Organic Chemistry

To synthesize 2-methyl-2-butene from 2-bromo-2-methylbutane, you would use

conditions (a high concentration of in an aprotic polar solvent) because

a tertiary alkyl halide gives onlythe elimination product under those conditions. If

conditions were used (a low concentration of in water), both elimination

and substitution products would be obtained.

PROBLEM 23

Identify the three products that are formed when 2-bromo-2-methylpropane is dissolved in

a mixture of 80% ethanol and 20% water.

PROBLEM 24

a. What products (including stereoisomers if applicable) would be formed from the reac-

tion of 3-bromo-2-methylpentane with under conditions and under

conditions?

b. Answer the same question for 3-bromo-3-methylpentane.

11.10 Consecutive E2 Elimination Reactions

Alkyl dihalides can undergo two consecutive dehydrohalogenations, giving products

that contain two double bonds. In the following example, Zaitsev’s rule predicts the

most stable product of the first dehydrohalogenation, but not the most stable product

of the second. The reason Zaitsev’s rule fails in the second reaction is that a conjugat-

ed diene is more stable than an isolated diene.

If the two halogens are on the same carbon (geminal dihalides) or on adjacent car-

bons (vicinal dihalides), the two consecutive E2 dehydrohalogenations can result in

the formation of a triple bond. This is how alkynes are commonly synthesized.

−NH 2 −NH 2

a geminal dibromide a vinylic bromide

RCH 2 CR

Br

Br

Br

RCH CR ++2 NH 3 2 Br−

an alkyne

RC CR

−NH 2 −NH 2

a vicinal dichloride a vinylic chloride

RCHCHR

ClCl Cl

RCH CR ++2 NH 3 2 Br−

an alkyne

RC CR

SN 1 >E1

HO- SN 2 >E2

SN 1 >E1 HO-

SN 2 >E2 HO-

Section 11.10 Consecutive E2 Elimination Reactions 427

CH 3

2-bromo-2-methylbutane

2-methyl-2-butene

2-methyl-2-butanol

SN2/E2

conditions

SN1/E1

conditions

CH 3 CCH 2 CH 3 HO−

CH 3

Br

+

CH 3 C CHCH 3 + H 2 O + Br−

CH 3

CH 3 C

CH 3

OH

CHCH 3 + CH 3 CCH 2 CH 3 + H 2 O + Br−

HO− HO−

3,5-dichloro-2,6-dimethylheptane 5-chloro-2,6-dimethyl-2-heptene

CH 3 CHCHCH 2 CHCHCH 3

CH 3

Cl

CH 3

Cl

CH 3 CH 3

Cl

CH 3 C CHCH 2 CHCHCH 3

++2 H 2 O 2 Cl−

2,6-dimethyl-2,4-heptadiene

CH 3 CH 3

CH 3 C CHCH CHCHCH 3

BRUI11-400_436r3 26-03-2003 10:20 AM Page 427