July 20, 1057
PROBLEM OF SELECTIVITY IN DIALYSIS
[-lec-j+ [“-$-:-I I
I
I
I
+
rate-determ.
carbonium ion
[
H-i-i-]’
+ 2H20
I t I 1
+ HaO equil.
H-C-C-OH
+
Application of the I-strain hypothesis to the relative rates of olefin hydration listed in Table V provides further evidence of importance concerning the structure of the transition state. If the positions occupied by the nuclei in the reaction transition state were those expected of a classical carbonium ion (in which the added H has formed a conventional C-H bond), then by arguments paralleling those used above, the rate of hydration of the cyclobutene should be favored and the cyclopentene retarded relative to the open chain olefins should be (favorable)
H
CHa
should be (unfavorable)
However, as noted above, exactly the opposite order of rates is observed. We conclude therefore that the transition state does not possess the nuclear arrangement of a classical carbonium ion. On the
[CONTRIBUTION FROM THE
3729
other hand, the transition state is stabilized by those structural features which by electronic shifts (inductive or hyperconjugative electron-release) stabilize carbonium ions, as witnessed by the many powers of ten increase in rate in the series : ethylene
