Organic Chemistry

430 CHAPTER 11 Elimination Reactions of Alkyl Halides • Competition Between Substitution and Elimination

Example 1.How could you prepare 1,3-cyclohexadiene from cyclohexane?

Since the only reaction an alkane can undergo is halogenation, deciding what the first

reaction should be is easy. An E2 reaction using a high concentration of a strong and

bulky base to encourage elimination over substitution will form cyclohexene. There-

fore,tert-butoxide ion is used as the base and tert-butyl alcohol is used as the solvent.

Bromination of cyclohexene will give an allylic bromide, which will form the desired

target molecule by undergoing another E2 reaction.

Example 2.Starting with methylcyclohexane, how could you prepare the following

vicinal trans-dihalide?

Again, since the starting material is an alkane, the first reaction must be a radical

substitution. Bromination leads to selective substitution of the tertiary hydrogen.

Under E2 conditions, tertiary alkyl halides undergo only elimination, so there will be

no competing substitution product in the next reaction. Because addition of in-

volves only anti addition, the target molecule (as well as its enantiomer) is obtained.

As you saw in Section 6.11, working backward can be a useful way to design a

synthesis—particularly when the starting material does not clearly indicate how to

proceed. Look at the target molecule and ask yourself how it could be prepared. Once

you have an answer, work backward to the next compound, asking yourself how it

could be prepared. Keep working backward one step at a time, until you get to a readily

available starting material. This technique is called retrosynthetic analysis.

Example 3.How could you prepare ethyl methyl ketone from 1-bromobutane?

At this point, you know only one way to synthesize a ketone—the addition of water

to an alkyne (Section 6.6). The alkyne can be prepared from two successive E2 re-

actions of a vicinal dihalide, which in turn can be synthesized from an alkene. The

desired alkene can be prepared from the given starting material by an elimination

reaction.

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

O

?

Br 2

?

CH 3 CH 3

Br

Br

?

Br

Br 2

Br

NBS

peroxide

high

concentration

tert-BuO−

tert-BuOH

high

concentration

tert-BuO−

h tert-BuOH

Br

CH 3 CH^3 CH 3

Br 2

CH 2 Cl 2

high

concentration

CH 3 CH 2 O−

CH 3 CH 2 OH

Br 2

h

CH 3

Br

Br

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