Page 412 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

a very close association between the cation and anion formed in the ionization step. 393
The solvent-separated ion pair retains an association between the two ions, but with
intervening solvent molecules. Only at the dissociation stage are the ions independent SECTION 4.1
and the carbocation symmetrically solvated. The tendency toward net inversion is Mechanisms for
Nucleophilic Substitution
believed to be due to electrostatic shielding of one face of the carbocation by the anion
in the ion pair. The importance of ion pairs is discussed further in Sections 4.1.3 and
4.1.4.
ionization dissociation
R–X R X R + X – R + + X –
+ –
contact solvent-
ion pair separated
ion pair

According to the ionization mechanism, if the same carbocation can be generated
from more than one precursor, its subsequent reactions should be independent of its
origin. But, as in the case of stereochemistry, this expectation must be tempered by
the fact that ionization initially produces an ion pair. If the subsequent reaction takes
place from this ion pair, rather than from the completely dissociated and symmetrically
solvated ion, the leaving group can influence the outcome of the reaction.

4.1.2. Substitution by the Direct Displacement S 2 Mechanism
N
The direct displacement mechanism is concerted and proceeds through a single
rate-determining TS. According to this mechanism, the reactant is attacked by a
nucleophile from the side opposite the leaving group, with bond making occurring
simultaneously with bond breaking between the carbon atom and the leaving group. The
TS has trigonal bipyramidal geometry with a pentacoordinate carbon. These reactions
exhibit second-order kinetics with terms for both the reactant and nucleophile:

rate = k R-X Nu

The mechanistic designation is S 2 for substitution, nucleophilic, bimolecular.
N
A reaction energy diagram for direct displacement is given in Figure 4.3. A symmetric
diagram such as the one in the figure would correspond, for example, to exchange of
iodide by an S 2 mechanism.
N
*I – + CH I CH *I + I –
3
3
The frontier molecular orbital approach provides a description of the bonding
interactions that occur in the S 2 process. The frontier orbitals are a filled nonbonding
N
orbital on the nucleophile Y: and the antibonding orbital associated with the
∗
carbon undergoing substitution and the leaving group X. This antibonding orbital has
3
a large lobe on carbon directed away from the C−X bond. Back-side approach by
the nucleophile is favored because the strongest initial interaction is between the filled
∗
orbital on the nucleophile and the antibonding orbital. As the transition state is
approached, the orbital at the substitution site has p character. The MO picture predicts
that the reaction will proceed with inversion of configuration, because the development
3
L. Salem, Chem. Brit., 5, 449 (1969); L. Salem, Electrons in Chemical Reactions: First Principles,
Wiley, New York, 1982, pp. 164–165.