Page 353 - Mechanism and Theory in Organic Chemistry

Table 7.2 RELATIVE RATES OF HYDRATION OF ALKENES AND ALKYNES
Alkene
Alkyne Relative Rate

SOURCE: K. Yates, G. H. Schmid, T. W. Regulski, D. G. Garratt, Hei-Wun Leung, and R. Mc-
Donald, J. Amer. Chem. Soc., 95, 160 (1973.). Reprinted by permission of the American Chemical
Society.

2-methylbutene-2 as would be expected if step 1 of Equation 7.1 were a rapid
equilibrium (see Equation 7.2).* Hydration, then, is an cxam~le-of-a.reaction
that proceeds via an Ad,2 (addition-electrophilic-second-ofder) mechanism.
Taft orGaFg=dsihat the mechanism for hydration involzan
additional step, namely the rapid, reversible formation of a n complex from a
proton and the olefin, and that this complex then rearranges, in the rate-deter-
mining step, to the carbocation as shown in Equation 7.3. This is corisistent with
all the data discussed so far, but it has recently been shown to be incorrect. The

concentration of the n complex, like the equilibrium concentration of any acid, is
determined solely by the pH. If its rearrangement is rate-determining, the
rate should depend on the concentrations of alkene and of H30+. If proton
abstraction is rate-determining, the rate should depend on the concentrations of
alkene and of all the acid present-hydronium ion and undissociated acid.
(Catalysis by H30+ only is called spec@ acid catalysis; catalysis by H,O+ and
undissociated acid is called general acid catalysis. These phenomena are discussed
in greater detail in Chapter 8.) Kresge and co-workers studied the hydration of
tram-cyclooctene and of 2,3-dimethyl-2-butene in phosphoric acid-bisulfate
buffer solutions in which the amount of undissociated acid varied and found that
the reactions exhibit general acid catalysis.1° This behavior parallels that ob-
served earlier in the hydration of styrene and substituted styrenes.ll

J. B. Levy, R. W. Taft, Jr., and L. P. Hammett, J. Amer. Chem. Soc., 75, 1253 (1953).
R. W. Taft, Jr., J. Amer. Chem. Soc., 74, 5372 (1952).
lo A. J. Kresge, Y. Chiang, P. H. Fitzgerald, K. S. McDonald, and G. H. Schmid, J. Amer. Chem.
SOC., 93, 4907 (1971).
l1 See note 3(a), (c), p. 338.