reading in the spectrophotometer; the reaction time was taken as the time of the reading. Rates were made by following the appearance of product. Absorbancy measurements were made at 410 mp where the product has maximum absorption and the reactant (ether) minimal absorption at the concentrations used (1.5 X M ) in the kinetic runs. The molarity of the methanol was calculated from the weight fractions and the densities of the mixtures at 25 and 35" calculated from Scatchard's formula.' Rate Constants. Initial rate studies at 25.07" showed that the rate was doubled when the methanol concentration was increased from 0.0991 to 0.1982 M , the ether concentration being fixed at 6.2 X M . Pseudofirst-order rate constants were calculated from leastsquares fitting of log ( A , - A ) , time data, after plots of log ( A , - A ) us. time were shown to be linear. Any data which showed significant scatter of points were rejected and another kinetic run was made. A is the absorbancy of the product, the diphenylamine. At least two to three runs at each of the nine mole fractions of methanol at 25 and 35" were made. In order to be certain, in the instances of the extremely slow reactions, that the true infinity absorbancy reading had been obtained, preliminary rate constants were estimated from a limited number of readings by the Guggenheim method and from these k's an infinity time was estimated. The true infinity reading was then taken a considerable time later than that estimated, The concentration of the ether M and rates was varied from 1 x 10-4 to 2 X followed over usually 2-4 half-lives; in some instances, rates were followed for 5 half-lives. I n this range of concentrations of ether, the rates were cleanly first order in ether. Plots of log ( A , - A,) us. time were
Figure 1. Spectra of 2',4'-dinitro-2-aminodiphenyl ether in methanol-carbon tetrachloride solution: (A) 0.01807 M MeOH, 0,01807 M ether; (B) 0.02711 M MeOH, 0.00904 M ether; (C) 0.03253 M MeOH, 0.003614 M ether. (7) G. Soatchard, S. E. Wood, and J. M. Moohel, J . Amer. Chem. Soc., 68, 1960 (1946).
Volume 72, Number 4 April 1968
1302
FRANCES WRIGHTBALBOUR AND THOMAS F. FAGLEY
1.300
1.100
0 goo I
12
OJO(
0,500
0.300
0.100
Figure 2. Spectra of ether and diphenylamine in dilute methanol solutions: (A) 0.00015 M diphenylamine, 0.0014 iM MeOH; (B) 0.00018 M ether, 0.0014 M MeOH.
linear; those of the reciprocal of ( A , - At) us. time definitely were not linear. Spectra and Continuous-Variations Study. When the enthalpies and entropies of activation were calculated from the second-order rate constants (first order each in ether and in alcohol), the cosolvent corrections, to be discussed later, suggested that three molecules of methanol were involved. We then attempted to make a continuous-variations study. Preliminary studies had shown a general increase in absorptivity of the ether with methanol concentration. Several difficulties were encountered that make such a study of limited accuracy, but, within these experimental limitations, the results strongly suggest that a complex of 1 ether-3 methanols does form. The difficulties were: (1) low solubility of the ether in pure carbon tetrachloride (ca. 0.018 M ) ; (2) at this concentration an autocatalytic isomerization of the ether to the diphenylamine was observed-a phenomenon not observed at the low concentrations (loM4 M ) used in the kinetic studies; (3) the necessity of making rapid measurements and at the same time eliminating contamination of the solutions by moisture; and (4)the ether at these concentrations does not obey Beer’s law, so that a different reference solution had to be made for each ether-methanol-carbon tetrachloride solution, containing ether in pure carbon tetrachloride at the same concentration as in the ternary system. The differences in absorption of ternary and binary solutions were plotted, as shown in Figure 1 at three The Journal of Physical Chemistry
,020
I
L:l
e:1 3:Z 4:l Mol.
I
5:l 6:1 7rl
I
8:l
I
9:l
Rat10 M O c n / E t h . r
Figure 3. Continuous-variation study of the ether-methanol complex.
wavelengths. That these differences are not due to a general solvent shift in the absorption spectra was demonstrated by separate studies of the spectra (Figure 2), in a concentration range where Beer’s law is obeyed. (See also Figure 3.) 111. Discussion
From the first-order rate constant at 25 and 35’ and the molarities of the alcohol at each temperature, the enthalpies and entropies of activation were calculated by use of the Eyring equation. The corrections for variations in these parameters, as well as in the rate constants, with a binary solvent composition were made in the manner described in previous workO4 AS seen from Tables I and I1 the assumption that the active molecular species is the trimer of methanol accounts for the ob-
REARRANQEMENT
ETHER
1303
O F 2’,4’-DINITRO-2-AMINODIPHENYL
Table I : Activation Parameters for Rearrangement of 2’,4’-Dinitro-2-aminodiphenyl Ether Corrected for Co Solvent Effect Xole fraction
AH
MeOH
(&to. 15)
*.spps
kcal
14.43 14.81 15.06 15.09 15.00 15.26 15.12 15.23 15.17
0.0994 0.2011 0.2999 0.3952 0.4999 0.5994 0.7002 0.8001 0.8991
LM~OH,
AH*,
kcal
kcal
0.251 0.092 0.035 0.028 -0.021 -0.038 -0.035 -0.022 -0.007
14.94 14.99 15.13 15.14 14.96 15.18 15.05 15.19 15.16 Mean
Table 11: Comparison of Observed and Predicted Changers in In ka at 35”
-
Mole fraction MeOH
0.0994 0.2011 0.2999 0.3952 0.4999 0.5994 0.7002 0.8001 0.8991
(ZAZ/RT) (A
In kdobsd
-
(2Ase/R)
...
...
-1.11 -1.80 -2.52 -3.06 -3.52 -3.98 -4.26 -4.66
-1.38 -2.19 -2.52 -3.10 -3.44 -3.67 -3.87 -3.97
served variations. One definition of a catalyst, in homogeneous reacting systems, is: a molecular species, the concentration of which appears in the rate equation to a higher power than in the stoichiometric equation. Methanol in the system described here satisfies such a definition. That the species kinetically involved is not the monomer is shown by the fact that the fraction of methanol in carbon tetrachloride solution existing as monomer decreases with total alcohol concentration.8 First-order participation of the methanol in the rate expression is not inconsistent with the 3: 1 complex surmised from the continuous-variations study, for the variations study was made in solutions dilute in both ether and methanol (0.01 M and less), while the initial rate study was made at 0.1 and 0.2 iM methanol, necessitated by the extremely slow reaction at low alcohol concentrations. Self-association constants for methyl and ethyl alcohols in “inert” solvents are known to be large (of the order of 1 or 10). Thus in the initial rate study, doubling the stoichiometric concentration of alcohol has the effectof nearly doubling the trimer concentration, giving an apparent order of unity. The assumption that, the trimer is the catalytic species is necessary, furthermore, to explain the change in the specific reaction-rate constant with the composition of
- A S *app,
eu (h0.50)
--*MeOH,
39.1 40.0 40.7 41.8 43.3 43.3 44.8 45.0 45.9
-AS*,
ell
eu
3.18 2.31 1.70 1.21 0.95 0.69 0.44 0.22 0.06
45.5 44.6 44.1 44.2 45.2 44.6 45.6 45.4 46.0 Mean 45.0rt0.6
15.08 rt 0.09
the binary solvent. Finally, a Fisher-HirschfelderTaylor molecular model of the ether supports the use of the trimer. The simultaneous rotation of the phenyl groups about the ethereal oxygen bonds and rotation of the amino group reveal one most favorable position in which the nitrogen and the carbon in the dinitrophenyl group are in closest contact; in this position, the amino hydrogens are in the least favorable position for transfer to the oxygen. However, a trimer of the methanol
H,-o~-R
!
Hat-Ob’-R I
can easily span the ethereal oxygen and the amino hydrogen through hydrogen bridges. The trimer’s hydrogen, Ha, bonds to the ether oxygen and the amino hydrogen bonds to the trimer’s oxygen, Ob”. The transfer of hydrogen to ether oxygen, to form the hydroxy diphenylamine, is then facilitated via the trimer, leaving the trimer unchanged at the end of the activation process. The partial molar enthalpies and entropies of methanol were calculated from the data of Moelwyn-Hughes and Misseng by both analytical differentiation and graphical differentiation, using, in the latter, the method suggested by Goates and Sullivan.’o Corrections to the activation parameters were made with these thermodynamic data, assuming that all changes in these kinetic parameters are reflections of the variations in the properties of the methan01.~ The mechanism for the rearrangement is
(8) I. Prigogine and R. Defay, “Chemical Thermodynamics,” Longmans Green and Co., New York, N. Y.,1954, Chapter XXVI. (9) E. A. Moelwyn-Hughes and R. W. Missen, J. Phgs. Chem., 61, 518 (1957). (10) J. R. Goates and R. J. Sullivan, ibid., 6 2 , 188 (1958). Volume 72, Number 4 April 1968
FRANCES WRIGHTBALFOUR AND THOMAS F. FAGLEY
1304
Table 11. is the pseudo-first-order rate constant divided by the cube of the stoichiometric methanol concentration. A In
~ / R T ( A ~ ? M ~ o-H )2/R(AS")
=
kq
Table 111: Rate Constants for the Isomerization at 25 and 35" Mole fraction MeOH
---25.07°-------.
(3=O,OOOl)
The specific role of methanol in the reaction can be represented by
E d(PDA) ~dt
+T
fast
c 2 DPA
- k(C) = kK(E)(T) = ko(E)
(1)
(2)
where DPA is diphenylamine, E is ether, T is trimer of MeOH, and C is the complex between ether and trimeric methanol ; k~ is the pseudo-first-order observed rate constant; K is the equilibrium constant for the trimerether complex. ko/(RfeOH)* = kK" = k4
(if* -. B E ) - 2RMeOH
(4)
A(AH*) = - 2 A g ~ , o ~
(5)
A(As*) = - ~ A S M ~ O H
(6)
and
These calculations give corrected enthalpy and entropy of activation for the isomerization, which represent parameters for the rearrangement in pure methyl alcohol; that is, the standard state for the methanol is pure liquid methanol. These thermodynamic corrections, interestingly, involve 2R and 2 9 , that is, two partial molar enthalpies and two partial molar excess entropies (not the total partial molar entropies). In the activation process only the hydrogen bridges at the ends of the trimer are reorganized, the central methyl alcohol unit is essentially undisturbed ; similarly, only the excess partial molar entropies (which are reflections of the restrictions on vibration and rotation of the monomer units imposed by the hydrogen bridges) are involved. The corrected activation parameters and the correlation of changes in In kq with those predicted from the thermodynamic properties of methanol are given in The Journal of Physical Chemistry
a
7
Molarity MeOH
106ka, M-1 min-1
lo%, M-1
min-1
1.088 2.345 3.743 5.274
3.260 i 0,019" 1.072 7.428 f 0.014a 4.926=t00.009 2.315 11.46 i 0 . 0 2 6 . 1 8 7 i 0 . 0 1 4 3.694 14.59 f 0 . 0 3 6.61 i 0 . 0 1 3 5.218 15.61 f O . 0 5 7.228 6.579 f 0 . 0 2 9 7.168 15.47 & 0 . 0 8 9,501 7.066 f 0.014 9.360 16.85 f O . 0 1 12.14 7.370 =t0.034 12.019 17.44 f 0 . 0 2 15.40 8 . 8 0 7 i 0 . 0 6 0 15.22 20.97 f 0 . 0 4 19.39 9 . 4 2 5 & 0 , 0 0 6 19.15 22.37 f 0 . 3 0
Standard deviation.
(3)
Differentiation of eq 3 with respect to temperature, according to the Eyring equation, will give an apparent enthalpy of activation, which must be corrected for changes in the partial molar enthalpy of the methanol with concentration; similar corrections must be made for the entropies
AH* =
0.0996 0.2011 0.3000 0.3945 0.3952 0.4987 0,4999 0.6002 0 I5994 0.6993 0.7002 0,8000 0.8991
---36,08°--
Molarity MeOH
Table IV: Analysis of a Typical Kinetic Run.? Time, min
0 10 20 30 40 52 80 90 105 121 125 140 150 160 171 185 190 200 220 230 240 251 281 320 351 379 389 __
Am
0.165 0.187 0.207 0.226 0.245 0.271 0.318 0.341 0.356 0.380 0.386 0.402 0.422 0.437 0.480 0.462 0.475 0.487 0,510 0.520 0.535 0.545 0.580 0,600 0.630 0.644 0.665
-
[nztiyi
-
a
log ( A m
0.693 0.671 0.651 0.632 0.613 0.587 0.540 0.517 0.502 0,478 0.472 0.456 0.436 0.421 0.408 0.396 0.383 0.371 0.348 0.338 0.323 0.313 0,278 0.258 0.228 0.214 0.193
4530 am =
Vi
A
A
- A)
-tiUi
til
-0.15927 -0.17328 -0.18642 -0,19928 -0.21254 -0.23136 0.26761 -0.28651 -0.29930 -0.32057 -0.32606 -0,34104 -0.36051 0.37572 -0.38934 -0.40230 -0.41680 -0.43063 -0.45842 -0.47108 0.49080 -0.50446 -0.58596 - 0,58838 - 0.64207 -0.66959 -0.71444
0 1.7328 3.7284 5.9784 8,5016 12.0307 21.4088 25.7859 31.4265 38.7890 40,7575 47.7456 54.0765 60.1152 66.5771 74,4255 79.1920 86.1260 100.8624 108.3484 117.7920 126.6195 156.2248 188.2816 225.3666 253.7746 277 9172
0 100 400 900 1,600 2 ,704 6,400 8,100 11,025 14 ,641 15,625 19,600 22,500 25 ,600 29,241 34,225 36,100 40,000 48,400 52,900 57,600 63,001 78,961 102,400 123, 201 143,641 151,321
- 10.47374
2213.5746
1,090,186
-
-
-
(ZtiZyi)]/[nZti'
I
-
(Zti)*(Zgi)'I,
1.3820 x 10-8 min-1, IC = 3.183 X 10-8 min-l. mole fraction of methanol, 0.7998.
m =
Temp, 35';
ADSORPTION A.ND OXIDATION OF CARBON MONOXIDE
1305
I n calculating the change in In Icq, differences in partial molar quantities are used; these, in turn, are calculated from differences (through graphical or analytical differentiation) between measured quantities, such as heats of mixing. The errors in the partial molar quantities, therefore, may vary from 10 to 20%. The agreement between observed and calculated changes in In I c d , then, is quite good. It is noted that Hudson and Loveday'l have previously observed specific solvation between alcohol and the transition state in nucleophilic substitutions involving alcohols and acid chlorides. The rates were found
to be approximately proportional to the concentration of the self-associated alcohol. More recently, Fletcher and Heller12 reported evidence for specific catalysis by the tetramer of octanol. (For data on the rate constants for the isomerization at 25 and 35" and an analysis of a typical kinetic run, see Tables I11 and IV, respectively.) (11) R. F. Hudson and G. W. Loveday, J . Chen. SOC.Sect. B, 766 (1966). (12)A. N. Fletcher and C. A. Heller, J . Phys. Chen., 71, 3742 (1967).
Adsorption and Oxidation of Carbon Monoxide on Platinized Platinum Electrodes by M. W. Breiter General Electric Reaearch and Development Center, Schenectady, New York 163001 (Received October d , 1967)
Carbon monoxide is adsorbed at open circuit on platinized platinum as two types that are oxidized in different potential regions in acid and alkaline electrolytes. The different mechanisms of the anodic oxidation of the two types of co&are discussed. The saturation coverage of H atoms decreases in a linear fashion with the C0,d coverage. The isotherms of hydrogen adsorption degenerate to Temkin-type isotherms with increasing coverage of cod. The C0,d coverage is equal to 1 between 0.1 and 0.4 V under potentiostatic or galvanostatic conditions in acid solutions stirred with carbon monoxide. The beginning of the rapid decrease of C0,d with potential depends upon the experimental conditions. The oxidation of dissolved carbon monoxide under steady-state conditions appears to involve the one type of COad as intermediate between 0.2 and 0.4 V and the other type above 0.4 V. Adsorption of the one type is the rate-determining step between 0.2 and 0.4 V. Diffusion of CO and adsorption of the other type are rate determining above 0.4 V.
Introduction I n recent years the adsorption of carbon monoxide has been extensively on smooth platinum electrodes in acid electrolytes. Carbon monoxide was allowed to interact with the platinum surface a t potentials between 0.2 and 0.4 V. Voltammetricl-4 and galvanostatic6J techniques were used to determine the adsorbed amount from the charge required for its anodic oxidation to COz. It was reported in two of the publications that the charge SQCO corresponding to is smaller than twice the saturation coverage with co&d charge SQH of a monolayer of H atoms on the same electrode ( ~ S & H / S & C O = 1.8 in ref 2, and 1.08 in ref 6). Gilman2 attributed this ratio to the presence of two forms of Coed in analogy to the interpretation' of infrared data of carbon monoxide adsorption on platinum in the gas phase. One molecule of the bridged or linear form occupies two or one platinum atoms, re-
spectively. Warner arid Schuldiner5 found that &&/ SQCO = 0.94. Physical adsorption of carbon monoxide on top of a chemisorbed monolayer was suggested5 as a reason that more than a monolayer was present. Codeposition of hydrogen atoms on a surface having the saturation coverage of G o a d led to the conclusion2 that carbon monoxide is only adsorbed on about SO$70 of the Pt atoms. I n contrast, a percentage of 98% was given in ref 6. The investigation^^^^ of carbon monoxide (1) S. Gilman, Phys. Chem., 66, 2657 (1962). (2) 8. Gilman, ibid., 67, 78 (1963). (3) S. Gilman, ibid., 67, 1898 (1963). (4) 8.Gilman, ibid., 68, 70 (1964). ( 5 ) T. B. Warner and S. Schuldiner, J . Electrochen. SOC.,111, 992 (1964). (6) S. B. Brummer and J. I. Ford, J . Phys. Chem., 69, 1355 (1965), (7) R. P.Eischens and W. Pliskin, Adwan. Catal., 10, 18 (1958). (8) A. B. Fasman, G. L. Padyukova, and D. V. Sokolskii, Dokl. Akad. Nauk SSSR, 150, 856 (1963). Volume 76,Number 4 April 1968
