Page 215 - Mechanism and Theory in Organic Chemistry

most of these reactions do not follow the reaction path implied in Equation 4.41.
Rather, the path usually followed includes a cyclic transition state (Equation
4.42) in which the electrophile enters as the metallic leaving group departs while

at the same time a ligand originally attached to the electrophile is transferred to
the leaving group. Thus, it is not necessary that the electrophile lack a pair of
electrons at the start of the reaction; loss of the ligand with its bonding electrons
during the course of the reaction enables the electrophile to take over the pair of
electrons on carbon formerly used for bonding to the leaving group. We shall
call the mechanism shown in Equation 4.42 the S,i mechanism (in which the
"i" is for "internal").
Although nucleophilic substitutions have been much studied for over 40
years, the corresponding electrophilic substitutions aroused little interest until the
1950's. Since then, because mercury usually forms truly covalent bondse2 and
because organomercurials can be prepared in optically active form and do not
subsequently racemize, the majority of mechanistic studies in this area have used
organomercurials as the substrate. Thus, to stay in the most brightly lighted area
of a field, at best dimly lighted, we shall limit our short discussion of bimolecular
electrophilic aliphatic substitutions almost entirely to reactions of organo-
mercurials.

Electrophilic Cleavage of Organomercurials
Mercury compounds are often used as electrophiles in displacements on
organomercurials. There are five possible combinations of reactants in such
mercury exchange" reactions. A dialkylmercurial substrate may be attacked
6 6
either by a mercury salt (Reaction 4.43) or by a monoalkylmercurial (Reaction
4.44) ; and likewise a monoalkylmercurial may react either with a mercury salt or
with another monoalkylmercurial (Reactions 4.45 and 4.46). Finally, a dialkyl-
mercurial may react with another dialkylmercurial (Reaction 4.47). (In Reac-
tions 4.43-4.47 and in other reactions in this section, one of the reactants, and
the fragments derived from that reactant in the products, are written in italics.
This is done to make it easier to follow the course of the reaction.)
RHgR + XHgX A RHgX + XHgR (4.43)
RHgR + RHgX d RHgR + XHgR (4.44)
RHgX + XHgX A RHgX + XHgX (4.45)
RHgX + RHgX d RHgR + XHgX (4.46)
RHgR + RHgR d RHgR + RHgR (4.47)

O2 In the gas phase, divalent mercury has been shown to be linear and therefore to be sp hybridized.
However, in solution the X-R-X, R-Hg-X, or R-Hg-R bond angle in divalrnt mercury
compounds varies from 130 to 180'. The variation in geometry is not yet entirely understood, so we
shall follow Jensen's example and assume that, even in solution, divalent mercury is sp hybridized
and that if a divalent mercury compound donates one empty orbital to coordinate with a Lewis base
it rehybridizes to sp2 (F. R. Jensen and B. Rickborn, Electrophilic Substitution of Organomercurials,
pp. 35, 36).