Chemistry Reference
In-Depth Information
9
Aldehydes and Ketones
Chapter Summary
The
carbonyl
group
,
C=O
, is present in both
aldehydes
(RCH=O) and
ketones
(R
2
C=O).
The IUPAC ending for naming aldehydes is -
al
, and numbering begins with the carbonyl
carbon. The ending for the names of ketones is
-one
, and the longest chain is numbered as
usual. Common names are also widely used. Nomenclature is outlined in Sec. 9.1.
Formaldehyde
,
acetaldehyde
, and
acetone
are important commercial chemicals,
synthesized by special methods. In the laboratory, aldehydes and ketones are most
commonly prepared by oxidizing alcohols, but they can also be prepared by hydrating
alkynes and by Friedel-Crafts acylation of arenes. Aldehydes and ketones occur widely in
nature (see Figure 9.1).
The carbonyl group is planar, with the
sp
2
carbon trigonal planar. The C=O bond is
polarized, with C partially positive and O partially negative. Many carbonyl reactions are
initiated by nucleophilic addition to the partially positive carbon and completed by addition of
a proton to the oxygen.
With acid catalysis, alcohols add to the carbonyl group of aldehydes to give
hemiacetals
[RCH(OH)OR']. Further reaction with excess alcohol gives
acetals
[RCH(OR')
2
]. Ketones react similarly. These reactions are reversible; that is, acetals can be
readily hydrolyzed by aqueous acid to their alcohol and carbonyl components. Water adds
similarly to the carbonyl group of certain aldehydes (for example, formaldehyde and chloral)
to give hydrates. Hydrogen cyanide adds to carbonyl compounds as a carbon nucleophile to
give
cyanohydrins
[R
2
C(OH)CN].
Grignard reagents add to carbonyl compounds. The products, after hydrolysis, are
alcohols whose structures depend on that of the starting carbonyl compound. Formaldehyde
gives
primary
alcohols, other aldehydes give
secondary
alcohols, and ketones give
tertiary
alcohols.
Nitrogen nucleophiles add to the carbonyl group. Often, addition is followed by
elimination of water to give a product with a R
2
C=NR group in place of the R
2
C=O group.
For example, primary amines (R'NH
2
) give
imines
(R
2
C=NR');
hydroxylamine
(NH
2
OH)
gives
oximes
(R
2
C=NOH); and
hydrazine
(NH
2
NH
2
) gives
hydrazones
(R
2
C=NNH
2
).
Aldehydes and ketones are easily reduced to primary or secondary alcohols,
respectively. Useful reagents for this purpose are various metal hydrides such as
lithium
aluminum hydride
(LiAlH
4
) or
sodium borohydride
(NaBH
4
).
Aldehydes are more easily oxidized than ketones. The
Tollens' silver mirror test
is
positive for aldehydes and negative for ketones.
