Aldehydes and Ketones
O
C
Both contain the functional group
Aldehyde:
R
O
C
Ketone
H
R
O
C
R'
Aldehydes
Nomenclature. IUPAC: drop the 'e' from the name of the alkane and add 'al'. There are also common
names which are often derived from the name of the corresponding carboxylic acid.
H
O
C
O
C
H3C H
ethanal
acetaldehyde
H
methanal
formaldehyde
O
C
O
C
H3 CH2CH2C H
butanal
butyraldehyde
H3CH2C H
propanal
propionaldehyde
The aldehyde group has to be at the end of the chain, so it is not necessary to designate its position. The
group to which the aldehyde group is attached is named according to the appropriate rules, e.g.
ClH2CH2C
O
C
H
3-chloropropanal
3-chloropropionaldehyde
Ketones
Nomenclature. IUPAC: drop the 'e' from the name of the corresponding alkane and add 'one'. Indicate the
position of the carbonyl group if necessary. Common names are similar to those of ethers using the names
of the groups attached to the carbonyl.
H3C
O
C
CH3
H3CH2C
propanone
dimethyl ketone
acetone
O
C
CH3
butanone
methylethylketone
H3CH2C
O
C
CH2CH3
3-pentanone
diethylketone
H3C CH
O
C
CH CH3
CH3 CH3
2,4-dimethyl-3-pentanone
diisopropylketone
The chain is numbered so that the carbonyl group has the lowest possible number.
C6H5
O
1
O
3
2
O
C
C
O
3-phenylcylcopentanone
O
diphenylketone
1,4-cyclohexanedione
C6H5 CH3
phenylethanone
methylphenylketone
acetophenone
O
C
C6H5 CH2CH2CH3
1-phenyl-1-butanone
phenylpropylketone
Preparation of Aldehydes and Ketones
1. Ozonolysis of alkenes
1) O 3
C C
2. Hydration of alkynes
C
C
+
O C
OH
H 2SO4
H 2O
+
C O
2) Zn, H2O
CH 3COOH
CH
HgSO4
CH 2
C
O
C
3. Oxidation of alcohols
OH
C
K 2Cr2O 7
ketones from secondary alcohols
aldehydes from primary alcohols
CrO 3/pyridine
or pyridinium
chlorochromate PCC
4. Rosenmunde reduction of acyl halides to aldehydes
O
R C
O
H2, Pd-S
Cl or LiAlH(OC(CH 3)3)3
R C
H
The sulfur partially deactivates the palladium catalyst. Lithium tert-butylaluminum hydride is a less
powerful reducing agent than LiAlH4 .
Acyl chlorides can be prepared from acids
O
O
SOCl 2
R C
OH
COOH
e.g.
thionyl chloride
R C
O
Cl
O
C Cl
SOCl 2
C H
H 2/Pd-S
or LiAlH(OC(CH 3)3)3
C2H 5
C2H 5
C2H 5
5. Corey reaction to form ketones
O
R C Cl
C
+
O
OH
(R')2 CuLi
SOCl 2
-80oC
R
C
O
C
R'
O (C2H5)2 CuLi
Cl
-80oC
C
O
C2H5
6. Aromatic Ketones by Friedel Crafts acylation
O
O
AlCl3
Ar C
ArH +
Cl C R
R
O
e.g.
+
O
Cl C CH2CH3
C CH 2CH3
AlCl3
excellent yield
CH 3
CH3
1-(4-methylphenyl)-1-propanone
Instead of an acyl chloride, the acid anhydride can be used.
e.g.
+
H3CH2C
O
O
H3C C O C CH3
O
O
4
C
AlCl3
O
+
AlCl3
O
C CH3
H3CH2C
3 2 O
C C C OH
H2 H2
150oC
O
succinic anhydride
4-phenyl-4-oxo-butanoic acid
Reactions of Aldehydes and Ketones
Many reactions involve addition to the carbon-oxygen double bond. Since the oxygen is electron
withdrawing, the carbon carries a partial positive charge and we get nucleophilic addition to the carbon.
O
C
O
O
C
C
O
+ Nu
C
H3O+
Nu
OH
C
Nu
Nu is a nucleophile which can be either an anion or a neutral
compound with unshared electrons. This reaction is written as if
it were reversible which is often true.
e.g.
Grignard reaction
O
C
+
(which we have already discussed)
OMgX
δ− δ+
C
R-MgX
R
H3O+
OH
C
R
Reduction to Alcohols (an important reaction)
O
C
1) LiAlH4 -ether
2) H 2O
OH
C
or 1) NaBH 4, CH 3OH
H
+
2) H 3O
The mechanism involves the transfer of a hydride ion 'H -' from the metal hydride to the ketone.
O
C
OBH3 Na+
C
H
+ H B- H Na+
H
OH
C
H
H3O+
H
Of the two reducing agents, sodium borohydride is the milder reagent and is the one of preference for
aldehydes and ketones since it is specific for these two functional groups. Lithium aluminum hydride will
reduce many types of compounds very quickly.
Reduction to hydrocarbons (we saw this used to reduce ketones to alkyl groups in the Freidel Crafts
reaction)
a) Clemmensen reduction.
This reaction requires strong acid
zinc amalgam
H
O
Zn/Hg
+ H2 O + Cl C
C
HCl
H
b) Wolff Kishner reduction
This reaction requires strong base
O
C
H
C
NH2 NH2
KOH ~190o C
HOCH2CH2OH
+ N2
+ H2 O
H
Therefore, use the Clemmensen reduction when it is undesirable to have strong base in the reaction
and the Wolff Kishner when it is undesirable to have strong acid in the reaction.
Example of the use of these reactions to make a fused ring system:
O
O
O
AlCl3
+
C CH2 CH2 C OH
H3CH2C
H3CH2C
O
ethylbenzene
O
CH2 CH2 CH2 C Cl
H3CH2C
Zn/Hg, HCl
O
SOCl2
AlCl3
NH2 NH2, KOH
H3CH2C
O
CH 2 CH2 CH2 C OH
H3CH2C
Pt
CH2
O
C
HOCH2CH 2OH
H3CH2C
CH 2
o
190
C
C
H2
H3CH2C
2-ethylnaphthalene
Pt, H2
H3CH2C
2-ethyldecalin
Cyanhydrin formation
O
+ Na+CN -
C
OH
H2SO4
HCN which is formed in situ
from sodium cyanide and sulfuric
acid is added across the C-O
double bond
C
CN
cyanhydrin
The cyanhydrin group is readily oxidized to a carboxylic acid and so this reaction can be used to make a
hydroxy acid.
OH
O
OH
NaCN
H3O+
C
C
C
H2SO4
CH3
H
C
CH
3
CH
C
heat
H
3
H3C
3
3
COOH
very mild conditions
CN
for oxidation
2-hydroxy-2-methylpropanoic acid
O
C6H5
C
H
NaCN
H2SO4
benzaldehyde
C6H5
OH
C H
CN
H3O+
heat
mandelonitrile, found in
almonds, peach buds, peach
seeds
millipede defence substance
found in laetrile
OH
C6H5 C H
COOH
mandelic acid, bacteriacide
used in treating urinary infections
Bisulfite Addition
This is a reversible reaction in which a solid product is formed which can be used to purify aldehydes and
ketones.
OH
soluble in water, insoluble in organic
O
C
solvents
+
+ Na HSO3
C
- Na+
+
SO
H3O or OH
3
The bisulfite addition compound cna be used to separate a mixture of an aldehyde and an alcohol
CH3CH2CH 2CH 2CHO
+
CH 3CH2CH2CH2CH 2OH
NaHSO3/H2 O
extract with ether
OH
CH3CH2CH 2CH 2CH
SO3 -Na +
in aqueous layer
CH 3CH2CH2CH2CH 2OH
in ether layer
Addition of ammonia, amines and their derivatives
O
pH 3-4
+
C
NH3
+
C
NH
H2 O
imine
Mechanism
O
C
H N H
H
OH
C
OH
C
N
N
H
H
H3O
OH2 +
C
+
C
N
N
H
H
H
H
-H2O
H
H
Substituted ammonias, NH2-Z, can be added where Z is a large variety of groups.
O
+
C
C
NH2 -Z
+
H2O
H2O
imine
NZ
Examples
NH2 -Z
Product
C
N-H +
C
N-R
H2N-OH hydroxylamine
C
N-OH
H2N-NH2 hydrazine
C
N-NH2 + H 2O
H2N-NH-C6H5 phenylhydrazine
C
N-NH-C6H 5 + H2O
C
N-NH
H2N-H ammonia
H2N-R
primary amine
NO2
H2N-NH
O2N
H3C
C
O
CH2CH3
NH2 OH
+ H2 O
O2N
2,4-dintrophenylhydrazine
O
H2N-NH-C-NH2 semicarbazide
Specific examples:
+ H2O
C
H3C
C
HO N
O
N-NH-C-NH2
CH2CH3
alkyl or aryl amine (Schiff's base)
oxime
hydrazone
phenylhydrazone
NO2
2,4-dinitrophenylhydrazone
+ H2O
semicarbazone
H 3C
CH2CH3
C
N OH
Z-butanone oxime
E-butanone oxime
The geometric isomerism of oximes is due to the restricted rotation about the C=N bond and the geometry of
the groups around the nitrogen.