Page 220 - Mechanism and Theory in Organic Chemistry

Bimolecular Electrophilic Substitution at Saturated Carbon 209

the S,i mechanism of mercury exchange and might be more important than the
coordinating power of the anions.
If all three alkyl groups are identical, mercury exchanges of the type shown
in Equation 4.44 are also identical in the forward and reverse directions. The
law of microscopic reversibility taken in conjunction with the kinetics of the
reaction again suggest a cyclic transition state (by the arguments outlined above),
but the observed order of reactivity of the electrophiles again is RHgBr <
RHg(OAc), < RHgNO,. It therefore seems likely that even reactions such as
those shown in Equation 4.49 and Equation 4.43, in which the forward and re-
verse reactions are different but in which the order of reactivity of the mercury
salts is the same as in Reaction 4.53,91 proceed through a cyclic S,i mechanism.
In fact, since almost all bimolecular electrophilic substitutions on carbon in
organometallic compounds of which the stereochemistry have been studied
proceed with retention of configuration, the transition state of Figure .4.9 may be
general for substitutions in organometallics.
Bimolecular Electrophilic Substitutions at Carbon-Hydrogen Bonds
Recently Olah has found a means of studying electrophilic bimolecular
substitutions on the C-H and the unstrained C-C bond and concluded that the
"triangular" transition state shown in Figure 4.9 is also involved here.92 For
example, in DF-SbF,, a superacid medium,93 adamantane rapidly exchanges
hydrogen for deuterium with great preference for the bridgehead positions, as
shown in Equation 4.55. In this rigid bicyclic system, backside attack is im-
probable. There is no strong base, so a carbanion cannot form. Structure 23 then
seems like the most likely representation of the transition state.

See note 83 (a), p. 205.
p2 G. A. Olah, Y. Halpern, J. Shen, and Y. K. Mo, J. Amer. Cham. Soc., 93, 1251 (1971). See also
G. A. Olah and J. A. Olah, J. Amer. Cham. Soc., 93, 1256 (1971) and G. A. Olah and H. Lin, J. Amr.
Cham. SOC., 93, 1259 (1971). For a discussion of attack of H+ on a strained (cyclopropane) C-C
bond, see Section 6.2.
93 Antimony pentafluoride (SbF,) is a strong Lewis acid, so the equilibrium
SbF, + HF -SbF, + H+
lies to the right, giving rise to unsolvated and therefore very reactive protons.