1584
R.
w.TAFT,JR.,
[CONTRIBUTION FROM
THE
E. L. PURLEE, PETERRIESZAND
COLLEGE
OF
c. A. DEFAZIO
VOl. 77
CHEMISTRY AND PHYSICS, THE PENNSYLVANIA STATE USIVERSITY]
r-Complex and Carbonium Ion Intermediates in Olefin Hydration and E l Elimination from &Carbinols. 11. Trimethylethylene, Methylenecyclobutane, Triptene and the Effect of Acidity on their Hydration R a t e l , '
w.TAFT,JR.,
B Y ROBERT
E. LEE PURLEE,' PETERRIESZAKD CHARLES A. DEFAZIO RECEIVED ALXXJST 31, 1954
The rates of hydration of gascous olefin a t unit pressure, kp, have been determined in one to five molar nitric acid with aliphatic olefins of varying sizes and shapes. Log k p values parallel precisely the acidity function, Ho,with slopes varying from 1.?0 to 1.30. These results show the activity coefficient of the hydration transition state to be approximately but not precisely independent of structure. The rates of hydration a t unit concentration of dissolved olefin, k,, follow the acidity function, h,, essentially with unit slopes. Accordingly, the hydration transition states must be free from firmly bound water molecules. Entropies of activation for olefin hydration and corresponding carbinol dehydration are reported which are in good accord with this conclusion. As pointed out earlier,' *-complex and carbonium ion intermediates are consistent with and apparently required by the nature of the transition state.
I n paper I, the rates of hydration of gaseous isobutene a t unit pressure, k,, by one-tenth to five molar aqueous nitric acid solutions a t 25' were reported. The rates of hydration of dissolved isobutene a t unit concentration, kc, were estimated using the Setschenow equation to obtain the olefin activity coefficients. The logarithm of kc plotted against the acidity function, Ho,gives a linear relationship of near unit slope. On the basis of this result, and the theory of Zucker and Hammett as to its significance, a hydration mechanism was proposed in which the rate-determining step is the isomerization of a a-complex to a carbonium ion. I n support of this mechanism i t was further shown that entropies of activation for olefin hydration and carbinol dehydration involving both ordinary aliphatic olefins fleading to tertiary carbinols) and a,p-unsaturated aldehydes correlate with the type of dependence on acidity of the hydration rate. We now report measurements similar to those given in paper I for gaseous aliphatic olefins of appreciably varying sizes and shapes, namely, trimethylethylene, methylenecyclobutane and triptene (asym-methyl-t-butylethylene). The results obtained are in accord with the ideas expressed in paper I. Experimental
TABLE I RATE CONSTANTS FOR THE HYDRATIOS OF GASEOUS TRIMETHYLETHYLENE, TRIPTENEAND METHYLENECYCLOBUT A S E AT UNIT PRESSURE, k p , BY XITRIC ACID SOLUTION I N USITS OF lo4 MOLE-L.-~-ATM.-'-MIK.-~ Trimethylethylen: a t 30.00 "03,
hf
0,973 1,500 2.011 2,995 4,000 1.941
kp
2.58 5.42 10.04 25.7 56.5 99.0
Methylenecyclobutane a t 25.17' kp
3.97 ....
16.14 45.0 109.2 209
Triptene a t 25.12' kp
1.22
...
... 15.08 ...
74.2
and for the dehydration of corresponding carbinols. The values for the ordinary aliphatic olefins have been calculated from recently obtained data which are reported el~ewhere.~-6Except in the cases noted these entropies of activation are accurate to within two or three e.u.
Discussion Structure and k, Values.-Paul,' and Long and McIntyre8 have found recently that in solutions of strong acids or acid and salt mixtures having total electrolyte concentrations in the region one to about six molar the activity coefficient, f B H + , of the conApparatus and Procedure.-These have been described jugate acid of a base, B, is more nearly (but not in detail in earlier papers.'~~ Materials.-Trimethylethylene4c and methylenecyclobu- exactly) independent of structure than is the activtane5 used in this work are described elsewhere. Triptene ity coefficient ratio fB/fHB+. The conjugate acid was refractionated from a sample of the olefin available a t may be a reaction transition state if this consists of Pennsvlvania State Univ.,. b.a. - 77.7" (760 mm.), e z o ~substrate plus a proton. The Hammett acidity 1.4036. function is based upon the term fB/fHB+, being inResults dependent of structure.$ Long and McIntyre I n Table I are listed the rates of hydration of the point out that the distinction between the above three gaseous olefins a t unit pressure obtained in two conditions is rarely seen in acid solutions alone, one to five molar nitric acid solutions. because of the relatively small "salting-in" or Table V lists entropies of activation a t unit acid "salting-out" parameters of these electrolytes. Preand substrate concentrations for olefin hydration sumably for this and the reason given in the next (1) Paper I, R. W.T a f t , J r . , THISJOURNAL, 74, 5372 (1932). section, the present results do not distinguish (2) T h e work reported herein was carried out on Project NRO55-295 as to which is the better general approximation. between t h e Office of Naval Research and T h e Pennsylvania State However, the present results do provide a precise University. (3) Allied Chemical and Dye Corp. Fellow for 1953-1954; taken in indication of whether the activity coefficients of the part from t h e Ph.D. Thesis of E. Lee Purlee, T h e Pennsylvania State hydration transition states in the moderately con1954. University, June, (4) (a) 1. B. Levy, R . W. Taft, Jr., D. Aaron and L. P. Hammett, THISJ O U R N A L , 73,3792 (1951); 76, 3955 (1953); (b) R . W. Taft, J r . , J . B. Levy, D . Aaron, and L. P. H a m m e t t , ibid., 74, 4735 (1932): ic) E. L. Purlee, R . W.Taft, J r . , and C . IDeFazio, i b i L , 77, 837 ( 1 9 3.5). ( 3 ) 1'. lliesz and I
