Organic Chemistry

identical, two ketones are formed because the initial addition of the proton can occur

to either of the spcarbons.

Terminal alkynes are less reactive than internal alkynes toward the addition of

water. Terminal alkynes will add water if mercuric ion is added to the acidic

mixture. The mercuric ion acts as a catalyst to increase the rate of the addition

reaction.

The first step in the mercuric-ion-catalyzed hydration of an alkyne is formation of a

cyclic mercurinium ion. (Two of the electrons in mercury’s filled 5datomic orbital are

shown.) This should remind you of the cyclic bromonium and mercurinium ions

formed as intermediates in electrophilic addition reactions of alkenes (Sections 4.7

and 4.8). In the second step of the reaction, water attacks the most substituted carbon

of the cyclic intermediate (Section 4.8). Oxygen loses a proton to form a mercuric

enol, which immediately rearranges to a mercuric ketone. Loss of the mercuric ion

forms an enol, which rearranges to a ketone. Notice that the overall addition of water

follows both the general rule for electrophilic addition reactions and Markovnikov’s

rule: The electrophile ( in the case of Markovnikov’s rule) adds to the spcarbon

bonded to the greater number of hydrogens.

PROBLEM 10

What ketones would be formed from the acid-catalyzed hydration of 3-heptyne?

PROBLEM 11

Which alkyne would be the best reagent to use for the synthesis of each of the following

ketones?

a. b. c.

O

CH 3 C

O

CH 3 CH 2 CCH 2 CH 2 CH 3

O

CH 3 CCH 3

H+

CH 3 CH 2 CCH+ H 2 O

H 2 SO 4

HgSO 4

OH O

CH 3 CH 2 CCH 2 CH 3 CH 2 CCH 3

an enol a ketone

(Hg^2 +)

O

CH 3 CH 2 C CCH 2 CH 3 H 2 OCH 3 CH 2 CCH 2 CH 2 CH 3

H 2 SO 4

+

H 2 SO 4

O O

CH 3 C CCH 2 CH 3 + H 2 OCH 3 CCH 2 CH 2 CH 3 + CH 3 CH 2 CCH 2 CH 3

Section 6.6 Addition of Water 247

Tutorial:

Mercuric-ion-catalyzed

hydration of an alkyne

+

H 2 O + Hg^2 +

Hg+

H 3 O

+

H +

2 O

Hg^2 +

CH 3 CCH H

2 O

Hg+

OH

CH 3 CCH

H

Hg+

OH

CH 3 CCH

OH

CH 3 CCH 2

O

O+

H

CH 3 CCH 2

HH

O

CH 3 CCH 3

mechanism for the mercuric-ion-catalyzed hydration of an alkyne

a mercuric enol

a mercuric ketone

Hg^2 +

CH 3 CCH

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