Functional Classes II, Reactions - Organic Chemistry Concepts: An EFL Approach - page 113

FIGURE 7.16
Summary mechanisms of nucleophilic substitution.

There are stereochemical outcomes for the above mechanisms. In the S N 2 pro-
cess, an inversion of the configuration occurs during the movement from sp 3 -
tetrahedral → trigonal bipyramidal → sp 3 -tetrahedral.

The S N 1 process passes through a carbocation. In Chapter 5 we saw that an
sp 2 -carbocation was planar. Because of this, the nucleophile can approach from
either side of the plane. This gives a mixture of products from either inversion or
retention of the configuration.

Appendix 9 gives more information about the stereochemistry of nucleophilic
substitution reactions.

7.5.3 Elimination Reactions
As mentioned earlier, alkyl halides can undergo elimination reactions in compe-
tition with substitution. Elimination causes an increase in the bond order, and
is the reverse of the electrophilic addition reactions of alkenes and alkynes seen
in Section 7.3.1 . The highly polarized carbon-halogen bond is again important
in these reactions.

As with substitution, there are two main mechanisms for elimination. The elim-
ination equivalents for nucleophilic substitution are:

E2 for S N 2; n

E1 for S N 1.

n

These mechanisms show the same rate-determining concentration dependen-
cies as for the substitutions.

Elimination by E2 is a concerted process. A base removes a proton from the
carbon next to the halide attachment, and the halide is lost at the same time.
Therefore, the rate depends on both the substrate and base concentration.

Elimination by E1 is stepwise. A carbocation is formed by heterolysis of the car-
bon-halogen bond. Then, the carbocation loses a proton from a neighboring