Organic Chemistry

Section 17.1 Nomenclature 671

Class II carbonyl compounds are those in which the acyl group is attached to a
group that cannotbe readily replaced by another group. Aldehydes and ketones belong
to this class. The and alkyl or aryl or groups of aldehydes and ke-
tones cannot be replaced by a nucleophile.

In Chapter 10, we saw that the likelihood of a group’s being replaced by another group
depends on the relative basicities of the two groups:The weaker the basicity of a group,
the better its leaving ability. Recall from Section 10.3 that weak bases are good leaving
groups because weak bases do not share their electrons as well as strong bases do.
The values of the conjugate acids of the leaving groups of various carbonyl
compounds are listed in Table 17.1. Notice that the acyl groups of Class I carbonyl com-
pounds are attached to weaker bases than are the acyl groups of Class II carbonyl
compounds. (Remember that the lower the the stronger the acid and the weaker its
conjugate base.) The of an aldehyde and the alkyl or aryl or group of
a ketone are too basic to be replaced by another group.
This chapter discusses the reactions of Class I carbonyl compounds. We will see
that these compounds undergo substitution reactions because they have an acyl group
attached to a group that can be replaced by a nucleophile. The reactions of aldehydes
and ketones will be considered in Chapter 18. Because aldehydes and ketones have an
acyl group attached to a group that cannotbe replaced by a nucleophile, we can cor-
rectly predict that these compounds do notundergo substitution reactions.

17.1 Nomenclature

Carboxylic Acids
In systematic nomenclature, a carboxylic acidis named by replacing the terminal “e”
of the alkane name with “oic acid.”For example, the one-carbon alkane is methane,so
the one-carbon carboxylic acid is methanoic acid.

¬H (¬R ¬Ar)

pKa,

pKa

an aldehyde

H

a ketone

R

C

R

O

C

R′

O

cannot be replaced by a nucleophile

¬H (¬R ¬Ar)

an acyl chloride an acyl bromide

acyl halides

NH 2

amides

Cl NHR′ NR′ 2

C

R

O

Br

C

R

O

C

R

O

C

R

O

C

R

O

C

H OH

O

C

CH 3 CH 2 CH 2 CH 2 OH

O

C

CH 3 CH 2 CH 2 CH 2 CH 2 OH

O

C

OH

O

C

CH 3 OH

O

C

CH 3 CH 2 OH

O

C

CH 3 CH 2 CH 2 OH

O

C

OH

O

CH 2 CH

methanoic acid

formic acid

propenoic acid

acrylic acid

benzenecarboxylic acid

benzoic acid

ethanoic acid

acetic acid

propanoic acid

propionic acid

butanoic acid

butyric acid

pentanoic acid

valeric acid

hexanoic acid

caproic acid

systematic name:

common name:

compounds with groups that can be replaced by a nucleophile