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Chapter 34
Carbonyl Compounds
34.1
34.2
34.3
34.4
34.5
34.6
1
Introduction
Nomenclature of Carbonyl Compounds
Physical Properties of Carbonyl Compounds
Preparation of Carbonyl Compounds
Reactions of Carbonyl Compounds
Uses of Carbonyl Compounds
New Way Chemistry for Hong Kong A-Level Book 3B
34.1 Introduction (SB p.2)
Aldehydes and ketones :
carbonyl compounds, contain
group
General formula of aldehydes:
Examples:
2
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34.1 Introduction (SB p.2)
General formula of ketones:
Examples:
3
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34.1 Introduction (SB p.3)
• Carbonyl carbon is sp2-hybridized
• The  bond is formed by the head-on overlap of an
sp2 hybrid orbital of C and one p prbital of O
• The  bond is formed by the side-way overlap of p
orbitals of C and O
4
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34.1 Introduction (SB p.3)
5
•
The three atoms that are bonded to the carbonyl carbon
forms a trigonal planar structure
•
The bond angles between three attached atoms are 120
New Way Chemistry for Hong Kong A-Level Book 3B
34.1 Introduction (SB p.3)
• Oxygen is more electronegative
• The carbonyl oxygen bears a partial negative charge
and the carbonyl carbon bears partial positive charge
6
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34.2 Nomenclature of Carbonyl Compounds (SB p.3)
Aldehydes
Aldehydes are named by replacing the final “-e” of the
name of the corresponding alkane with “-al”
Examples:
7
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34.2 Nomenclature of Carbonyl Compounds (SB p.4)
Ketones
•
Ketones are named by replacing the final “-e” of the name of
the corresponding alkane with “-one”.
•
The parent chain is then numbered in the way that gives the
carbonyl carbon atom the lowest possible number, and this
number is used to indicate its position.
Examples:
8
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34.2 Nomenclature of Carbonyl Compounds (SB p.4)
Check Point 34-1
(a) Draw the structural formulae of all carbonyl compounds
having the molecular formula C4H8O. Give their IUPAC
names.
Answer
9
New Way Chemistry for Hong Kong A-Level Book 3B
34.2 Nomenclature of Carbonyl Compounds (SB p.4)
Check Point 34-1
(b) Draw the structural formulae of all straight-chain
carbonyl compounds having the molecular formula
C5H10O.
Answer
10
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34.2 Nomenclature of Carbonyl Compounds (SB p.4)
Check Point 34-1
(c) Explain why there is no such a compound called
“ethanone”.
Answer
(c) Ketones are compounds with the
group
situated between two carbon chains. Therefore, the
simplest ketone is the one with three carbon atoms.
“Ethanone”, however, suggests that there are two
carbon atoms in it and it does not exist.
11
New Way Chemistry for Hong Kong A-Level Book 3B
34.3 Physical Properties of Carbonyl Compounds (SB p.5)
•
Simple aldehydes and ketones are gases or liquids at room
temperature
•
Aliphatic aldehydes have unpleasant and pungent smell
•
Ketones and benzaldehyde have a pleasant and sweet odour
Carbonyl
compound
Aldehydes:
Methanal
Ethanal
Propanal
Butanal
Methylpropanal
Benzaldehyde
12
Formula
Boiling
point (°C)
HCHO
CH3CHO
CH3CH2CHO
CH3(CH2)2CHO
(CH3)2CHCHO
C6H5CHO
–21
20.8
48.8
75.7
64.2
179
Melting
Density at
point (°C) 20°C (g cm–3)
–92
–124
–81
–99
–65.9
–26
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—
0.783
0.807
0.817
0.790
1.046
34.3 Physical Properties of Carbonyl Compounds (SB p.5)
Carbonyl
compound
Formula
Ketones:
Propanone
Butanone
Pentan-3-one
Pentan-2-one
3-Methylbutan-2-one
Hexan-2-one
Phenylethanone
13
CH3COCH3
CH3COCH2CH3
CH3CH2COCH2CH3
CH3CO(CH2)2CH3
CH3COCH(CH3)2
CH3CO(CH2)3CH3
C6H5COCH3
Boiling
Melting
Density at
point (°C) point (°C) 20°C (g cm–3)
56.2
79.6
102
102
95
127
202
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–95.4
–86.9
–39.9
–77.8
–92
–56.9
19.6
0.791
0.806
0.814
0.811
0.803
0.812
1.028
34.3 Physical Properties of Carbonyl Compounds (SB p.5)
Boiling Point and Melting Point
14
•
Carbonyl compounds have
higher b.p. and m.p. than
hydrocarbons of similar relative
molecular masses
∵ the presence of dipole-dipole
interactions
•
Carbonyl compounds have
lower b.p. and m.p. than the
corresponding alcohols
∵ dipole-dipole interactions are
weaker than intermolecular
hydrogen bonds
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34.3 Physical Properties of Carbonyl Compounds (SB p.6)
Density
• The densities of aliphatic carbonyl compounds are
lower than that of water at 20°C
• Aromatic carbonyl compounds are slightly
denser than water 20°C
• Densities increase with increasing relative
molecular masses
15
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34.3 Physical Properties of Carbonyl Compounds (SB p.6)
Solubility
• Aldehydes and ketones of low molecular masses
show appreciable solubilities in water
∵ carbonyl oxygen can form strong hydrogen
bonds with water molecules
• e.g. propanone and ethanal are soluble in water in
all proportions
16
New Way Chemistry for Hong Kong A-Level Book 3B
34.3 Physical Properties of Carbonyl Compounds (SB p.7)
Example 34-1
(a) In each pair of compounds below, select the one you
would expect to have a higher boiling point.
(i) A: CH3CH2CHO
B: CH3CH2CH2OH
(ii) C:
D:
(iii)E: CH3CH2CH2CHO
F: CH3CH2CH2CH3
(iv) G:
Solution:H:
(a) (i)
(ii)
(iii)
(iv)
17
B
D
E
H
New Way Chemistry for Hong Kong A-Level Book 3B
Answer
34.3 Physical Properties of Carbonyl Compounds (SB p.7)
Example 34-1
(b) Propanone, CH3COCH3, is completely soluble in
water, but octan-4-one,
CH3CH2CH2COCH2CH2CH2CH3, is almost insoluble
in soluble in water. Explain their difference in
solubility.
Answer
Solution:
(b) This is because the solubility in water decreases
as the hydrophobic hydrocarbon portion lengthens
18
New Way Chemistry for Hong Kong A-Level Book 3B
34.4 Preparation of Carbonyl Compounds (SB p.7)
Dehydrogenation of Alcohols
Industrially, lower members of aldehydes and ketones are
prepared by passing alcohol vapour over hot silver catalyst
19
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34.4 Preparation of Carbonyl Compounds (SB p.8)
Oxidation of Alcohols
Example of oxidizing agents: acidified K2Cr2O7
• Aldehydes are prepared by oxidation of 1° alcohols
• Ketones are prepared by oxidation of 2 ° alcohols
20
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34.4 Preparation of Carbonyl Compounds (SB p.8)
Oxidative Cleavage of Alkenes (Ozonolysis)
• Ozone reacts with alkenes vigorously to from ozonides
• Ozonides are reduced by Zn and H2O to give aldehydes
and/or ketones
21
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34.4 Preparation of Carbonyl Compounds (SB p.8)
Decarboxylation of Acid Salts
Aldehydes can be prepared by heating a mixture of
calcium methanoate and calcium carboxylate
e.g.
22
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34.4 Preparation of Carbonyl Compounds (SB p.9)
Ketones can be prepared by heating calcium carboxylate
e.g.
23
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34.4 Preparation of Carbonyl Compounds (SB p.9)
Reduction of Acyl Chlorides
• Aldehydes can be prepared by reducing acyl chlorides
by treatment with H2 in the presence of Pd / BaSO4
catalyst and S
• The purpose of adding sulphur is to poison the catalyst,
so that the reduction does not proceed to produce
alcohols
24
New Way Chemistry for Hong Kong A-Level Book 3B
34.5 Reactions of Carbonyl Compounds (SB p.9)
Nucleophilic Addition Reactions
• Carbonyl group is susceptible to nucleophilic attack
∵ carbonyl carbon bears a partial positive charge
• Nucleophiles use its lone pair electrons to form a bond
with carbonyl carbon
• One pair of bonding electrons of the carbon-oxygen
bond shift out to the carbonyl oxygen
25
New Way Chemistry for Hong Kong A-Level Book 3B
34.5 Reactions of Carbonyl Compounds (SB p.10)
• The electron-rich oxygen transfers its electron pair to
a proton
 addition of Nu – H to the carbonyl group
• The carbonyl carbon changes from a trigonal planar
geometry (i.e. sp2-hybridized) to a tetrahedral
geometry (i.e. sp3-hybridized)
26
New Way Chemistry for Hong Kong A-Level Book 3B
34.5 Reactions of Carbonyl Compounds (SB p.10)
• Aldehydes are more reactive than ketones
∵ inductive effect and steric effect
1. The inductive effect
The carbonyl carbon in ketones is less electrondeficient because two alkyl groups release electrons
whereas only one present in aldehydes
27
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34.5 Reactions of Carbonyl Compounds (SB p.10)
2. The steric effect
•
Aldehyde molecules are relatively open to the attack
of nucleophiles
∵ one group being attached to the carbonyl carbon
is a small hydrogen atom
• In ketones, the
two alkyl or aryl
substituents cause
a greater steric
hindrance to the
nucleophiles
28
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34.5 Reactions of Carbonyl Compounds (SB p.11)
• Due to the above 2 factors, the general order of
reactivity of carbonyl compounds:
• The delocalization of  electrons from the benzene ring
reduce the electron deficiency of the carbonyl carbon
atom and makes aromatic carbonyl compounds even
less reactive than aliphatic ketones
29
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34.5 Reactions of Carbonyl Compounds (SB p.11)
Addition of Hydrogen Cyanide
Addition of hydrogen cyanide to the carbonyl group to form
2-hydroxyalkanenitriles (also known as cyanohydrins)
30
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34.5 Reactions of Carbonyl Compounds (SB p.11)
Examples:
31
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34.5 Reactions of Carbonyl Compounds (SB p.11)
Mechanism for the nucleophilic addition of hydrogen
cyanide to the carbonyl group:
32
New Way Chemistry for Hong Kong A-Level Book 3B
34.5 Reactions of Carbonyl Compounds (SB p.11)
• HCN is a poor nucleophile while CN– is much
stronger
 the reaction can be catalyzed by a base stronger
than CN–, as the base can increase the concentration
of CN–
HCN + OH–  CN– + H2O
• As HCN is very toxic and volatile, it is safer to
generate it in the reaction mixture
• Mixing KCN or NaCN with dilute H2SO4 at 10 – 20°C
gives HCN:
2KCN + H2SO4  2HCN + K2SO4
2NaCN + H2SO4  2HCN + Na2SO4
33
New Way Chemistry for Hong Kong A-Level Book 3B
34.5 Reactions of Carbonyl Compounds (SB p.12)
• 2-Hydroxyalkanenitriles are useful intermediates in
organic synthesis
• On acid hydrolysis, 2-hydroxyalkanenitriles are converted
to 2-hydroxycarboxylic acids or 2-alkenoic acids
e.g.
34
New Way Chemistry for Hong Kong A-Level Book 3B
34.5 Reactions of Carbonyl Compounds (SB p.12)
• With the use of reducing agents (e.g. LiAlH4),
2-hydroxyalkanenitriles are reduced to amines
e.g.
35
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34.5 Reactions of Carbonyl Compounds (SB p.13)
Addition of Sodium Hydrogensulphate(IV)
Carbonyl compounds react reversibly with excess 40%
aqueous hydrogensulphate(IV) solutions at room temperature
36
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34.5 Reactions of Carbonyl Compounds (SB p.13)
Examples:
37
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34.5 Reactions of Carbonyl Compounds (SB p.13)
The reaction mechanism:
• The reaction is initiated by the attack of nucleophile, HSO3–
38
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34.5 Reactions of Carbonyl Compounds (SB p.13)
• This reaction is very sensitive to steric hindrance and is
limited to aliphatic aldehydes and sterically unhindered
ketones
• This reaction can be used for the separation and
purification of the aldehydes and ketones, as they can be
regenerated by treating the bisulphite addition product with
aqueous alkalis or dilute acids.
39
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34.5 Reactions of Carbonyl Compounds (SB p.14)
Example 34-2
Outline how you are going to separate a mixture of
butanone (b.p. 79.6°C) and 1-chlorobutane (b.p. 78.5°C) in
diethyl ether.
Answer
40
New Way Chemistry for Hong Kong A-Level Book 3B
34.5 Reactions of Carbonyl Compounds (SB p.14)
Solution:
The mixture of butanone and 1-chlorobutane cannot be separated by
distillation as their boiling points are too close. However, they can be
separated through the nucleophilic addition reaction of sodium
hydrogensulphate(IV). With the addition of sodium hydrogensulphate(IV)
to the mixture, only butanone reacts to give the bisulphite addition
product which is soluble in water. Then the organic later (containing 1chlorobutane) and the aqueous layer (containing the bisulphite addition
product of butanone) are separated using a separating funnel. 1Chlorobutane can be obtained by distilling off the ether. On the other
hand, with the addition of a dilute acid, the bisulphite addition product is
converted to the carbonyl compound (i.e. butanone) which dissolves in
diethyl ether. The organic layer (containing butanone) is separated from
the aqueous layer by means of a separating funnel. Butanone is
obtained after distilling off the ether.
41
New Way Chemistry for Hong Kong A-Level Book 3B
34.5 Reactions of Carbonyl Compounds (SB p.14)
Addition – Elimination (Condensation) Reactions
42
•
Addition – elimination reactions involve the first
addition of two molecules to form an unstable
intermediate followed by the spontaneous
elimination of the elements of water
•
e.g. reaction of aldehydes or ketones with the
derivatives of ammonia
New Way Chemistry for Hong Kong A-Level Book 3B
34.5 Reactions of Carbonyl Compounds (SB p.14)
Reaction with Hydroxylamine
• Carbonyl compounds react with hydroxylamine
(NH2OH) to form oximes
43
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34.5 Reactions of Carbonyl Compounds (SB p.15)
Examples:
44
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34.5 Reactions of Carbonyl Compounds (SB p.15)
Reaction with 2,4-dinitrophenylhydrazine
• Carbonyl compounds react with 2,4-dinitrophenylhydrazine
to form 2,4-dinitrophenylhydrazones
45
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34.5 Reactions of Carbonyl Compounds (SB p.15)
Examples:
46
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34.5 Reactions of Carbonyl Compounds (SB p.16)
47
•
The oximes and 2,4-dinitrophenylhydrazones are used to
identify unknown aldehydes and ketones
•
They are insoluble solids and have
sharp characteristic melting points
•
The products are purified by
recrystallization from ethanol and
then filtered and washed under
suction
•
Their melting points are determined
and compared with that in data book
to identify the original aldehyde or
ketone
New Way Chemistry for Hong Kong A-Level Book 3B
34.5 Reactions of Carbonyl Compounds (SB p.16)
Carbonyl
compound
Aldehydes:
Methanal
Ethanal
Propanal
Butanal
Benzaldehyde
Ketones:
Propanone
Butanone
Pentan-2-one
Pentan-3-one
Hexan-2-one
Phenylethanone
48
Formula
Melting point of
2,4-dinitrophenylhydrazone (°C)
HCHO
CH3CHO
CH3CH2CHO
CH3CH2CH2CHO
C6H5CHO
167
146, 164 (2 forms)
156
123
237
CH3COCH3
CH3CH2COCH3
CH3CH2CH2COCH3
CH3CH2COCH2CH3
CH3CH2CH2CH2COCH3
C6H5COCH3
128
115
141
156
107
250
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34.5 Reactions of Carbonyl Compounds (SB p.17)
Check Point 34-2
(a) Carbonyl
compoundsreactions
always undergo
nucleophilic
addition
(a) Compare
the addition
of carbonyl
compounds
andreactions.
alkenes.As the carbon atom in the carbonyl group bears a partial
positive charge, the carbonyl group is susceptible to nucleophilic
Answer
attack.
In the case of alkenes, they always undergo electrophilic addition
reactions. As the  bonding electrons of the carbon-carbon double bond
are only loosely held by the carbon atoms and are exposed, the carboncarbon double bond is susceptible to electron-loving reagents (i.e.
electrophiles)
49
New Way Chemistry for Hong Kong A-Level Book 3B
34.5 Reactions of Carbonyl Compounds (SB p.17)
Check Point 34-2
(b) Describe briefly how you can distinguish between two
carbonyl compounds having similar boiling points.
Answer
(b) The two compounds can be distinguished by
determining the melting points of their
2,4-dinitrophenylhydrazone derivatives.
50
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34.5 Reactions of Carbonyl Compounds (SB p.17)
Oxidations
• Aldehydes can be oxidized to carboxylic acids by strong
oxidizing agents such as KMnO4 and K2Cr2O7, and
also by mild oxidizing agents such as Tollen’s reagent
and Fehling’s reagent
51
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34.5 Reactions of Carbonyl Compounds (SB p.18)
Reaction with Potassium Manganate(VII)
and Potassium Dichromate(VI)
• Aldehydes are oxidized readily by common oxidizing
agents such as KMnO4/H+ and K2Cr2O7/H+
52
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34.5 Reactions of Carbonyl Compounds (SB p.18)
• Generally, ketones do not undergo oxidation as their
oxidation involves the cleavage of the strong carboncarbon bond
• More severe conditions are required to bring about the
oxidation
53
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34.5 Reactions of Carbonyl Compounds (SB p.18)
Reaction with Tollens’ Reagent (Silver Mirror Test)
• Tollens’ reagent contains Ag(NH3)2+
• Ag(NH3)2+ oxidizes aldehydes to carboxylic acids
while it is reduced to metallic silver which deposits
on the wall of the reaction vessel as silver mirror
54
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34.5 Reactions of Carbonyl Compounds (SB p.19)
• Aldehydes are mixed with Tollens’ reagent in a clean test
tube and placed in water bath kept at 60°C
• If the wall of the reaction vessel is not
clean enough, a silver mirror cannot be
formed and a black precipitate is
deposited instead
• All ketones give a negative result of the
silver mirror test
• This reaction can be used to distinguish
aldehydes from ketones
55
New Way Chemistry for Hong Kong A-Level Book 3B
34.5 Reactions of Carbonyl Compounds (SB p.18)
Reaction with Fehling’s Solution (Fehling’s Test)
• Fehling’s solution is an alkaline solution of copper(II)
tartrate. It is a blue solution
• Aliphatic aldehydes reduce the Cu2+ ion in Fehling’s
solution to form a brick-red precipitate of Cu2O
56
New Way Chemistry for Hong Kong A-Level Book 3B
34.5 Reactions of Carbonyl Compounds (SB p.18)
Addition of
an aliphatic
aldehyde
Fehling’s
solution
• Ketones and aromatic aldehydes give a negative
result of Fehling’s test
• This reaction can be used to distinguish aliphatic
aldehydes from ketones and aromatic aldehydes
57
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34.5 Reactions of Carbonyl Compounds (SB p.20)
Reductions
• Aldehydes and ketones undergo reduction reactions
forming 1° and 2° alcohols respectively
58
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34.5 Reactions of Carbonyl Compounds (SB p.20)
Reaction with Lithium Tetrahydridoaluminate
• Lithium tetrahydridoaluminate (also called lithium
aluminium hydride, LiAlH4) is a powerful reducing
agent
• It reduces aldehydes to 1° alcohols and ketones to 2°
alcohols
59
New Way Chemistry for Hong Kong A-Level Book 3B
34.5 Reactions of Carbonyl Compounds (SB p.20)
• LiAlH4 is able to reduce carboxylic acid and esters to
give alcohols
• LiAlH4 reacts violently with water, therefore the
reaction must be carried out in anhydrous solutions,
usually in dry ether
60
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34.5 Reactions of Carbonyl Compounds (SB p.21)
Reaction with Sodium Tetrahydridoborate
In practice, the reduction of aldehydes and ketones to
alcohols is carried out by sodium tetrahydridoborate
(also called sodium borohydride, NaBH4)
61
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34.5 Reactions of Carbonyl Compounds (SB p.21)
• NaBH4 is a less powerful reducing agent than
LiAlH4
• NaBH4 reduces only aldehydes and ketones
• The reduction by NaBH4 can be carried out in water
or alcohol solutions
62
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34.5 Reactions of Carbonyl Compounds (SB p.21)
Triiodomethane Formation (Iodoform Reaction)
Aldehydes or ketones having the
group react
with iodine in aqueous sodium hydroxide solution to
give a bright yellow precipitate of iodoform (CHI3)
63
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34.5 Reactions of Carbonyl Compounds (SB p.21)
•
Ethanol and secondary alcohols with the
group
also give a positive result of iodoform test
∵ the group is first oxidized to
group and
further oxidized to give the carboxylate and the iodoform
•
64
Iodoform test is test for the presence of
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or
34.5 Reactions of Carbonyl Compounds (SB p.22)
Check Point 34-3
Draw the structural formulae of the major organic products
A to H in the following reactions:
(a) A:
(a)
(b)
B:
KCN/H2SO4
conc. HCl
CH3CH2CHO 
A  B
20°C
2,4-dinitrophenylhydrazine
CH3CH2CHO  C
(b) C:
(c)
1. LiAlH4 / dry ether
CH3CH2CHO 
D
+
2. H3O
(d)
(c) D: CH3CH2CH2OH
NaBH4
CH3CH = CHCH2CHO 
E
HO
(d) E: CH3CH = CHCH22 CH2OH
65
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Answer
34.5 Reactions of Carbonyl Compounds (SB p.22)
Check Point 34-3
Draw the structural formulae of the major organic products
A to H in the following reactions:
(e)
(e)
(f)
F:
(f) G:
Answer
H: CHI3
66
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34.6 Uses of Carbonyl Compounds (SB p.22)
As Raw Materials for Making Plastics
Urea-methanal
Urea-methanal is produced by condensation
polymerization of urea
and methanal
under heat and pressure with the elimination of a water
molecule
67
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34.6 Uses of Carbonyl Compounds (SB p.23)
In the presence of
excess methanal, cross
linkages will be formed
68
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34.6 Uses of Carbonyl Compounds (SB p.24)
Urea-methanal
• thermosetting polymer (cannot be softened and
insoluble in any solvents)
• excellent electrical insulator
• resistant to chemical attack
• used for moulding electrical sockets
69
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34.6 Uses of Carbonyl Compounds (SB p.24)
Perspex
Propanone is converted to methyl 2-methylpropenoate,
which is the monomer for the production of perspex
70
•
Perspex is a dense, transparent
solid
•
Used to make safety goggles,
advertising signs and carside
light protectors
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34.6 Uses of Carbonyl Compounds (SB p.24)
As Solvents
Propanone
• Liquid with a boiling point of 56.2°C
• Can dissolve a variety of organic compounds
• Important solvent used in industry and in the laboratory
71
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The END
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