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Bimolecular Electrophilic Substitution at Saturated Carbon 205
R
Figure 4.8 Transition state for backside SE2 displacement.
Figure 4.9 Transition state for frontside SE2 displacement.
We shall only discuss experiments that shed light on the first three types
(Reactions 4.43-4.45), but the characteristics of all five types of mercury exchange
reactions seem to be very similar.
The first question we shall ask is whether bimolecular mercury exchanges
proceed with retention or inversion of configuration at the central carbon. We
shall then see if we can decide whether the transition state is open (S,2 mecha-
nism) or cyclic (S,i mechanism). If the reactions proceed with inversion of con-
figuration, then the mechanism must be S,2. The geometry of the activated
complex leading to inversion would be very similar to that leading to S,2 substi-
tution. The electrophile, attacking the carbon from the back side, would cause
carbon to rehybridize from sp3 to sp2 so that the remaining P orbital could be
shared by it and the leaving group (Figure 4.8). The difference between this
transition state and the transition state for backside displacement in the S,2 reac-
tion is, of course, that in the SE2 reaction the three participating atomic orbitals
share a total of two electrons whereas in the S,2 reaction they share a total of
four. Retention ofconfiguration could result from either an SE2 or an S,i mechan-
ism. In either case the electrophile would attack the sp3 orbital used in bonding
with the leaving group from the front side (Figure 4.9).
The first detailed study of the stereochemistry of a mercury exchange reac-
tion that was known to be bimolecular was carried out as
Di-s-butyl-
mercury was prepared by reacting optically active s-butylmercuric bromide with
racemic s-butylmagnesium bromide as shown in Equation 4.48.
O3 (a) H. B. Charman, E. D. Hughes, and C. K. Ingold, J. Chem. Soc., 2530 (1959): (b) F. R. Jensen,
J. Amrr. Chem. Soc., 82, 2469 (1960); (c) 0. A. Reutov and E. V. Uglova, BII/[. Acad. Sri. USSR,
Chem. Div. Sci., 1628 (1959).
