Organic Chemistry

Because the first step is the rate-determining step, the rate of an E1 reaction de-

pends on both the ease with which the carbocation is formed andhow readily the leav-

ing group leaves. The more stable the carbocation, the easier it is formed because more

stable carbocations have more stable transition states leading to their formation.

Therefore, the relative reactivities of a series of alkyl halides with the same leaving

group parallel the relative stabilities of the carbocations. A tertiary benzylic halide is

the most reactive alkyl halide because a tertiary benzylic cation—the most stable

carbocation—is the easiest to form (Sections 7.7 and 10.8).

Notice that a tertiary alkyl halide and a weak base were chosen to illustrate the E1 re-

action in this section, whereas a tertiary alkyl halide and a strong base were used to il-

lustrate the E2 reaction in Section 11.1. Tertiary alkyl halides are more reactive than

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

Free energy

Progress of the reaction

CH 3

Cl

CH 3 CH 2 CCH 3

CH 3

+

Cl−

CH 3 CH 2 CCH 3

CH 3

CH 3 CH 2 CCH 2 + H 3 Ο+

CH 3

CH 3 CH CCH 3 + H 3 Ο+

+ H 2 Ο

+ H 2 Ο

+ Cl−

+ Cl−

Figure 11.2

Reaction coordinate diagram for the E1 reaction of 2-chloro-2-methylbutane. The major

product is the more substituted alkene because its greater stability causes the transition

state leading to its formation to be more stable.

When two elimination products can be formed in an E1 reaction, the major product

is generally the more substituted alkene.

The more substituted alkene is the more stable of the two alkenes formed in the preced-

ing reaction, and therefore, it has the more stable transition state leading to its formation

(Figure 11.2). As a result, the more substituted alkene is formed more rapidly, so it is

the major product. To obtain the more substituted alkene, the hydrogen is removed from

the -carbon bonded to the fewest hydrogens, in accordance with Zaitsevb ’s rule.

3-D Molecule:

2-Chloro-2-methyl-butane

CH 3 CH 2 CCH 3 + CH 3 CH CCH 3 + CH 3 CH 2 C CH 2 + H 3 O+ + Cl−

Cl

CH 3 CH 3 CH 3

2-chloro-2-methylbutane

H 2 O

2-methyl-2-butene

major product

2-methyl-1-butene

minor product

3 ° benzylic 3 ° allylic

relative reactivities of alkyl halides in an E1 reaction = relative stabilities of carbocations

≈>≈ 2 ° benzylic ≈ 2 ° allylic ≈ 3 ° > 1 ° benzylic ≈ 1 ° allylic 2 ° >> 1 ° vinyl

most

stable

least

stable

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