Organic Chemistry

Section 10.9 Competition Between SN 2 and SN 1 Reactions 387

Therefore, when both reactions occur simultaneously, increasing the concentration of
the nucleophile increases the fraction of the reaction that takes place by an path-
way. In contrast, decreasing the concentration of the nucleophile decreases the fraction
of the reaction that takes place by an pathway.
The slow (and only) step of an reaction is attack of the nucleophile on the alkyl
halide. Increasing the reactivityof the nucleophile increases the rate of an reac-
tion by increasing the value of the rate constant because more reactive nucle-
ophiles are better able to displace the leaving group. The slow step of an reaction
is the dissociation of the alkyl halide. The carbocation formed in the slow step reacts
rapidly in a second step with any nucleophile present in the reaction mixture. Increas-
ing the rate of the fast step does not affect the rate of the prior slow, carbocation-form-
ing step. This means that increasing the reactivity of the nucleophile has no effect on
the rate of an reaction. A good nucleophile, therefore, favors an reaction over
an reaction. A poor nucleophile favors an reaction, not by increasing the rate
of the reaction itself, but by decreasing the rate of the competing reaction. In
summary:

• An reaction is favored by a high concentration of a good nucleophile.


• An reaction is favored by a low concentration of a nucleophile or by a poor
  nucleophile.
  Look back at the reactions in previous sections and notice that they all use poor
  nucleophiles whereas the reactions use good nucleophiles
  In other words, a poor nucleophile is used to encourage an reac-
  tion, and a good nucleophile is used to encourage an reaction. In Section 10.10,
  we will look at the third factor that influences whether an or an reaction will
  predominate—the solvent in which the reaction is carried out.

PROBLEM-SOLVING STRATEGY

This problem will give you practice determining whether a substitution reaction will take

place by an or an pathway. (Keep in mind that good nucleophiles encourage

reactions, whereas poor nucleophiles encourage reactions.)

Give the configuration(s) of the substitution product(s) that will be obtained from the reac-

tions of the following secondary alkyl halides with the indicated nucleophile:

a.

Because a high concentration of a good nucleophile is used, we can predict that the

reaction is an reaction. Therefore, the product will have the inverted configuration

relative to the configuration of the reactant. (An easy way to draw the inverted product

is to draw the mirror image of the reacting alkyl halide and then put the nucleophile in

the same location as the leaving group.)

b.

Because a poor nucleophile is used, we can predict that the reaction is an reaction.

Therefore, we will obtain two substitution products, one with the retained configuration

and one with the inverted configuration, relative to the configuration of the reactant.

c.

The relatively poor nucleophile suggests that the reaction is an reaction. However,

the product does not have an asymmetric carbon, so it does not have stereoisomers.

SN 1

CH 3 CH 2 CHCH 2 CH 3 + NH 3

I

CH 3 CH 2 CHCH 2 CH 3

NH 2

SN 1

C + CH 3 OH

CH 3

CH 2 CH 3

CH 3

Br OCH 3

C + H

CH 2 CH 3

H CH 3 H

CH 3 O

C

CH 2 CH 3

SN 2

high concentration

C + CH 3 O− CH

3

CH 2 CH 3

CH 3 H

Br CH 3 O

H C

CH 2 CH 3

SN 1

SN 1 SN 2 SN 2

SN 2 SN 1

SN 2

(HO-, CH 3 O-). SN 1

(H 2 O, CH 3 OH), SN 2

SN 1

SN 1

SN 2

SN 1 SN 2

SN 1 SN 1

SN 1 SN 2

SN 1

1 k 22 ,

SN 2

SN 2

SN 2

SN 2