Page 512 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

A study of several substituted alkenes in methanol developed some generaliza- 493
tions pertaining to the capture of bromonium ions by methanol. 72 For both E- and
−
Z-disubstituted alkenes, the addition of both methanol and Br was completely anti SECTION 5.3
stereospecific. The reactions were also completely regioselective, in accordance with Addition of Halogens
Markovnikov’s rule, for disubstituted alkenes, but not for monosubstituted alkenes. The
lack of high regioselectivity of the addition to monosubstituted alkenes can be inter-
preted as competitive addition of solvent at both the mono- and unsubstituted carbons of
the bromonium ion. This competition reflects conflicting steric and electronic effects.
Steric factors promote addition of the nucleophile at the unsubstituted position, whereas
electronic factors have the opposite effect.

+
Br
for mono- and solvent capture is
CC 1,2-disubstituted Br stereospecific but
alkenes not regiospecific
Br 2 CH 3 O
C C or
CH 3 OH solvent capture is
+ C CH 2 Br for disubstituted CH 3 O CH 2 Br regiospecific but not
alkenes stereospecific
Similar results were obtained for chlorination of several of alkenes in methanol. 73
Whereas styrene gave only the Markovnikov product, propene, hexene, and similar
alkenes gave more of the anti Markovnikov product. This result is indicative of strong
bridging in the chloronium ion.

Cl 2 RCHCH 2 Cl
RCHCH 2 OCH 3
RCH CH 2 +
CH 3 OH major minor
Cl OCH 3
We say more about the regioselectivity of opening of halonium ions in Section 5.8,
where we compare halonium ions with other intermediates in electrophilic addition
reactions.
Some alkenes react with halogens to give substitution rather than addition. For
example, with 1,1-diphenylethene, substitution is the main reaction at low bromine
concentration. Substitution occurs when loss of a proton is faster than capture by
bromide.

Br 2 [Br 2 ] Ratio
Ph 2 C CH 2 Ph 2 CCH 2 Br + Ph 2 C CHBr
–2
10 M >99:1
Br
–3
10 M 90:10
–4
10 M <1:99
Similarly, in chlorination, loss of a proton can be a competitive reaction of the cationic
intermediate. 2-Methylpropene and 2,3-dimethyl-2-butene give products of this type.

72 J. R. Chretien, J.-D. Coudert, and M.-F. Ruasse, J. Org. Chem., 58, 1917 (1993).
73
K. Shinoda and K. Yasuda, Bull. Chem. Soc. Jpn., 61, 4393 (1988).