Page 501 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

482 in the product was determined by NMR. The fact that 1 and 2 are formed in
unequal amounts excludes the possibility that the symmetrical bridged ion is the only
CHAPTER 5 22
intermediate.
Polar Addition
and Elimination D
Reactions
D
D–Cl D + Cl
AcOH Cl Cl +
1 57% 2 41% 3 2%

The excess of 1 over 2 indicates that some syn addition occurs by ion pair collapse
before the bridged ion achieves symmetry with respect to the chloride ion. If the
amount of 2 is taken as an indication of the extent of bridged ion involvement, one
can conclude that 82% of the reaction proceeds through this intermediate, which must
give equal amounts of 1 and 2. Product 3 results from the C 6 → C 2 hydride shift
that is known to occur in the 2-norbornyl cation with an activation energy of about
6 kcal/mol (see p. 450).
From these examples we see that the mechanistic and stereochemical details
of hydrogen halide addition depend on the reactant structure. Alkenes that form
relatively unstable carbocations are likely to react via a termolecular complex and
exhibit anti stereospecificity. Alkenes that can form more stable cations can react via
rate-determining protonation and the structure and stereochemistry of the product are
determined by the specific properties of the cation.

5.2. Acid-Catalyzed Hydration and Related Addition Reactions

The formation of alcohols by acid-catalyzed addition of water to alkenes is a
fundamental reaction in organic chemistry. At the most rudimentary mechanistic level,
it can be viewed as involving a carbocation intermediate. The alkene is protonated and
the carbocation then reacts with water.

H + H O
2
R C CHR' R CCH R' R CCH R' + H +
2
2
2
2
2
+
OH
This mechanism explains the formation of the more highly substituted alcohol from
unsymmetrical alkenes (Markovnikov’s rule). A number of other points must be
considered in order to provide a more complete picture of the mechanism. Is the
protonation step reversible? Is there a discrete carbocation intermediate, or does the
nucleophile become involved before proton transfer is complete? Can other reactions
of the carbocation, such as rearrangement, compete with capture by water?
Much of the early mechanistic work on hydration reactions was done with conju-
gated alkenes, particularly styrenes. Owing to the stabilization provided by the phenyl
group, this reaction involves a relatively stable carbocation. With styrenes, the rate
of hydration is increased by ERG substituents and there is an excellent correlation

22
H. C. Brown and K.-T. Liu, J. Am. Chem. Soc., 97, 600 (1975).