Organic Chemistry

In Section 4.5, we saw that an alkene is hydrated (adds water) in the presence of an

acid catalyst, thereby forming an alcohol. The hydration of an alkene is the reverse of

the acid-catalyzed dehydration of an alcohol.

To prevent the alkene formed in the dehydration reaction from adding water and

reforming the alcohol, the alkene can be removed by distillation as it is formed,

because it has a much lower boiling point than the alcohol. Removing a product

displaces the reaction to the right. (See Le Châtelier’s principle, Section 10.4.)

PROBLEM 6

Explain why the acid-catalyzed dehydration of an alcohol is a reversible reaction, whereas

the base-promoted dehydrohalogenation of an alkyl halide is an irreversible reaction.

Because the rate-determining step in the dehydration of a secondary or a tertiary

alcohol is formation of a carbocation intermediate, the rate of dehydration parallels the

ease with which the carbocation is formed. Tertiary alcohols are the easiest to dehydrate

because tertiary carbocations are more stable and therefore are easier to form than

secondary and primary carbocations (Section 4.2). In order to undergo dehydration,

tertiary alcohols must be heated to about 50°C in 5% secondary alcohols must

be heated to about 100°C in 75% and primary alcohols can be dehydrated only

under extreme conditions (170°C in 95% ) and by a different mechanism

because primary carbocations are too unstable to be formed (Section 10.5).

H 2 SO 4

H 2 SO 4 ,

H 2 SO 4 ,

dehydration

hydration

RCH 2 CHR RCH CHR H 2 O

OH

+ H+ + + H+

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

Carbocation stability: 3 °> 2 °> 1 °

When more than one elimination product can be formed, the major product is the

more substituted alkene—the one obtained by removing a proton from the -carbon

that is bonded to the fewest hydrogens. (Recall Zaitsev’s rule, Section 11.2.) The more

substituted alkene is the major product because it is the more stable alkene, so it has

the more stable transition state leading to its formation (Figure 12.1).

H 3 PO 4

CH 3 CCH 2 CH 3

CH 3

CH 3 C CHCH 3

CH 3

H 3 C OH CH 3 CH 2

CH 2 CCH 2 CH 3

CH 3

OH

+ + H 2 O

+ + H 2 O

∆

H 2 SO 4

∆

84%

93% 7%

16%

Movie: b

Dehydration

Free energy

Progress of the reaction

CH 3 CCH 2 R

CH 3

CH 3 CCH 2 R

CH 3

CH 2 CCH 2 R + H 3 O+

CH 3

CH 3 C CHR + H 3 O+

+OH CH 3

+^ H 2 O

+

H

Figure 12.1N

The reaction coordinate diagram

for the dehydration of a

protonated alcohol. The major

product is the more substituted

alkene because the transition state

leading to its formation is more

stable, allowing it to be formed

more rapidly.

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