Organic Chemistry

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

product obtained from the reaction of 2-iodobutane and tert-butoxide ion is 2-butene.

In other words, it takes a lot of hindrance for the less stable product to be the major

product.

PROBLEM 1

Which of the alkyl halides is more reactive in an E2 reaction?

PROBLEM 2

Draw a reaction coordinate diagram for the E2 reaction of 2-bromo-2,3-dimethylbutane

with sodium tert-butoxide.

Although the major product of an E2 dehydrohalogenation of alkyl chlorides, alkyl

bromides, and alkyl iodides is normally the more substituted alkene, the major product

of the E2 dehydrohalogenation of alkyl fluorides is the less substituted alkene

(Table 11.2).

CH 3 CH 2 CH 2 Br or

or

CH 3 CH 2 CHCH 3

Br

Br

CH 3 CHCH 2 CHCH 3 or CH 3 CH 2 CH 2 CCH 3

Br

CH 3 CH 3

CH 3

CH 3 CCH 2 Cl or CH 3 CCH 2 CH 2 Cl

CH 3

CH 3 CH 3

a.

b.

c.

d.

Cl Br

CH 3 CHCH 2 CH 3 + CH 3 CO− CH 3 CH CHCH 3 ++CH 2 CHCH 2 CH 3 + Br−

CH 3

CH 3

CH 3

I CH 3

2-iodobutane tert-butoxide ion 79%

2-butene

21%

1-butene

CH 3 CO−

More substituted product Less substituted product



CH 3 CHCH 2 CH 2 CH 2 CH 3 + CH 3 O− CH 3 CH CHCH 2 CH 2 CH 3 + CH 2 CHCH 2 CH 2 CH 2 CH 3

Leaving group Conjugate acid pKa

=IHI− 10 81% 19%

=Br HBr − 9 72% 28%

=Cl HCl − 7 67% 33%

=F HF 3.2 30% 70%

X

X

X

X

X

2-hexene

(mixture of E and Z)

1-hexene

Table 11.2 Products Obtained from the E2 Reaction of CH 3 O–and 2-Halohexanes

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