Page 502 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

+ 23
with
. A substantial solvent isotope effect k /k equal to 2 to 4 is observed. 483
H2O D2O
Both of these observations are in accord with a rate-determining protonation to give
SECTION 5.2
a carbocation intermediate. Capture of the resulting cation by water is usually fast
relative to deprotonation. This has been demonstrated by showing that in the early Acid-Catalyzed
Hydration and Related
stages of hydration of styrene deuterated at C(2), there is no loss of deuterium from the Addition Reactions
unreacted alkene that is recovered by quenching the reaction. The preference for nucle-
ophilic capture over elimination is also consistent with the competitive rate measure-
ments under solvolysis conditions, described on p. 438–439. The overall process is
reversible, however, and some styrene remains in equilibrium with the alcohol, so
isotopic exchange eventually occurs.

H + – D +
PhCH CD 2 PhCHCD H PhCH CHD
+ 2 slow
H O fast
2
H
PhCHCD 2
OH

Alkenes lacking phenyl substituents appear to react by a similar mechanism. Both
the observation of general acid catalysis 24 and solvent isotope effect 25 are consistent
with rate-limiting protonation of alkenes such as 2-methylpropene and 2,3-dimethyl-
2-butene.

2
R C CHR′ + H + slow R 2 CCH 2 R′ H O R 2 CCH 2 R′ + H +
2
+ fast
OH
Relative rate data in aqueous sulfuric acid for a series of alkenes reveal that the reaction
is strongly accelerated by alkyl substituents. This is as expected because alkyl groups
both increase the electron density of the double bond and stabilize the carbocation
7
12
intermediate. Table 5.1 gives some representative data. The 1 10 10 relative rates
for ethene, propene, and 2-methylpropene illustrate the dramatic rate enhancement by
alkyl substituents. Note that styrene is intermediate between monoalkyl and dialkyl
alkenes. These same reactions show solvent isotope effects consistent with the reaction
26
proceeding through a rate-determining protonation. Strained alkenes show enhanced
reactivity toward acid-catalyzed hydration. trans-Cyclooctene is about 2500 times as
reactive as the cis isomer, 27 which reflects the higher ground state energy of the
strained alkene.
Other nucleophilic solvents can add to alkenes in the presence of strong acid
catalysts. The mechanism is analogous to that for hydration, with the solvent
replacing water as the nucleophile. Strong acids catalyze the addition of alcohols

23 W. M. Schubert and J. R. Keefe, J. Am. Chem. Soc., 94, 559 (1972); W. M. Schubert and B. Lamm,
J. Am. Chem. Soc., 88, 120 (1966); W. K. Chwang, P. Knittel, K. M. Koshy, and T. T. Tidwell, J. Am.
Chem. Soc., 99, 3395 (1977).
24
A. J. Kresge, Y. Chiang, P. H. Fitzgerald, R. S. McDonald, and G. H. Schmid, J. Am. Chem. Soc., 93,
4907 (1971); H. Slebocka-Tilk and R. S. Brown, J. Org. Chem., 61, 8079 (1998).
25 V. Gold and M. A. Kessick, J. Chem. Soc., 6718 (1965).
26 V. J. Nowlan and T. T. Tidwell, Acc. Chem. Res., 10, 252 (1977).
27
Y. Chiang and A. J. Kresge, J. Am. Chem. Soc., 107, 6363 (1985).