Page 413 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

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CHAPTER 4
Nucleophilic Substitution

Potential energy Y X

Y: – X Y X: –

Reaction coordinate

Fig. 4.3. Reaction energy profile for nucleophilic substitution by the direct
displacement S N 2 mechanism.

of the TS is accompanied by rehybridization of the carbon to the trigonal bipyramidal
3
geometry. As the reaction proceeds on to product, sp hybridization is reestablished
in the product with inversion of configuration.

Y : C X Y : C : X Y : C + : X –

∗
Front-side approach is disfavored because the density of the orbital is less in the
region between the carbon and the leaving group and, as there is a nodal surface
between the atoms, a front-side approach would involve both a bonding and an
antibonding interaction with the orbital.
∗
C X
Y

The direct displacement S 2 mechanism has both kinetic and stereochemical
N
consequences. S 2 reactions exhibit second-order kinetics—first order in both reactant
N
and nucleophile. Because the nucleophile is intimately involved in the rate-determining
step, not only does the rate depend on its concentration, but the nature of the nucleophile
is very important in determining the rate of the reaction. This is in sharp contrast to
the ionization mechanism, in which the identity and concentration of the nucleophile
do not affect the rate of the reaction.

k
R–X + Y: – R–Y + X: –
–
–
rate = –d [R–X] = –d [Y: ] = k [R–X] [Y: ]
dt dt
Owing to the fact that the degree of coordination increases at the reacting carbon
atom, the rates of S 2 reactions are very sensitive to the steric bulk of the substituents.
N