Organic Chemistry

246 CHAPTER 6 Reactions of Alkynes • Introduction to Multistep Synthesis

PROBLEM 9

From what you know about the stereochemistry of alkene addition reactions, predict the

configurations of the products that would be obtained from the reaction of 2-butyne with

the following:

a. one equivalent of in b. one equivalent of

6.6 Addition of Water

In Section 4.5, we saw that alkenes undergo the acid-catalyzed addition of water. The

product of the reaction is an alcohol.

Alkynes also undergo the acid-catalyzed addition of water. The product of the reaction

is an enol. An enolhas a carbon–carbon double bond and an OH group bonded to one

of the carbons. (The ending “ene”signifies the double bond, and “ol”the OH.

When the two endings are joined, the final e of “ene”is dropped to avoid two consec-

utive vowels, but it is pronounced as if the ewere there,“ene-ol.”)

The enol immediately rearranges to a ketone. A carbon doubly bonded to an oxygen is

called a carbonyl (“car-bo-nil”)group. A ketoneis a compound that has two alkyl

groups bonded to a carbonyl group. An aldehydeis a compound that has at least one

hydrogen bonded to a carbonyl group.

A ketone and an enol differ only in the location of a double bond and a hydrogen.

Such isomers are called tautomers(“taw-toe-mers”). The ketone and enol are called

keto–enol tautomers. Interconversion of the tautomers is called tautomerization. We

will examine the mechanism of this reaction in Chapter 19. For now, the important

thing to remember is that the keto and enol tautomers come to equilibrium in solution,

and the keto tautomer, because it is usually much more stable than the enol tautomer,

predominates at equilibrium.

Addition of water to an internal alkyne that has the same group attached to each

of the spcarbons forms a single ketone as a product. But if the two groups are not

O

RCH 2 C

keto tautomer

tautomerization

R

OH

RCH C

enol tautomer

R

O

C

a carbonyl group

O

C

RR

a ketone

O

C

RH

an aldehyde

CH 3 C CCH 3 + H 2 O

H 2 SO 4

OH O

CH 3 C CHCH 3 CH 3 CCH 2 CH 3

an enol a ketone

sp^2

CH 3 CH 2 CH CH 2 + H 2 O CH 3 CH 2 CH CH 2

H 2 SO 4

OH H

2-butanol

1-butene

Br 2 CH 2 Cl 2 HBr+peroxide

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