8351 Table 11. Results of Finite Perturbation-INDO Calculations on Ethyl, Isopropyl, and tert-Butyl Compounds, CB-CA-X' -Isopropyl---
Ethyl*PA'
X
JCC~
C02H COCH3 CN
39.3 40.0d 40.7
0.0596 0.0593 0,0588
4.0186 4.0066 3.9397
3.9272 3.9338 3.9378
40.6 39.8 40.7
H
41.5
0.0620
3.9457
3.9457
CsHs NO? CH3 NHz OCH3
41.6 41.9 42.1 47.0 47.6d
0.0599 0.0624 0,0602 0.0644 0,0660
3.9410 3.9430 3.9391 3.8223 3.7478
OH F
49.6 49.6
0,0672 0.0679
3.7495 3.6787
PscAsc~'
PB
7
PA
PB
0.0574 0.0570 0,0566
4.0181 3.9935 3.9336
3.9315 3.9292 3.9382
42.1
0.0602
3.9391
3.9453
3.9417 2.9488 3.9453 3.9682 3.9857
44.1 44.1 42.0 47.2 50.8
0.0586 0.0610 0.0578 0.0620 0.0643
3.9422 3.9368 3.9398 3.8270 3.7657
3.9458 3.9525 3.9439 3.9657 3.9860
3.9798 4.0000
48.7 50.1
0.0638 0.0655
3.7668 3.6890
3.9858 3.9968
JCC
PscBsoA2
,
JCC
39.2 40.5 39.9 (41.1). 42.0 (43.3) 42.0 43.0 41.0 45.5 48.9 (50.3) 47.6 48.7
--
tert-Butyl PacAmB2 PA
0.0545 0.0549 0,0539 (0.0548) 0.0578 (0.0589) 0.0553 0 0582 0,0550 0,0586 0,0610 (0.0620) 0,0608 0,0622
PB
4.0138 3.9996 3.9353
3.9294 3.9308 3.9374
3.9398
3.9439
3.9513 3.9325 3.9461 3.8432 3.7771
3.9404 3.9503 3.9418 3.9652 3.9809
3.7764 3.7053
3.9777 3.9933
+
a J C Cvalues in Hz. b Taken from ref 2 unless otherwise noted. Valence-shell atomic electron density of carbon A, Le., Pze2s P2p,2,,+ P2py2py P2p,2p,= P. d Determined in this study. e Values in parentheses correspond to calculations in which the CB-CA-CB angle is 115".
-+
carried out for selected substituents using CCC angles larger (by 5 . 5 " ) than the tetrahedral value. The resulting shifts in Jcc, shown in Table 11, are of the appropriate sense so as to bring the computed results into more general qualitative agreement with experiment. It was found that the majority of the increase in computed Jcc upon expanding the CCC angle is accounted for, as expected, by altered carbon hybridization, as manifested by changes in PscAscB2, the square of the bond order between the 2s orbitals of the coupled carbons. 2 2 The experimental data and computed results of this study are consistent with the following view of substituent effects on 13C-13C coupling constants in the ethyl, isopropyl, and tert-butyl series. Within a given series, an electron-withdrawing substituent may be con(22) G. E. Maciel, J. W. McIver, Jr., N. J . Amer. Chem. Soc., 92, 11 (1970).
s. Ostlund, and J. A. Pople,
sidered qualitatively to enhance the s character of the C-C bond and thus to increase the Fermi contact coupling. For a given substituent, substitution of methyl groups for hydrogens at the a carbon tends to increase the s character in the C-C bond by sterically inducing minor changes in bond angles and hence in hybridization. Thus the coupling constant goes up with the number of methyl groups attached to the a carbon. Studies of 13C-13C coupling constants in n-propyl, isobutyl, and neopentyl compounds would be useful in testing this interpretation. Experiments to this end are under way. It can be noted that preliminary data on n-propyl compounds support the interpretation based upon steric effects. Acknowledgment. The authors are grateful to the National Science Foundation, which provided funds toward the purchase of the spectrometer and data system. They also acknowledge helpful discussions with Richard L. Elliott.
Anomalous Kinetic Hydrogen Isotope Effects on the Rate of Ionization of Some Dialkyl Substituted Ketones R. A. Lynch, S. P. Vincenti, Y. T. Lin, L. D. Smucker, and S. C. Subba Rao*
Contribution from the Department of Chemistry, Lincoln University, Lincoln University, Pennsylvania 19352. Received February 7, 1972 Abstract: The rates of hydroxide catalyzed H+, D+, and T+ transfer from 2,4-dimethyl-3-pentanone and 3 3 -
dimethyl-4-heptanone have been measured over a temperature range. The Arrhenius plots are linear and the deuterium isotope effects are normal. These results suggest that tunneling is not significant in these reactions. The rates of H+ and T+ transfers are similar and apparently there is no tritium isotope effect. The results suggest that in detritiation, T+ transfer is not the rate determining step. A mechanism for tritium-hydrogen exchange has been proposed. t was reported' that the Arrhenius plot for the bromination of 2,4-dimethyl-3-pentanonecatalyzed by hydroxide ion exhibited pronounced curvature at temperatures below 20". This deviation was ascribed to
I
(1) J. R. Hullet, J . Chem. Soc., 430 (1965).
tunneling of the proton through the energy barrier.' The Arrhenius plot for the hydroxide ion catalyzed detritiation of 2,4-dimethyl-3-pentanoneshowed a sharp break at approximately 40". This was attributed to a (2) J. R. Jones, Trans. Faraday Soc., 61,2456 (1965).
Lynch, Vincenti, Lin,Smucker, Subba Rao 1 Kinetic Hydrogen Isotope Effects
8352 Table I. Ionization Rate Coefficients (1. mol-1 sec-1). Compound
Temp, "C
2,4-Dimethyl3-pentanone
0.2 20.0 25 .O 30.0 35 .O 40.0 0.2 20.0 25.0 30.0 35.0 40.0
3,SDimethyl4-heptanone
a
104k~oH.-
1 0 4 ~ ~ ~ -
2.4 f 0 . 0 2 12.2 f 0.05 17.8 f 0 . 0 2 24.6 f 0 . 1 3 36.3 f 0.18 51.2 f 0.25 0.205 f 0.002 1.210 i 0.002 1.850 i 0.007 2.720 i 0.015 4.066 f 0.025
104
0.372 i. 0.005 1.950 f 0.008 2.920 f 0.02 4.420 f 0.03 6.610 f 0.03 9.340 f 0.04 0.0537 f 0.0003 0.3360 f 0.0028 0.5030 f 0.0030 0.7780 f 0.005 1.170 f 0.008
~
-
0
~
1.14 f 0.01 6 . 0 3 f 0.03 8.90 f 0.05 12.90 f 0.05 18.20 f 0.08 26.10 f 0.14 0.110 f 0.001 0.660 ?C 0.001 0.998 f 0.003 1.450 f 0.010 2.140 f 0.020 3.230 f 0.020
Obtained from base-catalyzed bromination.
dynamic steric effect. The above results of bromination in conjunction with detritiation disclose the following features: (a) kHoH-/2kTo~-= 0.98 at 30" and remains the same in the temperature range 0-40", (b) A H / ~ A T= 0.43, (c) very small difference between the activation energies (ET - EH = 500 cal mol-'). None of these features are indicative3 of tunneling. Thus, the situation regarding 2,4-dimethyl-3-pentanone is not clear and further investigation is required to explain the results more satisfactorily. The current work was undertaken t o determine whether or not a nonlinear Arrhenius plot would be obtained for the hydroxide ion catalyzed bromination of 2,4dideuterio-2,4-dimethyl-3-pentanone.Further, it was considered interesting to investigate the rates of H+, D+,and T+ transfers from substrates similar to 2,4dimethyl-3-pentanone but in which the ionizable proton is surrounded by larger nonionizable groups. 3,5Dimethyl-4-heptanone was chosen for this purpose. Since our method of measuring the rate of bromination of 2,4-dideuterio-2,4-dimethyl-3-pentanone differed from that adopted by Hullet for 2,4-dimethyl-3-pentanone, the bromination rates of 2,4-dimethyl-3-pentanone were measured by our method to ensure that comparison of H+ and D+ transfer would have significance. In the current work, significant deuterium isotope effects were observed for 2,4-dimethyl-3-pentanone whereas in the previous study2 no tritium isotope effects were observed. Hence it was considered necessary to measure the rates of T+ transfer to confirm the absence of tritium isotope effects. Results 2,4-Dimethyl-3-pentanone and 3,5-dimethyl-4-heptanone react with alkaline solutions of bromine according to the general equation RiRKHCOCHRiRz
0
I1
OH + HzO + -
(CH&CDCCD(CH&
0
I/
(CH&CHCCD(CH&
+ HDO
The two rates are similar and confirm the above assumption. Table I contains the ionization coefficients kHoH-, kDoH- (both obtained from bromination), and kToH-. Table I1 contains the ionization rate coefficient kDoH(obtained from the exchange reaction). Table I11 Table 11. Ionization Rate Coefficient kDoa- (1. mol-' sec-lp
a
Compound
Temp, "C
2,4-Dimethyl-3-pentanone
25.0
1 0 4 ~ ~ ~ 2.14 4~ 0.024
Obtained from base-catalyzed deuterium-hydrogen exchange.
Table 111. Arrhenius Parameters for H +, D +, and T + Transfersa Compound EH ED
ET AH AD AT
2,4-Dimethyl3-pentanone 13.04 13.82 13.36 (6.38 (4.06 (5.54
f 0.03 f 0.01 f 0.19 f 0.02)106 f 0.04)106 f 0.04)106
3,5-Dimethyl4-heptanone 14.35 i 0 . 3 2 14.72 f 0.23 14.35 f 0 . 1 6 (6.03 f 0.06)106 ( 6 . 3 f 0.03)106 (3.31 + 0.02)106
nE in kcal mol-' and A in 1. mol-' sec-l.
+ OBr----t R1RzCHCOC(Br)RiRz
+ OH-
(1)
Cullis and Hashmi4 reported that in the bromination of 2,4-dimethyl-3-pentanone, 2 mol of OBr- are consumed by 1 mol of ketone. Monobromoisobutyric acid and 2-propanol were found to be the products. The second step of the bromination appeared to be
+
Our work established the uptake of 1 mol of OBr- by 1 mol of ketone. It seems that under the current experimental conditions only simple monobromination is important and the second step is very slow. This has been confirmed by comparing the rate of bromination and the rate of the first deuterium-hydrogen exchange reaction.
OH -
(CH3)2CHCOCBr(CH3)z OBr- + (CH3)2CBrCOO- (CH3),CHOH
+
+ Br-
(2)
(3) R. P. Bell, "Proton in Chemistry," Cornell University Press, New York, N.Y.,1959, p 211. (4) (a) C. F. Cullis and M. H. Hashmi, J . Chem. SOC.,3080 (1957); (IJ) ibid., 1548 (1957).
Journal of the American Chemical Society
contains the Arrhenius parameters calculated by the method of least squares. Tables IV and V contain the deuterium and tritium isotope effects, respectively. In order to obtain the tritium isotope effects, it is necessarys to introduce a statistical factor of 2 since in the substrates there are two ionizable protons but only one ionizable triton per molecule. Experimental Section Materials. 2,4-Dimethyl-3-pentanoneand 3,5-dimethyl-4-heptanone were purchased from Aldrich Chemical Co., Milwaukee, (5) D. M. Bishop and I
