Organic Chemistry

770 CHAPTER 18 Carbonyl Compounds II

Nucleophiles that form stable addition products—that is, nucleophiles that are

strong bases, thereby making direct addition irreversible—can form eitherdirect addi-

tion products or conjugate addition products. Nucleophiles in this group include hy-

dride ion and carbanions. The reaction that prevails is the one that is faster, so the

product that is formed will depend on the reactivity of the carbonyl group. Compounds

with reactive carbonyl groups form primarily direct addition products because for

those compounds, direct addition is faster, whereas compounds with less reactive car-

bonyl groups form primarily conjugate addition products because for thosecom-

pounds, conjugate addition is faster. For example, aldehydes have more reactive

carbonyl groups than do ketones, so sodium borohydride forms primarily direct addi-

tion products with aldehydes. Compared with aldehydes, ketones form less of the di-

rect addition product and more of the conjugate addition product.

Notice that a saturated alcohol is the final product of conjugate addition in the preced-

ing reaction because the carbonyl group of the ketone will react with a second equiva-

lent of hydride ion.

If direct addition is the desired outcome of hydride addition, it can be achieved by car-

rying out the reaction in the presence of cerium chloride, a Lewis acid that activates the

carbonyl group toward nucleophilic attack by complexing with the carbonyl oxygen.

O

H OH

O OH O OH

product of direct addition

97%

1. NaBH 4


2. EtOH


3. NaBH 4


4. EtOH


5. NaBH 4


6. EtOH

product of

direct addition

51%

product of

conjugate addition

49%

+

O

CH 3

C

O

CH 3

C

CH 3 CH CH 2

CH 3

CH 3

N

CH 3

SCH 3

OO

+ CH 3 NH

CH 3 CH

+ CH 3 SH

CH

O

CH 3

C

CH

O

CH 3

C

BrCH 2 CH 2

CN

OO

CH 2 + HBr

+ −CNHCl

Tutorial:

1,2- vs 1,4-Additions to

, -unsaturated carbonyl

compounds

ab