Organic Chemistry

794 CHAPTER 19 Carbonyl Compounds III

When an reaction takes place under basic conditions, a base removes

a proton from the and the nucleophilic enolate ion then reacts with an elec-

trophile. Enolate ions are much better nucleophiles than enols because they are nega-

tively charged.

The resonance contributors of the enolate ion show that it has two electron-rich

sites: the and the oxygen. The enolate ion is an example of an ambident nu-

cleophile(ambiis Latin for “both”; dentis Latin for “teeth”). An ambident nucleo-

phileis a nucleophile with two nucleophilic sites (“two teeth”).

Which nucleophilic site (C or O) reacts with the electrophile depends on the elec-

trophile and on the reaction conditions. Protonation occurs preferentially on oxygen

because of the greater concentration of negative charge on the more electronegative

oxygen atom. However, when the electrophile is something other than a proton,

carbon is more likely to be the nucleophile because carbon is a better nucleophile

than oxygen.

Notice the similarity between keto–enol interconversion and Actu-

ally, keto–enol interconversion is an reaction in which hydrogen

serves as both the electrophile that is removed from the and the elec-

trophile that is added to the (when the enol or enolate reverts back to the

keto tautomer).

As various reactions are discussed in this chapter, notice that they

all follow basically the same mechanism: A base removes a proton from an

and the resulting enol or enolate reacts with an electrophile. The reactions differ

only in the nature of the base and the electrophile—and in whether the reactions are

carried out under acidic or basic conditions.

a-carbon

a-substitution

a-carbon

a-carbon

a-substitution

a-substitution.

RCH C

O−

RC

O

R

resonance contributors of an enolate ion

electron-rich oxygen

electron-rich -carbon

RCH

−

a-carbon

a-carbon

a-substitution

3-D Molecule:

Enolate of acetone

RCH C

OH

R RCH C

+OH

RCH C R

+OH

RCH R

H

C

O

R RCH

E

E

C

O

RH 3 O

HO

H

H

HEE+

H 2 O

-substituted product

= Electrophile

acid-catalyzed α-substitution reaction

enol

H 2 OH+ 3 O+

+

+

RCH C

O−

RCH CR R

H

C

OO

R RCH

E

C

O

R

E+

enolate ion – substituted product

HO−

+ H 2 O

RCH

−

base-catalyzed α-substitution reaction