Organic Chemistry

448 CHAPTER 12 Reactions of Alcohols, Ethers, Epoxides, and Sulfur-Containing Compounds

While the dehydration of a tertiary or a secondary alcohol is an E1 reaction, the de-

hydration of a primary alcohol is an E2 reaction because of the difficulty encountered

in forming primary carbocations. Any base ( ) in the reaction mixture (ROH,

can remove the proton in the elimination reaction. An ether is also obtained as

the product of a competing reaction.

PROBLEM 9

Heating an alcohol with sulfuric acid is a good way to prepare a symmetrical ether such as

diethyl ether. It is not a good method for preparing an unsymmetrical ether such as ethyl

propyl ether.

a. Explain.

b. How would you synthesize ethyl propyl ether?

Although the dehydration of a primary alcohol is an E2 reaction and therefore does

not form a carbocation intermediate, the product obtained in most cases is identical to

the product that would be obtained if a carbocation had been formed in an E1 reaction

and then had rearranged. For example, we would expect 1-butene to be the product of

the E2 dehydration of 1-butanol. However, we find that the product is actually

2-butene, which would have been the product if an E1 reaction had occurred and the

initially formed primary carbocation intermediate had rearranged to a more stable

secondary carbocation. 2-Butene is the product of the reaction, not because an E1

reaction occurred, but because, after the E2 product (1-butene) is formed, a proton

from the acidic solution adds to the double bond—adding to the carbon bonded to

the greater number of hydrogens (in accordance with Markovnikov’s rule) to form a

carbocation. Loss of a proton from the carbocation—from the -carbon bonded to the

fewest hydrogens (in accordance with Zaitsev’s rule)—gives 2-butene, the final

product of the reaction.

The stereochemical outcome of the E1 dehydration of an alcohol is identical to the

stereochemical outcome of the E1 dehydrohalogenation of an alkyl halide. That is,

both the Eand Zisomers are obtained as products. More of the isomer with the bulky

b

sp^2

SN 2

HSO 4 - )

B≠ H 2 O,

CH 3 CH 2 OH H OSO 3 H

H

H

CH 2 CH 2 OH CH 2 CH 2

B

B

+ + H 2 O + HB+

−OSO

+ 3 H

+

HB+

mechanism of dehydration (E2) and competing substitution (SN2)

E2

elimination product

SN 2

substitution product

H

CH 3 CH 2 OH ++CH 3 CH 2 OH+ CH 3 CH 2 OCH 2 CH 3 CH 3 CH 2 OCH 2 CH 3

H

+

protonation of the

most basic atom

removal of a proton

from a β-carbon

back-side attack by

the nucleophile

Primary alcohols undergo dehydration

by an E2 pathway.

Secondary and tertiary alcohols undergo

dehydration by an E1 pathway.

H+

CH 3 CH 2 CH 2 CH 2 OH CH 3 CH 2 CHCH+ 3 + H+

1-butanol

CH 3 CH 2 CH CH 2

+ H 2 O

1-butene

CH 3 CH CHCH 3

2-butene

H 2 SO 4

∆

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