Organic Chemistry

Key Terms 433

the conditions favor or reactions. When

reactions are favored, primary alkyl halides form

primarily substitution products unless the nucleophile/base

is sterically hindered, in which case elimination products

predominate. Secondary alkyl halides form both substitution

and elimination products; the stronger and bulkier the base,

the greater is the percentage of the elimination product. Ter-

tiary alkyl halides form only elimination products. When

conditions are favored, secondary and tertiary alkyl

halides form both substitution and elimination products;

primary alkyl halides do not undergo SN 1 >E1reactions.

SN 1 >E1

SN 2 >E2

SN 2 >E2 SN 1 >E1 If the two halogens are on the same or adjacent carbons,

two consecutive E2 dehydrohalogenations can result in the

formation of a triple bond. The Williamson ether synthesis

involves the reaction of an alkyl halide with an alkoxide ion.

If the two functional groups of a bifunctional molecule

can react with each other, both intermolecular and

intramolecular reactionscan occur. The reaction that is

more likely to occur depends on the concentration of the

bifunctional molecule and the size of the ring that will be

formed in the intramolecular reaction.

Summary of Reactions

1. E2 reaction: a one-step mechanism

Relative reactivities of alkyl halides:

Anti elimination; both Eand Zstereoisomers are formed. The isomer with the bulkiest groups on opposite sides of the double bond

will be formed in greater yield. If the -carbon from which the hydrogen is removed is bonded to only one hydrogen, only one elim-

ination product is formed. Its configuration depends on the configuration of the reactant.

2. E1 reaction: a two-step mechanism with a carbocation intermediate

Relative reactivities of alkyl halides:

Anti and syn elimination; both Eand Zstereoisomers are formed. The isomer with the bulkiest groups on opposite sides of the dou-

ble bond will be formed in greater yield.

Competing and E2 Reactions

Primary alkyl halides: primarily substitution

Secondary alkyl halides: substitution and elimination

Tertiary alkyl halides: only elimination

Competing and E1 Reactions

Primary alkyl halides: cannot undergo or E1 reactions

Secondary alkyl halides: substitution and elimination

Tertiary alkyl halides: substitution and elimination

SN 1

SN 1

SN 2

3 ° 72 ° 71 °

C

H

CC

H

X C+ CC

B

BH

+ X−

+

+

b

3 ° 72 ° 71 °

C

H

B− + C X CC+ BH + X−

Key Terms

anti elimination (p. 413)

anti-periplanar (p. 413)

bifunctional molecule (p. 428)

dehydrohalogenation (p. 402)

deuterium kinetic isotope effect (p. 421)

elimination reaction (p. 400)

reaction (p. 402)

E1 reaction (p. 408)

E2 reaction (p. 401)

1,2-elimination reaction (p. 402)

intermolecular reaction (p. 428)

intramolecular reaction (p. 428)

b-elimination syn elimination (p. 413)

syn-periplanar (p. 413)

target molecule (p. 429)

Williamson ether synthesis (p. 425)

Zaitsev’s rule (p. 404)

BRUI11-400_436r3 26-03-2003 10:20 AM Page 433