Organic Chemistry

conversion of the alcohol to the ether via the alkyl tosylate involves only one

reaction: attack of on the alkyl tosylate. Therefore, the final product and the

starting material have opposite configurations.

PROBLEM 5

Show how 1-butanol can be converted into the following compounds:

a. d.

b. e.

c. f.

12.5 Elimination Reactions of Alcohols: Dehydration

An alcohol can undergo an elimination reaction, forming an alkene by losing an OH

from one carbon and an H from an adjacent carbon. Overall, this amounts to the elim-

ination of a molecule of water. Loss of water from a molecule is called dehydration.

Dehydration of an alcohol requires an acid catalyst and heat. Sulfuric acid

and phosphoric acid are the most commonly used acid catalysts.

An acid always reacts with an organic molecule in the same way: It protonates the

most basic (electron-rich) atom in the molecule. Thus, in the first step of the dehydra-

tion reaction, the acid protonates the oxygen atom of the alcohol. As we saw earlier,

protonation converts the very poor leaving group into a good leaving group

In the next step, water departs, leaving behind a carbocation. A base in the

reaction mixture (water is the base in highest concentration) removes a proton from a

-carbon (a carbon adjacent to the positively charged carbon), forming an alkene and

regenerating the acid catalyst. Notice that the dehydration reaction is an E1 reaction of

a protonated alcohol.

b

(H 2 O).

(-OH)

OH

CH 3 CH 2 CHCH 3 CH 3 CH CHCH 3 H 2 O

∆

+

H 2 SO 4

(H 3 PO 4 )

(H 2 SO 4 )

CH 3 CH 2 CH 2 CH 2 OCH 3 CH 3 CH 2 CH 2 CH 2 C‚N

CH 3 CH 2 CH 2 CH 2 SH

O

CH 3 CH 2 CH 2 CH 2 OCCH 2 CH 3

CH 3 CH 2 CH 2 CH 2 Br CH 3 CH 2 CH 2 CH 2 NHCH 2 CH 3

CH 3 O-

SN 2

Section 12.5 Elimination Reactions of Alcohols: Dehydration 445

An acid protonates the most basic atom

in a molecule.

OH

CH 3 CHCH 3

OH

H

HOSO 3 H CH 3 CHCH 3 HCH 2 CHCH 3 CH 2 CHCH 3

−OSO

3 H

+

+

+

H 2 OH 3 O+

+

mechanism of dehydration (E1)

+ +

formation of

a carbocation

protonation of the

most basic atom

a base removes a

proton from a β-carbon

H

R OH

CH 3

H

Br R

CH 3

H

R OPBr 3

CH 3

H

R OCH 3

CH 3

PBr 3

pyridine

Br−

H

R OH

CH 3

H

CH 3 O R

CH 3

H

R OTs

CH 3

TsCl

pyridine

CH 3 O−

CH 3 O−

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