Rotation-isomerization scrambling

In aqueous 90% acetone at 99.6°C, At for /r Ai.s-a,y-dimethylallyl p-nitro-benzoate is 2.6x10-6 ec" [(II), R = R = CH3 Y = p-O-.NQH.CO., SOH = HoO]. The rate is not affected by addition of small amounts of lithium p-nitrobenzoate or p-nitrobenzoic acid. In aqueous 60% acetone at 60 C, At = 1.5 X lO secBy using carbonyl- o-labeled ester, it was shown that 60 scrambling is about twice as fast as allylic rearrangement, which implies that the carboxyl group of the p-nitrobenzoate ion in the tight ion pair rotates relative to the allylic carbonium ion in some instances before the ions recombine to form enantiomeric esters . This and similar observations on other allylic esters (see below) rules out the concerted Sf i mechanism as the only mechanism for allylic ester isomerization under solvolytic conditions. 

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