Transcript Redox reactions_organic - Welcome to SALEM

33
33.1
33.2
33.3
33.4
33.5
33.6
33.7
33.8
1
Redox
Reactions
Organic Synthesis
Redox Reactions
Oxidation of Alkylbenzenes
Oxidation of Alcohols
Redox Reactions of Aldehydes and Ketones
Redox Reactions of Carboxylic Acids
Redox Reactions of Alkenes
Autooxidation of Fats and Oils
33.1
2
Organic
Synthesis
33.1 Organic Synthesis (SB p.51)
Organic Synthesis
• In planning syntheses,
 we need to think backwards
 think backwards from the desired
product to simpler molecules
(precursors)
Target
molecule
3
Precursors
33.1 Organic Synthesis (SB p.51)
Organic Synthesis
• A synthesis usually involves more than
one step
Target
molecule
1st
Precursor
2nd
Precursor
Starting
material
4
33.1 Organic Synthesis (SB p.51)
Organic Synthesis
• Usually more than one way to carry out
a synthesis
2nd Precursor a
1st Precursor A
Target
molecule
2nd Precursor b
2nd Precursor c
1st Precursor B
2nd Precursor d
1st Precursor C
2nd Precursor e
2nd Precursor f
5
33.1 Organic Synthesis (SB p.52)
Number of Steps Involved in the
Synthesis
•
Most organic reactions are
 reversible reactions
 seldom proceed to completion
 impossible to have a 100% yield
of the product from each step of
the synthetic route
6
33.1 Organic Synthesis (SB p.52)
Number of Steps Involved in the
Synthesis
•
Consider the following synthetic
route:
 each step has a yield of 60 %
60 %
60 %
60 %
60 %
conversion conversion conversion conversion
A  B  C  D  E
7
What is the yield of the
desired product?
33.1 Organic Synthesis (SB p.52)
Number of Steps Involved in the
Synthesis
60 %
60 %
60 %
60 %
conversion conversion conversion conversion
A  B  C  D  E
Yield of the desired product
= 60 %  60 %  60 %  60 %
= 12.96 %
8
33.1 Organic Synthesis (SB p.52)
Number of Steps Involved in the
Synthesis
9
•
An efficient route of synthesis should
consist of a minimal number of steps
•
Limit the total number of reaction steps
in a synthesis to not more than four
33.1 Organic Synthesis (SB p.52)
Availability of Starting Materials
and Reagents
• Only a restricted number of simple,
relatively cheap starting materials is
available
• Include:
 simple haloalkanes and alcohols of
not more than four carbon atoms
 simple aromatic compounds (e.g.
benzene and methylbenzene)
10
33.1 Organic Synthesis (SB p.52)
Duration of the Synthetic Process
•
Many organic reactions proceed at a
relatively low rate
•
e.g. the acid-catalyzed esterification
requires refluxing the reaction mixture of
alcohols and carboxylic acids for a whole
day
•
Inclusion of these slow reactions in a
synthetic route is impractical
Check Point 33-1
11
33.2
12
Redox
Reactions
33.2 Redox Reactions (SB p.53)
Redox Reactions
• Redox reactions are reactions that
involve a change of oxygen or hydrogen
content in organic compounds
13
33.2 Redox Reactions (SB p.53)
Oxidation
•
Oxidation of an organic compound usually
corresponds to:
 an increase in oxygen content
 a decrease in hydrogen content
14
33.2 Redox Reactions (SB p.53)
Oxidation
•
e.g.
The change of ethanol to ethanoic acid is
an oxidation
 the oxygen content of ethanoic acid is
higher than that of ethanol
15
33.2 Redox Reactions (SB p.53)
Oxidation
•
e.g.
Converting ethanol to ethanal is also an
oxidation process
 the hydrogen content of ethanal is lower
than that of ethanol
16
33.2 Redox Reactions (SB p.53)
Oxidation
Common oxidizing agents used in organic
reactions include:
17
•
Acidified potassium manganate(VII)
(KMnO4/H+)
•
Alkaline potassium manganate(VII)
(KMnO4/OH–)
•
Acidified potassium dichromate(VI)
(K2Cr2O7/H+)
•
Ozone (O3/CH3CCl3, Zn/H2O)
33.2 Redox Reactions (SB p.54)
Reduction
•
Reduction of an organic compound usually
corresponds to:
 an increase in hydrogen content
 a decrease in oxygen content
18
33.2 Redox Reactions (SB p.54)
Reduction
•
e.g.
Converting ethanoic acid to ethanal is a
reduction
 the oxygen content of ethanal is lower
than that of ethanoic acid
19
33.2 Redox Reactions (SB p.54)
Reduction
•
e.g.
Converting ethanal to ethanol is also a
reduction process
 the hydrogen content of ethanol is
higher than that of ethanal
20
33.2 Redox Reactions (SB p.54)
Reduction
Common reducing agents used in organic
reactions include:
•
Lithium tetrahydridoaluminate in dry
ether (LiAlH4/ether, H3O+)
•
Sodium tetrahydridoborate (NaBH4/H2O)
•
Hydrogen with palladium (H2/Pd)
Check Point 33-2
21
33.3
Oxidation of
Alkylbenzenes
22
33.3 Oxidation of Alkylbenzenes (SB p.55)
Alkylbenzenes
23
•
A group of aromatic hydrocarbons in
which an alkyl group is bonded directly
to a benzene ring
•
Sometimes called arenes
33.3 Oxidation of Alkylbenzenes (SB p.55)
Alkylbenzenes
•
24
Examples of alkylbenzenes:
33.3 Oxidation of Alkylbenzenes (SB p.55)
Oxidation of Alkylbenzenes
•
Oxidation of alkylbenzenes
 carried out by the action of hot
alkaline potassium manganate(VII)
solution
•
25
In the oxidation process, a benzoate is
formed
33.3 Oxidation of Alkylbenzenes (SB p.55)
Oxidation of Alkylbenzenes
•
Benzoic acid can be recovered
 by adding a mineral acid such as
dilute H2SO4 to the benzoate
•
26
This method gives benzoic acid in
almost quantitative yield
33.3 Oxidation of Alkylbenzenes (SB p.55)
Oxidation of Alkylbenzenes
27
33.3 Oxidation of Alkylbenzenes (SB p.55)
Oxidation of Alkylbenzenes
28
33.3 Oxidation of Alkylbenzenes (SB p.56)
Oxidation of Alkylbenzenes
•
All alkylbenzenes are oxidized to
benzoic acid
 except the alkylbenzenes with a
tertiary alkyl group
 they do not have a benzylic
hydrogen atom
29
33.3 Oxidation of Alkylbenzenes (SB p.56)
Oxidation of Alkylbenzenes
• In the above oxidation processes,
 the alkyl groups of alkylbenzenes are
oxidized, rather than the benzene ring
• In the first step, the oxidizing agent
abstracts a benzylic hydrogen atom
• The oxidizing agent oxidizes the side
chain to a carboxyl group
30
33.3 Oxidation of Alkylbenzenes (SB p.56)
Oxidation of Alkylbenzenes
31
•
Side-chain oxidation by KMnO4 is not
restricted to alkyl groups
•
C = C bonds and C = O groups in the side
chain are also oxidized by hot alkaline
KMnO4
33.3 Oxidation of Alkylbenzenes (SB p.56)
Oxidation of Alkylbenzenes
•
32
e.g.
33.3 Oxidation of Alkylbenzenes (SB p.56)
Check Point 33-3
33
33.4
Oxidation of
Alcohols
34
33.4 Oxidation of Alcohols (SB p.56)
Alcohols
• A group of compounds with one or more
hydroxyl groups (OH) attached to an
alkyl group
• For alcohols having only one hydroxyl
group,
 their general formula is CnH2n+1OH
35
33.4 Oxidation of Alcohols (SB p.56)
Alcohols
• Examples of alcohols:
36
33.4 Oxidation of Alcohols (SB p.57)
Alcohols
•
Depending on the number of alkyl
groups attached to the carbon to which
the hydroxyl group is linked,
 alcohols can be classified as primary,
secondary and tertiary alcohols
37
33.4 Oxidation of Alcohols (SB p.57)
Alcohols
• Differentiating an alcohol as a 1o alcohol,
a 2o alcohol or a 3o alcohol is extremely
important
when oxidized, these alcohols give
different products
38
33.4 Oxidation of Alcohols (SB p.57)
Alcohols
Primary
alcohols
• Can be
oxidized to
aldehydes
• Further
oxidized to
carboxylic
acids
39
Secondary
alcohols
• Can be
oxidized to
ketones
• Cannot be
further
oxidized to
carboxylic
acids
Tertiary
alcohols
• Generally
resistant to
oxidation
33.4 Oxidation of Alcohols (SB p.57)
Oxidation of Primary Alcohols
40
•
Primary alcohols are firstly oxidized to
aldehydes and subsequently to
carboxylic acids
•
Using oxidizing agents like acidified
KMnO4 and acidified K2Cr2O7
33.4 Oxidation of Alcohols (SB p.57)
1. Oxidation of Primary Alcohols to Aldehydes
• The oxidation of alcohols is difficult to stop at
the aldehyde stage
 aldehydes are a reducing agent
• One way of solving this problem
 remove the aldehyde as soon as it is formed
 by distilling off the aldehydes from the
reaction mixture
41
33.4 Oxidation of Alcohols (SB p.57)
1. Oxidation of Primary Alcohols to Aldehydes
• e.g.
Ethanal can be synthesized from ethanol
using acidified K2Cr2O7
 ethanal is removed by distillation
42
33.4 Oxidation of Alcohols (SB p.58)
1. Oxidation of Primary Alcohols to Aldehydes
A typical
laboratory set-up
for the oxidation
of ethanol to
ethanal
43
33.4 Oxidation of Alcohols (SB p.58)
2. Oxidation of Primary Alcohols to Carboxylic
Acids
• Primary alcohols can be oxidized to
carboxylic acids by acidified KMnO4
• Acidified KMnO4 is a powerful oxidizing
agent
 the oxidation of the alcohols does
not stop at the aldehydes
 but directly to the carboxylic acids
44
33.4 Oxidation of Alcohols (SB p.58)
2. Oxidation of Primary Alcohols to Carboxylic
Acids
• e.g.
Ethanol can be oxidized to ethanoic acid
by acidified KMnO4
Ethanol
45
Ethanoic acid
33.4 Oxidation of Alcohols (SB p.59)
2. Oxidation of Primary Alcohols to Carboxylic
Acids
A reflux apparatus
used for the
oxidation of
ethanol to
ethanoic acid
46
33.4 Oxidation of Alcohols (SB p.59)
2. Oxidation of Primary Alcohols to Carboxylic
Acids
A distillation
apparatus used
for the
separation of
ethanoic acid
from the
reaction mixture
47
33.4 Oxidation of Alcohols (SB p.59)
2. Oxidation of Primary Alcohols to Carboxylic
Acids
• The oxidation of ethanol by acidified
K2Cr2O7
 the basis of the breathalyser used
by the police
 to rapidly estimate the ethanol content
of the breath of suspected drunken
drivers
48
33.4 Oxidation of Alcohols (SB p.59)
2. Oxidation of Primary Alcohols to Carboxylic
Acids
• When the drunken driver blows into the bag
 the ethanol molecules reduce the
orange Cr2O72- ions to green Cr3+ ions
• If more than a certain amount of the orange
crystal changes colour,
 the driver is likely to be “over the limit”
49
33.4 Oxidation of Alcohols (SB p.59)
2. Oxidation of Primary Alcohols to Carboxylic
Acids
Demonstration
of the use of the
breathalyser
50
33.4 Oxidation of Alcohols (SB p.59)
Oxidation of Secondary Alcohols
• Secondary alcohols can be oxidized to
ketones by either acidified K2Cr2O7 or
acidified KMnO4
51
33.4 Oxidation of Alcohols (SB p.59)
Oxidation of Secondary Alcohols
• The reaction usually stops at the ketone
stage
 further oxidation requires the breaking
of a carbon-carbon bond
 difficult to proceed
52
33.4 Oxidation of Alcohols (SB p.60)
Oxidation of Secondary Alcohols
• e.g.
Propan-2-ol can be oxidized to form
propanone
53
33.4 Oxidation of Alcohols (SB p.60)
Oxidation of Tertiary Alcohols
•
Tertiary alcohols are generally resistant to
oxidation unless they are subjected to
severe oxidation conditions
 any oxidation would immediately
involve the cleavage of the strong
C  C bonds in the alcohol molecule
54
33.4 Oxidation of Alcohols (SB p.60)
Oxidation of Tertiary Alcohols
•
Tertiary alcohols can be oxidized by
acidified KMnO4
 give a mixture of ketones and
carboxylic acids
 both with fewer carbon atoms than
the original alcohol
55
33.4 Oxidation of Alcohols (SB p.60)
Oxidation of Tertiary Alcohols
• e.g.
heat
2-Methylbutan-2-ol
Propanone Ethanoic acid
Check Point 33-4
56
33.5
Redox Reactions
of Aldehydes
and Ketones
57
33.5 Redox Reactions of Aldehydes and Ketones (SB p.62)
Aldehydes and Ketones
•
58
Carbonyl compounds that contain the
carbonyl group
33.5 Redox Reactions of Aldehydes and Ketones (SB p.62)
Oxidation of Carbonyl Compounds
• Aldehydes are readily oxidized by
acidified KMnO4 or K2Cr2O7 to form
carboxylic acids
59
33.5 Redox Reactions of Aldehydes and Ketones (SB p.62)
Oxidation of Carbonyl Compounds
• Ketones do not undergo oxidations
readily
 their oxidation involves the cleavage
of the strong CC bond
 more severe conditions are required
to bring about the oxidation
60
33.5 Redox Reactions of Aldehydes and Ketones (SB p.62)
Oxidation of Carbonyl Compounds
•
With the action of hot acidified KMnO4,
 the CC bonds in ketones would be
broken
 a mixture of carboxylic acids would
be formed
61
33.5 Redox Reactions of Aldehydes and Ketones (SB p.63)
Reduction of Carbonyl Compounds
• Both aldehydes and ketones undergo
reduction reactions readily
 forming 1o and 2o alcohols respectively
• Reducing agents:
 lithium tetrahydridoaluminate (LiAlH4)
 sodium tetrahydridoborate (NaBH4)
62
33.5 Redox Reactions of Aldehydes and Ketones (SB p.63)
Reduction of Carbonyl Compounds
• LiAlH4 is a powerful reducing agent
 it reacts violently with water
• Those reduction reactions using LiAlH4
must be carried out in anhydrous solutions
 usually in dry ether
63
33.5 Redox Reactions of Aldehydes and Ketones (SB p.63)
Reduction of Carbonyl Compounds
64
33.5 Redox Reactions of Aldehydes and Ketones (SB p.63)
Reduction of Carbonyl Compounds
•
The reduction of aldehydes and ketones to
alcohols is most often carried out by
NaBH4
•
NaBH4 is a less powerful reducing agent
 it does not react with water
 the reduction reactions using NaBH4
can be carried out in water or alcohols
65
33.5 Redox Reactions of Aldehydes and Ketones (SB p.63)
Reduction of Carbonyl Compounds
Check Point 33-5
66
33.6
Redox Reactions
of Carboxylic
Acids
67
33.6 Redox Reactions of Carboxylic Acids (SB p.64)
Carboxylic Acids
68
•
A group of organic compounds containing
the carboxyl group
•
Examples:
33.6 Redox Reactions of Carboxylic Acids (SB p.64)
Reduction of Carboxylic Acids
•
Reductions of carboxylic acids are
difficult to carry out
•
Can be achieved with the use of very
powerful reducing agents (e.g. LiAlH4)
•
LiAlH4 reduces carboxylic acids to
1o alcohols in excellent yields
Check Point 33-6
69
33.7
Redox Reactions
of Alkenes
70
33.7 Redox Reactions of Alkenes (SB p.65)
Alkenes
•
Alkenes are unsaturated hydrocarbons
containing C = C bonds
•
The C = C bonds are readily oxidized
 alkenes are able to undergo oxidation
reactions
71
33.7 Redox Reactions of Alkenes (SB p.65)
Alkenes
• Alkenes can accept hydrogen to
form alkanes
 alkenes are also able to
undergo reduction reactions
72
33.7 Redox Reactions of Alkenes (SB p.65)
Oxidation of Alkenes by
Potassium Manganate(VII)
•
Alkenes react with alkaline KMnO4
 form 1,2-diols called glycols
73
33.7 Redox Reactions of Alkenes (SB p.65)
Oxidation of Alkenes by
Potassium Manganate(VII)
74
•
Ethene is oxidized to ethane-1,2-diol
•
The manganate(VII) ions are reduced
to manganese(IV) oxide
33.7 Redox Reactions of Alkenes (SB p.65)
Oxidation of Alkenes by
Potassium Manganate(VII)
•
A change from the purple colour of
manganate(VII) ions to the brown
precipitate of manganese(IV) oxide
 a chemical test to distinguish
between alkenes and alkanes
75
33.7 Redox Reactions of Alkenes (SB p.66)
Oxidation of Alkenes by Ozone
•
Alkenes react rapidly and quantitatively
with ozone
 form an unstable compound, known as
ozonide
•
Ozonides are very unstable
 they are not usually isolated
 treated directly with a reducing agent
(Zn/H3O+)
76
33.7 Redox Reactions of Alkenes (SB p.66)
Oxidation of Alkenes by Ozone
•
The reduction produces carbonyl compounds
 can be safely isolated and identified
77
33.7 Redox Reactions of Alkenes (SB p.66)
Oxidation of Alkenes by Ozone
•
The net result of this reaction is
 the breaking of the C = C bond to
form two carbonyl groups
•
This process is called ozonolysis
 can be used to locate the position of
the C = C bond in an alkene
78
33.7 Redox Reactions of Alkenes (SB p.66)
Oxidation of Alkenes by Ozone
•
e.g.
Ozonolysis of but-1-ene gives two different
aldehydes
79
33.7 Redox Reactions of Alkenes (SB p.66)
Oxidation of Alkenes by Ozone
•
e.g.
Ozonolysis of but-2-ene gives one
aldehyde
Example 33-7
80
33.7 Redox Reactions of Alkenes (SB p.68)
Reduction of Alkenes
(Hydrogenation of Alkenes)
• Alkenes react with hydrogen in the
presence of metal catalysts (Ni, Pd and Pt)
 form alkanes
81
33.7 Redox Reactions of Alkenes (SB p.68)
Reduction of Alkenes
(Hydrogenation of Alkenes)
•
The atoms of the hydrogen molecule
add to each carbon atom of the C = C
bond of the alkene
 the alkene is converted to an alkane
82
33.7 Redox Reactions of Alkenes (SB p.68)
Reduction of Alkenes
(Hydrogenation of Alkenes)
• Useful in analyzing unsaturated
hydrocarbons (alkenes or alkynes)
• By measuring the number of moles of
hydrogen reacted with one mole of an
unsaturated hydrocarbon
 the number of double or triple bonds
83
present in an unsaturated hydrocarbon
molecule can be deduced
Check Point 33-7
33.8
Autooxidation
of Fats and Oils
84
33.8 Autooxidation of Fats and Oils (SB p.69)
Oxidation of Fats and Oils
Oils and fats as esters of propane-1,2,3triol and fatty acids
O
Mixed
triglycerides
H 2C
O
C
R
O
CH
O
C
R'
O
H 2C
O
C
R"
(R, R' and R" are hydrocarbon chains)
85
Rancidity : Bad taste or smell caused by spoilage
of fats/oils
(i) Hydrolytic rancidity
(ii) Oxidative rancidity
86
(i) Hydrolytic rancidity
CH 2 OH
CHCO
2 R'
+
RCO
2H
+
RCO
2H
+
R'CO
2H
+
RCO
2H
+
R'CO
+
R"CO
O
CH 2 CO 2 R"
H 2C
O
C
R
O
CH
O
C
R'
H2O
CH 2 OH
CHOH
O
CH 2 CO 2 R"
H 2C
O
C
R"
CH 2 OH
CHOH
CH 2 OH
Catalyzed by micro-organisms
87
2H
2H
Foul
smelling
(i) Hydrolytic rancidity
Potato chip
frying
Fat/oil + water
water
frying
foul smelling acids
Human sweat
 A mixture of RCOOH
Each of us has a unique blend of RCOOH
 Easily tracked by dogs
88
(ii) Oxidative rancidity (Autooxidation)
Unsaturated fats/oils are more susceptible
Odorless/flavorless
to autooxidation
hydroperoxide
O2
Catalyzed by trace
metals, light or free
radical initiators
Foul smelling
aldehydes,
ketones,
carboxylic acids
89
chain reaction
Hydroperoxide
free radical
Autooxidation can be slowed down by :
(i)
Food additives (antioxidants)
E.g. BHA, BHT
90
Principle of BHA/BHT as Antioxidants
Foods containing BHA and BHT
91
Autooxidation can be slowed down by :
(i)
Food additives (antioxidants)
E.g. BHA, BHT
(ii)
Naturally occurring antioxidants
Vitamins C and E
(iii) Exclusion of O2
Potato chips sealed in packets with N2
92
Principle of BHA/BHT as Antioxidants
AH + ROO •  ROOH + A •
BHA/BHT work by
donating a hydrogen atom to the
hydroperoxide free radical,
thus, stopping the chain reactions in
oxidative spoilage
93
O
OH
+
R
O
O
+
R
O
O
H
+
R
O
O
H
X
BHA
O
O
OH
O
+
R
O
O
Y
BHT
BHA : Butylated hydroxyanisole
BHT : Butylated hydroxytoluene
94
Q.87
Electron-donating
Electron-donating
95
96
Quickfit Set
23BU/M
Semi-micro
scale
97
27BU/M
1
98
Pear shaped flask
27BU/M
2
99
Stillhead
27BU/M
3
100
Liebig condenser
27BU/M
4
101
Screwcap adapter
27BU/M
5
102
Receiver adapter
27BU/M
6
103
Thermometer
27BU/M
7
104
Dropping funnel, 50 ml
with Rotaflo tap
27BU/M
8
105
Stopper
27BU/M
9
106
Air leak/steam inlet tube
27X/M
10 Round bottom flask/receiver
107
27X/M
11 Air condenser/drying tube
108
27X/M
12 Sintered glass funnel
109
27X/M
13 Drying(Absorption) tube
110
27X/M
14 Pear shaped flask, with
angled side neck
111
27X/M
15 Air leak/steam inlet tube
112
27X/M
16 Adapter with ‘T’ connection
113
27X/M
17 Screwcap adapter
114
A. Preparation
CH3CH2OH
115
Cr2O72/H+
CH3CHO
T.
Preparation/distillation
with stirring
Stirrer
116
U.
117
Distillation with air condenser
C.
CH3CH2OH
118
Reflux
Cr2O72/H+
CH3COOH
Conc. H2SO4
CH3(CH2)3OH
119
H/L. Reflux with
addition
Conc. H2SO4
NaBr(s)
CH3(CH2)3Br
D/J. Gas evolution
Water-soluble gases are
passed into water or
sink with flowing tap
water
H2SO4(l) + NaBr(s)  SO2(g)
+ Br2(g)
CH3(CH2)3OH
120
Conc. H2SO4
NaBr(s)
CH3(CH2)3Br
Input of steam from
steam generator
B. Steam distillation
121
E. Steam distillation
Water and the organic
product can be separated
122
H2O(g)
P. Steam distillation
Temperature of vapour
is monitored
123
Air leak
O. Vacuum distillation
To pump
Heating in a closed system is strictly forbidden!
124
Q. Fractional distillation
125
N. Vacuum filtration
To pump
126
Cl2
R. Reaction with
gas inlet
Soda lime to
absorb excess Cl2
P(s) + CH3OH
CH3OH
127
P, Cl2
CH3Cl
The END
128
33.1 Organic Synthesis (SB p.52)
Why are simple alcohols and simple aromatic compounds
relatively cheap starting materials for organic
syntheses?
Answer
They can be made from alkanes and benzene
which can be obtained directly from fractional
distillation of petroleum.
Back
129
33.1 Organic Synthesis (SB p.53)
(a) What are the main reasons for carrying out an organic
synthesis?
Answer
(a) To make new substances such as medicines,
dyes, plastics or pesticides
To make new organic compounds for studying
reaction mechanisms or metabolic pathways
130
33.1 Organic Synthesis (SB p.53)
(b) What are the factors that determine the feasibility of an
organic synthesis?
Answer
(b) Number of steps involved in the synthesis
Availability of starting materials and reagents
Duration of the synthetic process
Back
131
33.2 Redox Reactions (SB p.54)
(a) State two common oxidizing agents used in organic
reactions.
Answer
(a) Acidified potassium manganate(VII) (KMnO4/H+)
Alkaline potassium manganate(VII) (KMnO4/OH–)
Acidified potassium dichromate(VI) (K2Cr2O7/H+)
Ozone (O3/CH3Cl3, Zn/H2O)
(Any two)
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33.2 Redox Reactions (SB p.54)
(b) State two common reducing agents used in organic
reactions.
Answer
(b) Lithium tetrahydridoaluminate in dry ether
(LiAlH4/ether, H3O+)
Sodium tetrahydridoborate (NaBH4/H2O)
Hydrogen with palladium (H2/Pd)
(Any two)
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33.2 Redox Reactions (SB p.54)
Back
(c) State whether each of the following reactions is an
oxidation or a reduction.
(i)
Conversion of ethanol to ethanal
(ii) Conversion of ethene to ethanol
(iii) Conversion of nitrobenzene to phenylamine
(iv) Conversion of propene to propane
(v) Conversion of propan-2-ol to propanone
(c) (i) Oxidation
(iii) Reduction
(v) Oxidation
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(ii) Oxidation
(iv) Reduction
Answer
33.3 Oxidation of Alkylbenzenes (SB p.56)
Why is tert-butylbenzene resistant to side-chain
oxidation?
Answer
tert-Butylbenzene does not have a
benzylic hydrogen atom.
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135
33.3 Oxidation of Alkylbenzenes (SB p.56)
State the conditions under which ethylbenzene can be
converted to benzoic acid in the laboratory.
Reagents: 1. potassium manganate(VII),
sodium hydroxide
2. dilute sulphuric acid
Condition: heating under reflux
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136
Answer
33.4 Oxidation of Alcohols (SB p.60)
Back
Draw the structural formulae for the major organic
products in the following reactions:
K2Cr2O7/H+
(a) Propan-1-ol 
reflux
K2Cr2O7/H+
(b) Propan-2-ol 
reflux
(a)
137
(b)
Answer
33.5 Redox Reactions of Aldehydes and Ketones (SB p.63)
What is the species responsible for the reducing
property of LiAlH4 and NaBH4?
Answer
Hydride ion, H–
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138
33.5 Redox Reactions of Aldehydes and Ketones (SB p.64)
Give the structural formulae for the major organic products
of the following reactions:
(a)
(b)
(c)
(d)
Answer
139
33.5 Redox Reactions of Aldehydes and Ketones (SB p.64)
(a)
(b)
(c) CH3CH2OH
(d)
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140
33.6 Redox Reactions of Carboxylic Acids (SB p.65)
Give the structural formulae for the major organic products,
if any, in the following reactions:
(a)
(b)
(a)
(b)
(c) No reaction
(c)
141
Answer
33.6 Redox Reactions of Carboxylic Acids (SB p.65)
Back
Give the structural formulae for the major organic products,
if any, in the following reactions:
(d)
(d)
(e)
(e)
(f)
(f)
142
Answer
33.7 Redox Reactions of Alkenes (SB p.67)
Predict the structures of the following hydrocarbons A, B
and C using the information given below:
143
Hydrocarbon
Molecular
formula
A
C3H6
B
C6H10
C
C10H16
Products after
ozonolysis
Answer
33.7 Redox Reactions of Alkenes (SB p.67)
A: As C3H6 can be expressed as CnH2n, the hydrocarbon is a
compound with one C = C double bond. When A undergoes
ozonolysis,
∴
144
and
are formed.
The possible structure of A is CH3CH = CH2.
33.7 Redox Reactions of Alkenes (SB p.67)
B: As C6H10 can be expressed as CnH2n–2 and only one dicarbonyl
compound is formed on ozonolysis, the hydrocarbon is an
alicyclic compound with one C = C double bond.
∴ The possible structure of B is
145
.
33.7 Redox Reactions of Alkenes (SB p.67)
C: C10H16 can be expressed as CnH2n–4. Two products with totally
five carbon atoms are formed on ozonolysis. So the original
hydrocarbon is an acyclic compound with three C = C double
bonds.
ozonolysis
∴
The possible structure of C is
CH3CH = CHCH2CH = CHCH2CH = CHCH3.
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146
33.7 Redox Reactions of Alkenes (SB p.68)
Give the structural formulae for the major organic products,
if any, in the following reactions:
(a)
(b)
(c)
Answer
147
33.7 Redox Reactions of Alkenes (SB p.68)
(a)
(b)
(c)
148
Back
33.8 Autooxidation of Fats and Oils (SB p.71)
(a) What causes fats and oils to go rancid?
(b) Explain how BHA and BHT can slow down the
oxidative spoilage of fats and oils.
Answer
(a) Carbon-carbon double bonds in fats and oils as
well as oxygen in air
(b) BHA and BHT donate the hydrogen atoms of their
hydroxyl group to the free hydroperoxide radical
involved in the autooxidation of fats and oils.
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149