Page 326 - Advanced Organic Chemistry Part B - Reactions & Synthesis

298 carbon that is closer to the substituent, which is attributed to a favorable polar effect
that stabilizes the negative charge on the mercurated carbon.
CHAPTER 4
Electrophilic Additions
to Carbon-Carbon EWG favored site
Multiple Bonds for mercuration
EWG

Visual models, additional information and exercises on Oxymercuration can be
found in the Digital Resource available at: Springer.com/carey-sundberg.

Scheme 4.1 includes examples of oxymercuration reactions. Entries 1 and 2
illustrate the Markovnikov orientation under typical reaction conditions. The high exo
selectivity in Entry 3 is consistent with steric approach control on a weakly bridged
(or open) mercurinium ion. There is no rearrangement, indicating that the intermediate
is a localized cation.

+
CH Hg II
2
Entries 4 and 5 involve formation of ethers using alcohols as solvents, whereas the
reaction in Entry 6 forms an amide in acetonitrile. Entries 7 and 8 show use of other
nucleophiles to capture the mercurinium ion.

4.1.4. Addition of Halogens to Alkenes

The addition of chlorine or bromine to an alkene is a very general reaction.
Section 6.3 of Part A provides a discussion of the reaction mechanism. Bromination of
simple alkenes is extremely fast. Some specific rate data are tabulated and discussed
in Section 6.3 of Part A. As halogenation involves electrophilic attack, substituents on
the double bond that increase electron density increase the rate of reaction, whereas
EWG substituents have the opposite effect. Considerable insight into the mechanism
of halogen addition has come from studies of the stereochemistry of the reaction.
Most simple alkenes add bromine in a stereospecific manner, giving the product of
anti addition. Among the alkenes that give anti addition products are Z-2-butene,
E-2-butene, maleic and fumaric acid, and a number of cycloalkenes. 30 Cyclic,
positively charged bromonium ion intermediates provide an explanation for the
observed anti stereospecificity.

+
CH 3 CH 3 Br Br CH 3
+ Br 2 CH 3 CH 3 + Br – CH H
H H H H 3 H Br

30
J. H. Rolston and K. Yates, J. Am. Chem. Soc., 91, 1469, 1477 (1969).