Organic Chemistry

384 CHAPTER 10 Substitution Reactions of Alkyl Halides

There are two reasons that vinylic halides and aryl halides do not undergo reac-

tions. First, vinylic and aryl cations are even more unstable than primary carbocations

(Section 10.5) because the positive charge is on an spcarbon. Since spcarbons are

more electronegative than the carbons that carry the positive charge of alkyl carboca-

tions,spcarbons are more resistant to becoming positively charged. Second, we have seen

that carbons form stronger bonds than do carbons (Section 1.14). As a result, it

is harder to break the carbon–halogen bond when the halogen is bonded to an carbon.

PROBLEM-SOLVING STRATEGY

Which alkyl halide would you expect to be more reactive in an solvolysis reaction?

When asked to determine the relative reactivities of two compounds, we need to compare

the values of their rate-determining steps. The faster reacting compound will be the

one with the smallest difference between its free energy and the free energy of its rate-

determining transition state; that is, the faster reacting compound will have the smaller

value. Both alkyl halides have approximately the same stability, so the difference in

their reaction rates will be due to the difference in stability of the transition states of their

rate-determining steps. The rate-determining step is carbocation formation, so the com-

pound that forms the more stable carbocation will be the one that has the faster rate of

solvolysis. The compound on the left forms the more stable carbocation; it has three reso-

nance contributors, whereas the other carbocation has only two resonance contributors.

Now continue on to Problems 21–23.

PROBLEM 21

Which alkyl halide would you expect to be more reactive in an solvolysis reaction?

CC

CH 2 CH 3

H

or

CH 3 CHCH 2

H

Br

CC

CH 3

CH 3 CH 2 CH H

H

Br

SN 1

¢G‡

¢G‡

CH 3 CH CH CH 2 Br CH 2 CH 2 Br

OCH 3

O or C

SN 1

vinylic cation

too unstable to be formed

aryl cation

too unstable to be formed

RCH

Br Br−

CH RCH CH + Cl−

+

+

+

sp^2 hybridized sp hybridized

sp^2 hybridized sp hybridized

Cl

sp^2

sp^2 sp^3

sp^2

SN 1

CH 3 O CH CH CH 2 CH 3 O CH CH CH 2

++

CH 2 CCH 2

OCH 3

+

CH 2 CCH 2

OCH 3

+

CH 3 O CH CH CH 2

+ +