Carbonyl compounds
Introduction
• Aldehydes and ketones are known as carbonyl
compounds because they have the carbonyl
O
group,
C
• An aldehyde has one hydrogen atom attached
to the carbonyl carbon atom. In a ketone, the
carbonyl carbon atom is attached to two alkyl
or aryl groups.
O
R
C
Aldehyde
O
H
R
C
Ketone
R'
Nomenclature
1. Aldehydes are named by replacing the –e of the
corresponding alkane by –al. For example
O
O
C
H
H
C
H3C
O
H
ethanal
methanal
H
C
benzenecarbaldehyde
(benzaldehyde)
2. Ketones are named by replacing the –e of the corresponding
alkane with –one. For example
O
H3C
C
propanone
O
O
CH3
H3C
C
phenylethanone
H3C
C
CH2CH2CH3
pentan-2-one
Example
O
CH3CH2C
H
CH3
O
CH3CH2CH
C
H
2-methylbutanal
Propanal
O
O
CH3CH2C
CH3
butan-2one
CH3CH
C
CH3
O
CH3
3-methylbutan-2-one
Cyclohexanone
CH3CH2 C
O
pentan-3-one
CH2CH3
Exercise
• Name the following compound
CH3CH2CH(CH3)CHO
CH3COCH(CH3)CH2CH2CH3
CH3C(CH3)2CH2CH2COCH3
Physical properties
• Methanal is a gas at room temperature. The
other simple aliphatic aldehydes and ketones are
all colourless liquids
• Aldehydes and ketones have higher boiling points
than alkanes. However, molecules of aldehydes
and ketones cannot form hydrogen bonds with
each other. Therefore, they are more volatile and
have lower boiling points than alcohols and
carboxylic acids with similar relative molecular
mass
Compound
Example
Relative molecular
mass
Boiling point (°C)
Alkane
Butane
58
1
Aldehyde
Propanol
58
48
Ketone
Propanone
58
56
Alcohol
Propan-1-ol
60
97
Carboxylic acid
Ethanoic acid
60
118
• The lower aliphatic aldehydes and ketones are soluble in water because they
are able to form hydrogen bonds with water molecules
•Higher members of aliphatic carbonyl compounds with more than five carbon
atoms and the aromatic carbonyl compounds are insoluble in water. This is due
to the presence of the large hydrophobic hydrocarbon group.
R


H
 O
hydrogen
C
bond
Ketone R'

O
H
Preparation of aldehydes
1. Oxidation of primary alcohols
• By reacting primary alcohols with pyridinium
chlorochromate, PCC.
O
R
CH2OH
PCC in CH2Cl2
Room temperature
R
C
H
2. Dehydrogenation of primary alcohols
• Be prepared by the catalytic
dehydrogenation of the primary alcohol
vapour over hot copper. For example
O
Cu, 300 oC
H3C
CH2OH
H3C
C
H
+
H2
Preparation of aromatic aldehydes
Benzaldehyde
1. Oxidation of the methyl group by using
chromium dichloride dioxide.
O
CH3
CrO2Cl2
heat
+
C
benzaldehyde
H
H2O
2. Halogenation of the methyl group
Cl
CH3
+ 2Cl2
UV light
C
H
+
2 HCl
Cl
dichloromethylbenzene
• Hydrolysis of dichloromethylbenzene by
refluxing with solution of sodium hydroxide
Cl
C
O
H
+ 2 OH-
reflux
C
Cl
dichloromethylbenzene
Benzaldehyde
H
+ 2 Cl- + H2O
Preparation of ketones
1. Oxidation of secondary alcohols
• This is carried out by using a secondary
alcohol with an acidified solution of
potassium dichromate (VI) or potasium
manganate (VII).
H
H3C
C
OH
CH3
+ [O]
KMnO4 / K2Cr2O7
H3C
C
O
propanone
CH3
+ H2O
2. Dehydrogenation of secondary alcohols
• This is carried out by passing the secondary
alcohol vapour over a heated copper catalyst
H
Cu, 300 oC
H3C
C
OH
CH3
H3C
C
O
propanone
CH3
+ H2
3. Decarboxylation of calcium carboxylate
• When a solid calcium salt of a carboxylic acid
is heated strongly, a ketone is formed.
(CH3COO)2Ca
400 oC
H3C
C
O
CH3
+ CaCO3
Preparation of aromatic ketones
1. Aromatic ketones can be prepared by the Friedal
Craft acylation of benzene in the presence of
anhydrous aluminium chloride catalyst.
2. The reaction can be carried out in two ways:
a) With ethanoyl chloride at room temperature
AlCl3
H
+ H3C
C
Cl
O
benzene
Ethanoyl chloride
C
O
Phenylethanoane
CH3
+ HCl
b) With ethanoic anhydride at 50 oC
H3C
O
C
H
+
AlCl3
O
C
50 oC
C
benzene
H3C
CH3
O
O
ethanoic anhydride
Phenylethanoane
+
H3C
C
O
OH
Question
• Write equations to show the reactions
occuring when
(a) CH3OH
(b) CH3CH2CH2OH
(c)
CH3CH(OH)CH2CH3
Is passed over a heated copper catalyst at 300oC
Chemical properties
• Aldehydes and ketones undergo the following
types of reaction:
a)
b)
c)
d)
Nucleophilic addition
Condensation or addition-elimination reaction
Oxidation
Reduction
1. Nucleophilic addition
• The carbonyl group is highly polar. The
carbonyl carbon is electron-deficient. This
makes it susceptible to attack by electronrich nucleophiles, Nu-.

O
R
C

R'
R'
+
Nu-
R
C
Nu
O-
a) Addition of hydrogen cyanide, HCN
• The addition of hydrogen cyanide to the
carbonyl group produces the hydroxynitrile
or cyanohydrin
O
H3C
C
OH
H
+ HCN
KCN / H2SO4
H3C
C
C
N
ethanal
H
2-hydroxypropanenitrile
b) Addition of sodium hydrogensulphate
• When an aldehyde or a ketone is shaken with
a saturated solution of aqueous sodium
hydrogensulphate (IV), NaHSO3, at room
temperature, colorless crystals of carbonyl
hydrogensulphate (IV) are formed.
O
H3C
C
CH3
room
temperature
CH3
+ NaHSO3
H3C
C
Propanone
OH
SO3-Na+
• When heated with a dilute acid, the carbonyl
sodium hydrogensulphate (IV) decomposes to
produce the free carbonyl compound.
CH3
H3C
C
O
SO3-Na+
+ H+
heat
OH
H3C
C
CH3
+ Na+ + H2O + SO2
Propanone
• Therefore this reaction provides a useful way
of purifying or separating aldehydes or
ketones from non-carbonyl compounds.
c) Addition of Grignard reagents
• This reaction can be used to prepare alcohols
from carbonyl compounds by which
methanal produces primary alcohols,
aldehydes produce secondary alcohols and
ketones produce tertiary alcohols.
O
R
C
R'
R'
+
R'
dry
R"MgX
R
C
ethoxyethane
R"
-
O MgX
+
H2O
NH4Cl (aq)
R
C
R"
OH
+ Mg(OH)X
2. Condensation (Addition-elimination) reactions
• Aldehydes and ketones can undergo
condensation reactions with substituted
ammonia compounds, H2N-G to form a
compound containing an imine group, C
together with the elimination of a water
molecule
N
R
R
C
R
O
+ H2N
G
C
R
N
G
+ H2O
• Reaction with hydroxylamine
– Aldehydes and ketones react with hydroxylamine to form
oximes on warming
R
R
C
O
+
H2NOH
R
C
N
OH
+ H2O
R
• Reaction with hydrazine
Oxime
– Aldehydes and ketones react with hydrazine to form the
hydrazone, a crystalline solid
R
R
C
R
O
+
H2NNH2
hydrazine
C
N
NH2 + H2O
R
Aldehyde or ketone
Hydrozone
3. Oxidation
• Potassium manganate (VII) or potassium
dichromate (VI), (KMnO4 or K2Cr2O7)
• Aldehydes are readily oxidised to produce
COOH
O
R
C
O
H
+ [O]
KMnO4 / K2Cr2O7
heat
R
C
OH
4. Reduction
• Common reagents Lithium
tetrahydridoaluminate (III), LiAlH4
• Aldehydes are reduced to primary alcohols
O
R
C
H
+ 2[H]
LiAlH4
heat
R
H2
C
OH
• Ketones are reduced to secondary alcohol
O
R
C
OH
R
+ 2[H]
LiAlH4
heat
R
C
H
R
5. Reaction with phosphorus (V) chloride
• Carbonyl compounds react with PCl5, under
anhydrous conditions to yield the
dichlorohydrocarbon
O
R
C
R
R
+ PCl5
R
C
Cl
Cl
+ POCl3
Exercise
• Write equations to show the reactions
between
a) Propanal and KMnO4
b) Phenylethanal and KMnO4
c) Propanal and LiAlH4
d) Methanal and HCN
e) Cyclopentanone and LiAlH4
Thank you