Organic Chemistry

738 CHAPTER 18 Carbonyl Compounds II

product will notbe stable. Water will be eliminated from the addition product. This is

called a nucleophilic addition–elimination reaction. We will see that the fate of the

dehydrated product depends on the identity of Z.

18.4 Reactions of Carbonyl Compounds

with Carbon Nucleophiles

Few reactions in organic chemistry result in the formation of new bonds. Con-

sequently, those reactions that do are very important to synthetic organic chemists

when they need to synthesize larger organic molecules from smaller molecules. The

addition of a carbon nucleophile to a carbonyl compound is an example of a reaction

that forms a new bond and therefore forms a product with more carbon atoms

than the starting material.

Reaction with Grignard Reagents

Addition of a Grignard reagent to a carbonyl compound is a versatile reaction that

leads to the formation of a new bond. The reaction can produce compounds

with a variety of structures because both the structure of the carbonyl compound and

the structure of the Grignard reagent can be varied. In Section 12.11, we saw that a

Grignard reagent can be prepared by adding an alkyl halide to magnesium shavings in

diethyl ether. We also saw that a Grignard reagent reacts as if it were a carbanion.

Attack of a Grignard reagent on a carbonyl carbon forms an alkoxide ion that is

complexed with magnesium ion. Addition of water or dilute acid breaks up the com-

plex. When a Grignard reagent reacts with formaldehyde, the addition product is a pri-

mary alcohol.

When a Grignard reagent reacts with an aldehyde other than formaldehyde, the addi-

tion product is a secondary alcohol.

_ +

Mg

CH 3 CH 2 Br Et 2 O CH 3 CH 2 MgBr

CH 3 CH 2 MgBr reacts as if it were CH 3 CH 2 MgBr

C¬C

C¬C

C¬C

Aldehydes and ketones undergo
nucleophilic addition–elimination
reactions with oxygen and nitrogen
nucleophiles.

RC

OH

R'

Z

+

Z

RC

OH

R'

+Z

R

C

R'

H 2 O

B

HB+

+

H

product of nucleophilic

addition is not stable;

Z = O or N

product of nucleophilic

addition–elimination

+ CH 3 CH 2 CH 2 CH 2 MgBr CH 3 CH 2 CH 2 CH 2 CH 2 O−MgBr H^3 O CH 3 CH 2 CH 2 CH 2 CH 2 OH

+ +

H H

C

O

butylmagnesium

bromide

formaldehyde 1 – pentanol

a primary alcohol

an alkoxide ion

+ CH 3 CH 2 CH 2 MgBr

OMgBr

H 3 O+

CH 3 CH 2 CHCH 2 CH 2 CH 3

OH

CH 3 CH 2 CHCH 2 CH 2 CH 3

− +

propanal propylmagnesium

bromide

3-hexanol

a secondary alcohol

CH 3 CH 2 H

C

O