Organic Chemistry

Section 18.3 How Aldehydes and Ketones React 737

Aldehydes and ketones undergo

nucleophilic addition reactions with

hydride ion and with carbon

nucleophiles.

Consequently, aldehydes and ketones are not as reactive as carbonyl compounds in
which is a very weak base (acyl halides and acid anhydrides), but are more reactive
than carbonyl compounds in which is a relatively strong base (carboxylic acids, es-
ters, and amides). A molecular orbital explanation of why resonance electron donation
decreases the reactivity of the carbonyl group is given in Section 17.15.

18.3 How Aldehydes and Ketones React

In Section 17.5, we saw that the carbonyl group of a carboxylic acid or a carboxylic
acid derivative is attached to a group that can be replaced by another group. These
compounds therefore react with nucleophiles to form substitution products.

In contrast, the carbonyl group of an aldehyde or a ketone is attached to a group that is
too strong a base ( or ) to be eliminated under normal conditions, so it cannot be
replaced by another group. Consequently, aldehydes and ketones react with nucleo-
philes to form addition products, not substitution products. Thus, aldehydes and
ketones undergo nucleophilic additionreactions, whereas carboxylic acid derivatives
undergo nucleophilic acyl substitutionreactions.

When a nucleophile adds to a carbonyl group, the hybridization of the carbonyl
carbon changes from in the carbonyl compound to in the addition product. In
Section 17.5, we saw that a compound that has an carbon bonded to an oxygen
atom generally will be unstable if the carbon is also bonded to another elec-
tronegative atom. Therefore, if the nucleophile that adds to the aldehyde or ketone is
one in which Z is notelectronegative (Z is an H or a C nucleophile), the tetrahedral
addition product will be stable. It will be an alkoxide ion that can be protonated either
by the solvent or by added acid. ( is any species that provides a proton; is any
species that removes a proton.)

If, on the other hand, the nucleophile that adds to the aldehyde or ketone is one in
which Z iselectronegative (Z is an O or an N nucleophile), the tetrahedral addition

C

O

CR'

O−

+

R

R

R'

Z

sp^2

sp^3

product of nucleophilic

addition is stable;

Z = C or H

CR'

OH

R

Z

HB+

B

Z−

HB+ ≠B

sp^3

sp^3

sp^2 sp^3

C

O

CR'

OH

+

R

R

R'

Z

HZ

product of

nucleophilic

addition

H- R-

Z

C

O

R CY

O−

+

RY

C

O

R

Z

Z− + Y−

product of

nucleophilic

substitution

Y-

Y-

Carboxylic acid derivatives undergo

nucleophilic substitution reactions

with nucleophiles.