J. R.MCNESBYAND 9. S. GORDON
3570
Calculations of the dissociation constant for the tetrakis-(diethyldithiocarbamat0)-uranate(V1) ion were made according to the method of Foley and Anderson, l 2 which utilizes solution pairs having the same concentration of the complex but differing in concentrations of the free complex forming ions. Such data were available directly from the Yoe and Jones type of plot, the continuous variation curves, or appropriate dilutions of these solutions in 1 AI sodium perchlorate. Calculated values for three solution pairs gave six values ranging from 1.3 x 10-l8 to 6.9 X a t 25' for the equilibrium defined by the equation where X is diethyldithiocarbamate. The free energy of formation of the complex as calculated from the values of K is about 23 to 23 kcal. 'mole. The values for the dissociation constants are a t best approximations. The hydrolysis of the uranyl ion, which is appreciable a t p H values above 3, and the hydrolysis of the dithio acid salt are both neglected as is also equilibrium with other complexes. In the region of low pH, where hydrolysis of uranyl ion does not occur, the dithio acids are decomposed thus precluding any measurements. The spectrum of the ethylxanthate complex was similar to that of the dithiocarbamate complex except that the maximum occurred in the vicinity of 360 mp and then rose very steeply as the wave (12) R . T Foley a n d R. C. Anderson, THISJ O U R N ~ L , 71, 909 (1949).
[CONTRIBUTION FROM
Vol. 7 8
length decreased beyond 340 mp. The complex absorbed much more strongly in ethanol than in water, indicating considerable dissociation in the latter solvent. For this reason the continuous variation curves were run in ethanol. Plotting according to the method of Yoe and Jones gave values for the extinction coefficient in ethanol of 2.5 X lo3 a t 345 mp and 2.2 X l o 3 a t 360 mp. Similar calculations made for solutions of the solid in alcohol give the values 2.2 X l o 3 a t 345 mp and 1.9 X l o 3 a t 360 mp. The insolubility of the solid complex in non-polar solvents limits the value of the spectral data insofar as obtaining information about the structure of the solid is concerned. The existence of UOzXz in solutions of the solid in ethanol suggests that it may be a double salt of the type KX.U02X2,where X is the ethylxanthate group. The data in no way preclude the possibility of the existence of tris- or tetrakis- complexes. No definite conclusions concerning the structure of the solid appear to be justified, on the basis of these data. Acknowledgments.-The author wishes to thank Dr. Robert B. Booth of the American Cyanamid Company for supplying the various xanthates and for his suggestions concerning their purification. I should also like to thank Dr. J. S.Johnson of ORNL for his assistance in the use of the Cary spectrophotometer. Also, I want to thank Dr. C. F. Coleman of ORNL for his comments on the manuscript. COLLEGE STATIOS,TEXAS
c.s. NAVAL ORDSASCE TESTST.4TIOXI
Photolysis of Acetone-d, in the Presence of n-Butane and of n-Butane-2,2,3,3-d4. A Study of the Abstraction of Primary and Secondary Hydrogen by Methyl Radicals BY JAXES R . MCNESBYAXD ALVIXS.GORDON RECEIVED FEBRUARY 20, 1956 Mixtures of n-butane-2,2,3,3-d4 and acetone-& and of n-butane and acetone-& were irradiated with a medium pressure mercury arc. From the analysis of the methanes produced the activation energies for the following reactions were deduced: -+ CD4 CH3CDCD2CD, CH&D2CD2CH3 + CD3H CH2CD2CD2CH3,E = 11.4 kcal ; CD3 CH3CD2CD2CH3 CH3CH2CH2CH3+ CD3H CH3CHCH2CH3,E = 9.3 kcal. I n the system acetone-&CH3, E = 11.4 kcal.; CDa butane-&, it was found that H is abstracted by CH3 and CD:, at about the same specific rate. The pre-exponential factors in the Arrhenius equations for abstraction of primary and secondary hydrogen from n-butane are in the ratio 1.5 compared with the statistical ratio of 1.5. In n-butane-d4 the ratio of pre-exponential factors for abstraction of primary hydrogen and secondary deuterium is 0.92 compared with a statistical ratio of 1.5.
+
+
+
+
+
+
Introduction The comparison of the relative rates of abstraction of D from acetone-d6and H from hydrocarbons by the CD3 radical has proved a valuable tool for precise measurement of the activation energies of such metathetical reactions. Measurements made upon hydrocarbons containing more than one kind of hydrogen, e.g., butane, cannot be interpreted unambiguously. For this reason we have prepared practically pure CH3CD2CD2CH,by the photolysis of a,a'-diethyl ketone-&l and studied the photolysis of acetone-d6in its presence.
Results and Discussion 1. Acetone-& and n-Butane-2,2,3,3-d4.-Mixtures of acetone-& and butane-& were photolyzed in the temperature range 356-450°, employing a previously described technique.2 The advantage of working in this temperature range is that the secondary butyl radical formed upon abstraction of D from butane-& is thermally unstable and decomposes to give propylene and a light methyl radical. It has therefore been possible to compare the relative rates of abstraction of H and D by the CH3 radical, as well as by the CD, radical, in the
(1) J . R. McNesby, C. M. Drew and Alvin S. Gordon, J . Phys. Chem., 59, 988 (1955).
(2) J R . McNesby and Alvin S . Gordon, THISJ O U R N A L , 76, 4190 (1954).
Aug. 5 , 1956
3571
PHOTOLYSIS OF ACETONE-d6
per cent. of the CD4 from acetone-d6 was subtracted from the total to correct for reactions (7) and (8). Since 2.7% of the CD4 appears as CDaH from reacCDsCOCDa +2CD3 + CO (1) tion (8) in the case of acetone-d6 alone, i t was asCD3 C H ~ C D ~ C D Z C+ H ~ CD? CH3CDCD2CH3 sumed that about the same percentage of the CDI (2) formed from the butane-d4 appeared as CD3H from CH3 CH~CDZCD~C + H ~ CH3D CH3CDCDzCH3 reaction (9). As previously noted, 1.3% of the Pa) radicals produced from acetone-ds are CDzH, and CD3 f C H ~ C D ~ C D Z C + H ~ CD3H f C H Z C D ~ C D ~ C H they ~ abstract H and D from butane-d4 with equal facility. Thus reaction (10) results in CD3H to the (3) extent of 1.3% of the CD4 from butane-d4. The CH3 $. CH3CDzCDzCH3 +CH, CHzCDzCD2CHa corrections for reactions (9) and (10) total 4% of (3a) the CD4arising from the butane-d4. CD3 CD3COCD3 +CD4 CDICOCDz (4)
same system. The reactions of present interest in this system are
+ +
+ +
+ +
+ +
CHI CDICOCDB+CH3D 4- CDZCOCDI (4a) CHaCDCDzCH3 +CH$CD=CDz CHI (5) CDzCH3 (6) C H Z C D Z C D ~ C+ H ~ CHz=CDz
+
+
The rates of formation of the various methanes are
+
R C D= ~ ~ ~ ( C D ~ ) ( C H ~ C D Z C D ~k,(CD3)(CD3COCD3) CH~) (11 RCD~H = ~ ~ ( C D ~ ) ( C H ~ C D ~ C D Z C H (~2)) R C H ~ D= kza(CH3)(CH3CDzCDzCH3) kha(CH3)(CD3-
+
COCD3) R C H= ~ k3,(CH3)(CH3CD*CD2CH3)
(3) (4)
Since the reactions have been carried out only to about 1% completion, i t is possible to write
and
where A = (acetone-d6) and B = (n-butane-2,2,3,3-d4). It is evident from equation 5 that the ratio CD4/CD3H, when plotted against the acetone2 d6/butane-d4 ratio, should give a straight line whose slope is k4/k3 and whose intercept on the CD4/CD3H 1 axis is kz/k3. Three master mixtures of acetone0 1 2 3 d6-n-butane-2,2,3,3-d4 were used. Each mixture Acetone-ds/Butane-dr. was photolyzed with a medium pressure mercury arc a t three temperatures and the methane frac- Fig. 1.-Photolysis of acetone-ds in the presence of butane-dd. tions analyzed with a high resolution mass spectrometer. The results are shown in Table I , and The corrections on CHI were made on a similar the plots of CD4/CD3H vs. A / B are shown in Fig. 1. basis, assuming that the total extraneous CH4 arises Four minor sources of CD3H are present because from the reactants are incompletely deuterated.
L 5 L y o 0
+ CD3COCD3 +CDiH + CDzCOCD3 + CDzHCOCD3 +CD3H CDzCOCD3
CDzH CD3
(7) (8)
+ CH3CDHCDzCH3 +CDBHf CH3CDCD2CH3 (9) CDiH + CH3CDzCDzCH3 + CD3H + CH3CDCDzCH3
CD3
(10)
Acetone-d6 alone gives CD3H/CD4 = 4%. From the analysis of the deuterated acetone, i t is calculated that 1.3y0 of the radicals are CD2H, so that reaction (8) must account for the remaining 2.7% of CD3H produced from the photolysis of the acetone-de alone. Since Fig. 1 indicates that kz/k3 is about unity and since practically all of the CD3H in the mixture came from butane-d4, the amount of CDI coming from butane-d4 was equal to the total CDaH, with the remaining CD4 coming from acetone-d6. Four
CHa CH3
+
+ CDzHCOCD3
+
CH3CDHCDzCH3
CHI
+ CD2COCDs (11) + CH3CDCDzCH3
+ CH4
(12)
Figures in parentheses in Table I are quite inaccurate due to the small amounts of CH3D and CH4 in the particular experiments indicated. High resolution mass spectrometry was employed i o distinguish mass 16, 17 and 18 peaks due to water from those due to methane fragments. The CHsD/CHd ratios are not significantly different from the corresponding CDJCDsH ratios. The corrected data indicate that CH3 abstracts H and D from the system with the same activation energy differences as does CD3. It is very difficult to see a trend in k2/k3 with temperature. From the data i t appears that E3 E4 = 0.1 f 0.1 kcal. Since i t has already been es-
J. R.MCNESBYAND A.
3572
S.GORDON
Vol. 78
TABLE I PHOTOLYSIS O F ACET0XE.de I N THE PRESEXCE O F ?&BUTANE 1 3 4 3 2 3
356 0.84 24.4 48.7 9.4 20.2 2.00 (2.1) 23.3 10.7 25.4 9.5 9.4 25.0
356 1.65 46.8 138.8 12.1 39.5 2.99 (3.3) 90.9 27.2 48.9 12.3 12.1 48.1
401 0.84 26.3 52.4 21.8 43.0 1.99 1.97 25.0 20.9 27.4 22.1 21.8 27.0
356 2.45 26.5 99.3 5.0 20.1 3.74 (4.0) 71.6 15.1 27.7 5.0 5.0 27.3
40 1 1.65 50.3 146.1 23.9 71.7 2.91 3.00 93.6 47.5 52.5 24.2 23.9 51.7
2,2,3,3-d4"
401 2 45 13 6 50 8 6 0 19 7 3 74 (3 3 ) 36 6 13 6 14 2 6 1 6 0 13 9
1
1.5
1
450 0.84 35.9 69.9 48.9 97.7 1.95 2.00 32.5 48.1 37.4 49.6 48.9 36.9
450 1.65 44.2 124.3 26.2 76.9 2.62 2.94 78.1 50.5 46.2 26.6 26.2 45.5
450 2.45 26.5 96.3 12.9 49.9 3.63 (3.9) 68.6 36.8 27.7 13.1 12 9 27.3
1.20 1.97 1.86 2.18 3.34 4.74 2 69 0.68 1.55 1.24 2.11 2.17 2.67 3.98 5.5 2 33 0.75 1,74 (1.2) (1.2) 1.1 1.1 1.1 1.0 (1.1) (0 9) (1.2) 1.08 1.07 1.06 1.04 1.10 1.08 a CDJH and CH, have been corrected as indicated in the text. A/B = relative molar concentrations of acetone-d6 and butane-&. (CD,)A = CD, formed from acetone-&. (CDI)B= CDI formed from butane-&. Numbers are mass spectrometer peak heights. The total pressure was about 35 mm. for each run.
tablished that E , = 11.3 k ~ a l . , ~ -itj follows that E3 = 11.4 kcal. and E z = 11.4 kcal. From two independent the energy of activation E13 = 11.4 + 0.2. CD3
+ CzHs +CDIH + CzHj
(13)
Thus the activation energy for the abstraction of primary H is, within a "J experimental error, independent Of whether the is abstracted from ethane or butane-&. Table I includes the amounts Of the various methanes arising from each of the abstraction reactions. From these results the rate of reaction of CD3 and CH3 with the D atoms of butane-& may be compared. -RCDOA =-= (CWA kaA(CD3) RCH~D)A ( C H ~ D ) A K d ( C H 3
It has previously been shown that follows that
k4
=
CD3/CH3 = ( C D ~ ) A / ( C H I D ) A
k4a.4
It
(8)
anes analyzed as above. The literature cracking pattern of CD3H includes 51.17, of the parent at mass 17. In the 250" experiments where the secbutyl radical is quite stable, CD3H and CD, are the important methanes. h'egative residuals a t mass 17 were encountered under these conditions. If 49.570 is used rather than 51.1y,for the c D 3 H cracking patternat 17, no negative residuals were encountered and values of the CH4/CH3Dratio were commensurate with the CD3H/CD4 ratios. In the work with acetone-& and butane-d4 such an in the cracking pattern of CDBH would be negligible, since relatively large amounts ,,f C H ~ D were generated during the reaction. I n the acetone-da-n-butane system the methane forming reactions are
+
+
CD3 CDICOCDB+CD, CDzCOCD3 (4) CH3CH2CH2CH3 -+ CD3H CH2CH2CH2CH3 (14) CH3CH2CH2CH3 -+ CD3H CH3CHCH2CH3 CD3 CD3
+
+
+
+
+
[(CDI)B(CHSD)BI/[(CD,)A/(CH~D).~~ (10) CH3 + CH3CH2CH2CH3 + CH, CH2CH2CHsCH (17) It The values of k 2 / k z , were calculated in this way. is doubtful whether the ratio is significantly greater CHI + CH3CH2CH2CH3----f CHr + CH3CHCH2CH3 than 1.0, but it is certainly not less than 1.0. It (18) must be concluded that CH3 and CD3 abstract D The cH3 radicals are formed in from butane-d4 a t about equal rates over the temperature range studied. CH~CHCH~CH + B CH3 CH3CH=CHz (19) 2 . Acetone-d6 and n-Butane.-A mixture of I n the initial stapes of reaction acetone-da and n-butane was made up in the ratio CDaH/CDa = (h4B)/(k,A) (kisB)/(kaA) butanelacetone-de = 2.04. This mixture was From the data in Table I, i t is a sufficiently acphotolyzed from 250-450" and the product methcurate approximation to write k I 4 / k 4= 0.93 on the (3) J. R McNesby, T. W. Davis and A. S . Gordon, THISJ O U R N A L , assumption that the abstraction of primary H is un7 6 , 823 (1954). affected by the mass of the hydrogen atoms in the (4) J. R. McNesby a n d A S Gordon, i b i d . , 7 6 , 1416 (1954) ( 5 ) M. H. J Wijnen, J. Chem. Phys., 22, 1075 (1954). secondary positions. Since B / A = 2.04 kz/kza
=
+
+
(6) J. R. McNesby a n d A. S. Gordon, THISJOURNAL, 7 7 , 4719 (1955). ( 7 ) M . H. J. Wijnen, J. Chem. Phys., 23, 1357 (1955).
=
CDaH/CD4 - 1.90 2.04
Aug. 5, 1956
MOLECULAR STRUCTURE OF 1,1,1-TRIFLUOROETHANE
3573
TABLE I1 PHOTOLYSIS O F ACETONE-& I N THE PRESENCE O F %-BUTANE 2,
min.
5 10 4 10 3 6 3 6
4
T,"C. 250 250 303.5 303.5 355 355 397 397 449
CH4
C D4
CDsH
CDsH,
CHsD
C H4
CD2 Cor.
17.6 33.1 13.7 43.0 13.2 29.5 12.6 37.1 28.0
185.2 351.7 127.1 391.6 108.5 240.3 96.2 279.9 199.5
(0.0) 0.7 0.3 1.2 0.4 3.2 0.9 6.1 5.0
(0.4)
(5) (10) 104.3 249.1 256.3 530.6 392.0 1051.9 969.0
10.48 10.59 9.24 9.07 8.18 8.11 7.60 7.50 7.09
(1.5) 10.9 32.6 29.7 61.7 50.3 132.7 131.1
CHaD ... ... 9.57 9.17 8.63 8.60 7.53 7.93 7.39
kidka
4.21 4.26 3.60 3.51 3.08 3.04 2.79 2.75 2.54
The pertinent data are given in Table 11. From This is the ratio of the number of primary hydrogen the slope of the Arrhenius plot for k1b/k4, E4 - E15 is atoms to the number of secondary hydrogen atoms, calculated to be 2.0 kcal. Since E4 is 11.3 k ~ a l . , ~ *so * that the entropy of activation per hydrogen atom of the attack on primary hydrogen is the same as i t follows that E15 = 9.3 kcal. As in the acetone-&-n-butane-2,2,3,3-& mix- for the attack on secondary hydrogen. tures, these data indicate that CH3 abstracts H and We may also compare AB/Az, the ratio for the abD with the same activation energy difference as straction of primary hydrogen to the abstraction of does CD3. It is of interest to note that the differ- secondary deuterium in butane-&. From the intercepts in Fig. 1, ka/kz = 0.93, indeence in energy of activation for the abstraction of secondary D and H by CD3 from butane is 2.1 kcal., pendent of temperature. Thus A3/Az = 0.93. This a value in excess of the zero point energy difference ratio is far from the ratio of the number of primary of H z and Dz. Some previous work6V8also indi- hydrogen atoms to secondary deuterium atoms in cates that zero point energy differences are not suf- the butane-d4. We can offer no explanation for ficient to account for the difference in rate of ab- this discrepancy a t the present time. Since EI4= straction of D and H by methyl radicals. 11.4 kcal. and El6 = 9.3 kcal., i t is concluded that It is of interest to calculate the ratio of preexpo- the difference in activation energy for .abstraction nential factors in the Arrhenius equations, A3/A 15, by CD3 of primary and secondary H is 2.1 kcal. corresponding to the abstraction of primary and Considering the numerous approximations that secondary hydrogen. I n this discussion we assume were necessary in the work of Alleng a t a single temthat A3 = A14. perature, his estimate of 2.1 kcal. for the difference As may be seen in Fig. 1, kz/k4 varies very little in abstraction activation energies of primary and with temperature. From this observation AJA4 secondary H from propane is remarkably close to may be calculated as 0.93. our precisely measured value for the case of butane. From Fig. 2, A4/A15 = 1.61, so that Acknowledgment.-The authors wish to acknowledge the assistance of X r . Andreas V. Jensen and Mrs. Helen R. Young in mass spectrometer analy-
sis. (')
( 8 ) J. R. McNesby and A. S. Gordon, THISJOURNAL, 76, 1416
(1954).
A' O'A"en' ibid ' 638 708 '1941)'
CHINALAKE,CALIF.
[CONTRIBUTION FROM THE
DEPARTMENT O F CHEMISTRY AND
THE P U R D U E
RESEARCHFOUNDATIOS, PURDUE
UNIVERSITY]
An Electron Diffraction Investigation of the Molecular Structure of l,l,l-Trifluoroethanel BY JAMES L. BRANDT AND R. L. LIVINGSTON RECEIVED FEBRUARY 20, 1956 The interatomic distances in methylfluoroform have been determined by electron diffraction using the visual correlation procedure. Sectored electron diffraction photographs were used in the interpretation of some of the features. The following results were obtained: C-F = 1.33 & 0.02 A., C-C = 1.52 0.04 A., and L C C F = 111.5' Z!Z 1.5'.
*
Two previous electron diffraction investigation9 of the structure of methylfluoroform gave the following conflicting results: C-C = 1.45 Z!Z 0.04 fi. as compared t o 1.53 f 0.04 A., C-F 1.33 f 0.03 A. compared t o C-F 1.36 + 0.02 A. (1) From the Ph.D. thesis of James L. Brandt, Purdue Research Foundation Fellow in Chemistry, 1951-1952. (2) R. W. Allen and L. F. Sutton, Acto Cryst., 3, 46 (1950).
*
*
and L F C F = 108.5 2' against 107 3O. Edgell and R o b e r t ~in , ~a microwave spectroscopic study, obtained a moment of inertia perpendicular t o the threefold axis of 161.80 f 0.07 x 1 0 - 4 0 g. c m 2 . They showed that this moment of inertia was compatil$e with the following parameters : C-C = 1.54 A., C-F 1.33 K., C-H = 1.043 A., (3) W. F. Edgell and A. Roberts, J . Cizem. Phys., 16, 1002 (1948).
