Page 329 - Advanced Organic Chemistry Part B - Reactions & Synthesis

Chlorination can be accompanied by other reactions that are indicative of 301
carbocation intermediates. Branched alkenes can give products that are the result of
elimination of a proton from a cationic intermediate. 35 SECTION 4.1
Electrophilic Addition to
CH 3 Alkenes
Cl 2 +
(CH ) C CH Cl H C C CH Cl
2
CH 2 3 2 2 2
CH 3 CH 3 80%
Cl
Cl
CH 3 CH 3 Cl + H C CC(CH )
2
(CH ) C C(CH ) 2 3 2
3 2
3 2
CH 3 CH 3 H C 99%
3
Skeletal rearrangements are observed in systems that are prone toward migration.
H C Cl
) C H 2
(CH 3 3
Cl 2
CH CCHCHC(CH )
3
3 3
H C(CH )
3 3
CH 3 Ref. 35
Br 2 C CCH Br
2
3
Ph CCH CH 2 Ph CCHCH Br + Ph 2 2
3
Br Ph
Ref. 36
Nucleophilic solvents can compete with halide ion for the cationic intermediate.
For example, the bromination of styrene in acetic acid leads to significant amounts of
the acetoxybromo derivative.
CH CO H PhCHCH Br + PhCHCH Br
3
2
PhCH CH 2 + Br 2 2 2
Br O CCH 3
2
80% 20%
Ref. 30
The acetoxy group is introduced exclusively at the benzylic carbon. This is in accord
with the intermediate being a weakly bridged species or a benzylic cation. The addition
of bromide salts to the reaction mixture diminishes the amount of acetoxy compound
formed by shifting the competition for the electrophile in favor of the bromide ion.
Chlorination in nucleophilic solvents can also lead to solvent incorporation, as, for
example, in the chlorination of 1-phenylpropene in methanol. 37
OH
CH 3 PhCHCHCH 3 + PhCH CHCH 3
PhCH CHCH + Cl 2
3
CH O Cl Cl Cl
3
82% 18%
From a synthetic point of view, the participation of water in brominations, leading
to bromohydrins, is the most important example of nucleophilic capture of the interme-
diate by solvent. To favor introduction of water, it is desirable to keep the concentration

35 M. L. Poutsma, J. Am. Chem. Soc., 87, 4285 (1965).
36 R. O. C. Norman and C. B. Thomas, J. Chem. Soc. B, 598 (1967).
37
M. L. Poutsma and J. L. Kartch, J. Am. Chem. Soc., 89, 6595 (1967).