Page 321 - Advanced Organic Chemistry Part B - Reactions & Synthesis

are likely to react via the ion pair mechanism, which is not necessarily stereospecific, 293
as the carbocation intermediate permits loss of stereochemistry relative to the reactant
alkene. It might be expected that the ion pair mechanism would lead to a preference SECTION 4.1
for syn addition, since at the instant of formation of the ion pair, the halide is on the Electrophilic Addition to
Alkenes
same side of the alkene as the proton being added. Rapid collapse of the ion pair
intermediate would lead to syn addition. If the lifetime of the ion pair is longer and
the ion pair dissociates, a mixture of syn and anti addition products can be formed.
The termolecular mechanism is expected to give anti addition because the nucleophilic
attack occurs on the opposite side of the double bond from proton addition. Further
discussion of the structural features that affect the competition between the two possible
mechanisms can be found in Section 6.1 of Part A.

4.1.2. Hydration and Other Acid-Catalyzed Additions of Oxygen Nucleophiles
Oxygen nucleophiles can be added to double bonds under strongly acidic condi-
tions. A fundamental example is the hydration of alkenes in acidic aqueous solution.

H 2 O –H +
+
R C CH + H R CCH 3 R CCH 3 R CCH 3
2
2
2
2
2
+
+ OH 2 OH
Addition of a proton occurs to give the more-substituted carbocation, so addition is
regioselective and in accord with Markovnikov’s rule. A more detailed discussion
of the reaction mechanism is given in Section 6.2 of Part A. Owing to the strongly
acidic and rather vigorous conditions required to effect hydration of most alkenes,
these conditions are applicable only to molecules that have no acid-sensitive functional
groups. The reaction is occasionally applied to the synthesis of tertiary alcohols.
O O
H SO 4
2
(CH ) C CHCH CH CCH 3 (CH ) CCH CH CH CCH 3
2
2
2
3 2
2
2
3 2
H O
2
OH Ref. 9
Moreover, because of the involvement of cationic intermediates, rearrangements can
occur in systems in which a more stable cation can result by aryl, alkyl, or hydrogen
migration. Oxymercuration-reduction, a much milder and more general procedure for
alkene hydration, is discussed in the next section.
Addition of nucleophilic solvents such as alcohols and carboxylic acids can be
effected by using strong acids as catalysts. 10
HBF 4
(CH ) C CH + CH OH (CH ) COCH 3
3
3 3
3 2
2
HBF 4
CH CH CH + CH CO H (CH ) CHO CCH 3
2
2
3
3
2
3 2
9 J. Meinwald, J. Am. Chem. Soc., 77, 1617 (1955).
10
R. D. Morin and A. E. Bearse, Ind. Eng. Chem., 43, 1596 (1951); D. T. Dalgleish, D. C. Nonhebel, and
P. L. Pauson, J. Chem. Soc. C, 1174 (1971).