NOTES
4084
Table II: Isomerization of 4-Oxo-1-pentyl" R12 -
Downloaded by UNIV OF CAMBRIDGE on August 29, 2015 | http://pubs.acs.org Publication Date: December 1, 1966 | doi: 10.1021/j100884a510
a
T,OK
MBK
MEK
EPK
C4HlO
CZH6
RtlEPKl
502.5 502.9 479.8 457.5 438.5
10.77 11.29 8.27 5.07 3.21
36.6 47.5 42.9 34.1 23.8
8.27 9.89 6.58 4.38 2.38
19.4 22.4 22.1 20.5 15.6
41.9 63.8 79.7 94.2 91.8
0.831 0.754 0.840 0.767 0.958
Rates of formation of products are in moles ~ r n -sec-1 ~
x
7.8 10.6 10.6 12.6 10.0
lo'*.
tions where k-13 is of the order of magnitude kI4(Rf [CzHs]f"p/k~'p. This suggests that, in the temperature range used, k-13 is of the order of 10'4 X 8 X 10-6/106.7 = 170sec-1.
Discussion Reaction 7 is an intramolecular analog of the reaction R R'H = R H see-R'
+
RrIEPKl {RrlCnHsl)I" _______ Rin x 100
+
Most of the available data when R'H is a paraffin are for R = CHI and indicate that A is of the order of 1011 cms mole-' sec-' and E of the order 8.3 kcal mole-1,6 in comparison with A7 = 1.4 X lo7 and E, = 10.8. The low value of A , cannot be attributed solely to the loss of the entropy of rotation about the three C-C bonds which are incorporated into the cyclic activated complex. The A factor of a unimolecular reaction can be written, in terms of transition-state theory, in the form
The increase of ca. 2.5 kcal mole-' in activation energy above that for an intermolecular hydrogen abstraction can probably be accounted for by the strain energy on forming the cyclic complex. Kaarsemaker and Coops1ohave estimated a strain energy of 6.5 kcal mole-' for cyclopentane, in contrast to the strainfree configuration in cyclohexane. A quantitative comparison of inter- and intramolecular hydrogen abstraction is probably not justified since the dynamics of the two processes are quite different. Acknowledgments. The authors wish to express their appreciation of the assistance of Mr. Frank Safian. The financial assistance of the National Research Council of' Canada is gratefully acknowledged. (8) J. D. Kemp and C.J. Egan, J . Am. Chem. SOC.,60, 1521 (1938). (9) J. G.Aston and G. H. Messerly, ibid., 62, 1917 (1940). (10) S . Kaarsemaker and J. Coops, Rec. Truv. Chim., 71, 125 (1952).
A = K ekTy e AS'/R h
If the transmission coefficient K is assumed to be unity, then A S + = -31 eu at the mean temperature of 470°K. Kemp and Egan8 have estimated an entropy for the restricted rotation of the two methyl groups in propane of 3.84 eu at 298.2"K1 and Aston and Messerlys have estimated an entropy of 1.44 eu for the restricted rotation of the ethyl group in n-butane at 272.66"K. It is evident, therefore, that the removal of internal rotation alone cannot account for an entropy of acti1. I n order to treat the results vation of -31 eu if K in terms of transition-state theory it is necessary to assume that x
