Summary: 4258 J. Am. Chem. SOC.
exchange. For example, in the isoprene anion the four allylic
hydrogens exchange most rapidly ( k = 9 X 1O-'O cm3molecule-I`
s-I). A fifth proton, which we formulate as that in the 3 position,
exchanges more slowly but at a rate which is similar enough to
that of the first four to prevent a reliable dissection of its intrinsic
exchange rate constant. The final two hydrogens exchange more
slowly still with an estimated rate constant of 1 X lo-" cm3
molecule-' s-I (eq 7).'O Thus, by determining not only the
maximum number of exchangeable hydrogens but also their
relative ease of exchange, we can gain important information about
the structure of the carbanion. Further, in bracketing hydrocarbon
acidities the presence or absence of multiple exchanges may serve
to fix an upper or lower limit for certain types of hydrogens in
the molecule. For instance, we have found that in the 2-tert-
butylallyl anion only the four allylichydrogens exchangewith D20.
We are presently examining the potential of other compounds
for use as exchange agents in different hydrocarbon acidity ranges.
The details of our efforts in this area shall appear in a forthcoming