new way chemistry for hong kong a-level 3b 1 redox reactions 33.1organic synthesis 33.2redox...
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New Way Chemistry for Hong Kong A-Level 3B1
Redox Redox ReactionsReactions
33.133.1 Organic SynthesisOrganic Synthesis
33.233.2 Redox ReactionsRedox Reactions
33.333.3 Oxidation of AlkylbenzenesOxidation of Alkylbenzenes
33.433.4 Oxidation of AlcoholsOxidation of Alcohols
33.533.5 Redox Reactions of Aldehydes and KetonesRedox Reactions of Aldehydes and Ketones
33.633.6 Redox Reactions of Carboxylic AcidsRedox Reactions of Carboxylic Acids
33.733.7 Redox Reactions of AlkenesRedox Reactions of Alkenes
33.833.8 Autooxidation of Fats and OilsAutooxidation of Fats and Oils
3333
New Way Chemistry for Hong Kong A-Level 3B3
33.1 Organic Synthesis (SB p.51)
Organic Organic SynthesisSynthesis• In planning syntheses,
we need to think backwards
think backwards from the desired product to simpler molecules
(precursors)
Target molecule
Precursors
New Way Chemistry for Hong Kong A-Level 3B4
33.1 Organic Synthesis (SB p.51)
Organic Organic SynthesisSynthesis• A synthesis usually involves more than
one step
Target molecule
1st Precursor
2nd Precursor
Starting Starting materialmaterial
New Way Chemistry for Hong Kong A-Level 3B5
33.1 Organic Synthesis (SB p.51)
Organic Organic SynthesisSynthesis• Usually more than one way to carry out
a synthesis
Target molecule
1st Precursor A
1st Precursor B
1st Precursor C
2nd Precursor a
2nd Precursor b
2nd Precursor c
2nd Precursor d
2nd Precursor e
2nd Precursor f
New Way Chemistry for Hong Kong A-Level 3B6
33.1 Organic Synthesis (SB p.52)
Number of Steps Involved in Number of Steps Involved in the Synthesisthe 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
New Way Chemistry for Hong Kong A-Level 3B7
33.1 Organic Synthesis (SB p.52)
Number of Steps Involved in Number of Steps Involved in the Synthesisthe Synthesis
• Consider the following synthetic route:
each step has a yield of 60 %
A B C D E
60 % conversion
60 % conversion
60 % conversion
60 % conversion
What is the yield of the desired product?
New Way Chemistry for Hong Kong A-Level 3B8
33.1 Organic Synthesis (SB p.52)
Number of Steps Involved in Number of Steps Involved in the Synthesisthe Synthesis
A B C D E
60 % conversion
60 % conversion
60 % conversion
60 % conversion
Yield of the desired product
= 60 % 60 % 60 % 60 %
= 12.96 %
New Way Chemistry for Hong Kong A-Level 3B9
33.1 Organic Synthesis (SB p.52)
Number of Steps Involved in Number of Steps Involved in the Synthesisthe Synthesis
• 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
New Way Chemistry for Hong Kong A-Level 3B10
33.1 Organic Synthesis (SB p.52)
Availability of Starting Availability of Starting Materials and ReagentsMaterials 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)
New Way Chemistry for Hong Kong A-Level 3B11
33.1 Organic Synthesis (SB p.52)
Duration of the Synthetic Duration of the Synthetic ProcessProcess• 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 33-1Check Point 33-1
New Way Chemistry for Hong Kong A-Level 3B13
33.2 Redox Reactions (SB p.53)
Redox Redox ReactionsReactions• Redox reactions are reactions that
involve a change of oxygen or hydrogen content in organic compounds
New Way Chemistry for Hong Kong A-Level 3B14
33.2 Redox Reactions (SB p.53)
OxidationOxidation
• Oxidation of an organic compound usually corresponds to:
an increase in oxygen content
a decrease in hydrogen content
New Way Chemistry for Hong Kong A-Level 3B15
33.2 Redox Reactions (SB p.53)
OxidationOxidation
• e.g.
The change of ethanol to ethanoic acid is an oxidation
the oxygen content of ethanoic acid is higher than that of ethanol
New Way Chemistry for Hong Kong A-Level 3B16
33.2 Redox Reactions (SB p.53)
OxidationOxidation• e.g.
Converting ethanol to ethanal is also an oxidation process
the hydrogen content of ethanal is lower than that of ethanol
New Way Chemistry for Hong Kong A-Level 3B17
33.2 Redox Reactions (SB p.53)
OxidationOxidationCommon oxidizing agents used in organic reactions include:
• Acidified potassium manganate(VII) (KMnO4/H+)
• Alkaline potassium manganate(VII) (KMnO4/OH–)
• Acidified potassium dichromate(VI) (K2Cr2O7/H+)
• Ozone (O3/CH3CCl3, Zn/H2O)
New Way Chemistry for Hong Kong A-Level 3B18
33.2 Redox Reactions (SB p.54)
ReductionReduction
• Reduction of an organic compound usually corresponds to:
an increase in hydrogen content
a decrease in oxygen content
New Way Chemistry for Hong Kong A-Level 3B19
33.2 Redox Reactions (SB p.54)
ReductionReduction• e.g.
Converting ethanoic acid to ethanal is a reduction
the oxygen content of ethanal is lower than that of ethanoic acid
New Way Chemistry for Hong Kong A-Level 3B20
33.2 Redox Reactions (SB p.54)
ReductionReduction• e.g.
Converting ethanal to ethanol is also a reduction process
the hydrogen content of ethanol is higher than that of ethanal
New Way Chemistry for Hong Kong A-Level 3B21
33.2 Redox Reactions (SB p.54)
ReductionReduction
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 33-2Check Point 33-2
New Way Chemistry for Hong Kong A-Level 3B22
33.333.3Oxidation of Oxidation of AlkylbenzeneAlkylbenzene
ss
New Way Chemistry for Hong Kong A-Level 3B23
33.3 Oxidation of Alkylbenzenes (SB p.55)
AlkylbenzenesAlkylbenzenes
• A group of aromatic hydrocarbons in which an alkyl group is bonded directly to a benzene ring
• Sometimes called arenes
New Way Chemistry for Hong Kong A-Level 3B24
33.3 Oxidation of Alkylbenzenes (SB p.55)
AlkylbenzenesAlkylbenzenes
• Examples of alkylbenzenes:
New Way Chemistry for Hong Kong A-Level 3B25
33.3 Oxidation of Alkylbenzenes (SB p.55)
Oxidation of Oxidation of AlkylbenzenesAlkylbenzenes• Oxidation of alkylbenzenes
carried out by the action of hot alkaline potassium
manganate(VII) solution
• In the oxidation process, a benzoate is formed
New Way Chemistry for Hong Kong A-Level 3B26
33.3 Oxidation of Alkylbenzenes (SB p.55)
Oxidation of Oxidation of AlkylbenzenesAlkylbenzenes
• Benzoic acid can be recovered
by adding a mineral acid such as dilute H2SO4 to the benzoate
• This method gives benzoic acid in almost quantitative yield
New Way Chemistry for Hong Kong A-Level 3B27
33.3 Oxidation of Alkylbenzenes (SB p.55)
Oxidation of Oxidation of AlkylbenzenesAlkylbenzenes
New Way Chemistry for Hong Kong A-Level 3B28
33.3 Oxidation of Alkylbenzenes (SB p.55)
Oxidation of Oxidation of AlkylbenzenesAlkylbenzenes
New Way Chemistry for Hong Kong A-Level 3B29
33.3 Oxidation of Alkylbenzenes (SB p.56)
Oxidation of Oxidation of AlkylbenzenesAlkylbenzenes
• All alkylbenzenes are oxidized to benzoic acid
except the alkylbenzenes with a tertiary alkyl group
they do not have a benzylic hydrogen atom
New Way Chemistry for Hong Kong A-Level 3B30
33.3 Oxidation of Alkylbenzenes (SB p.56)
Oxidation of Oxidation of AlkylbenzenesAlkylbenzenes• 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
New Way Chemistry for Hong Kong A-Level 3B31
33.3 Oxidation of Alkylbenzenes (SB p.56)
Oxidation of Oxidation of AlkylbenzenesAlkylbenzenes• 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
New Way Chemistry for Hong Kong A-Level 3B32
33.3 Oxidation of Alkylbenzenes (SB p.56)
Oxidation of Oxidation of AlkylbenzenesAlkylbenzenes• e.g.
New Way Chemistry for Hong Kong A-Level 3B33
33.3 Oxidation of Alkylbenzenes (SB p.56)
Check Point 33-3Check Point 33-3
New Way Chemistry for Hong Kong A-Level 3B35
33.4 Oxidation of Alcohols (SB p.56)
AlcoholAlcoholss• 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
New Way Chemistry for Hong Kong A-Level 3B36
33.4 Oxidation of Alcohols (SB p.56)
AlcoholAlcoholss• Examples of alcohols:
New Way Chemistry for Hong Kong A-Level 3B37
33.4 Oxidation of Alcohols (SB p.57)
AlcoholAlcoholss• 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
New Way Chemistry for Hong Kong A-Level 3B38
33.4 Oxidation of Alcohols (SB p.57)
AlcoholAlcoholss
• Differentiating an alcohol as a 1o alcohol, a 2o alcohol or a 3o alcohol is extremely important
when oxidized, these alcohols give different products
New Way Chemistry for Hong Kong A-Level 3B39
33.4 Oxidation of Alcohols (SB p.57)
AlcoholAlcoholss
Primary alcohols
Secondary alcohols
Tertiary alcohols
• Can be oxidized to aldehydes
• Further oxidized to carboxylic acids
• Can be oxidized to ketones
• Cannot be further oxidized to carboxylic acids
• Generally resistant to oxidation
New Way Chemistry for Hong Kong A-Level 3B40
33.4 Oxidation of Alcohols (SB p.57)
Oxidation of Primary Oxidation of Primary AlcoholsAlcohols• Primary alcohols are firstly oxidized to
aldehydes and subsequently to carboxylic acids
• Using oxidizing agents like acidified KMnO4 and acidified K2Cr2O7
New Way Chemistry for Hong Kong A-Level 3B41
33.4 Oxidation of Alcohols (SB p.57)
1. Oxidation of Primary Alcohols to Alde1. Oxidation of Primary Alcohols to Aldehydeshydes
• 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
New Way Chemistry for Hong Kong A-Level 3B42
33.4 Oxidation of Alcohols (SB p.57)
1. Oxidation of Primary Alcohols to Alde1. Oxidation of Primary Alcohols to Aldehydeshydes• e.g.
Ethanal can be synthesized from ethanol using acidified K2Cr2O7
ethanal is removed by distillation
New Way Chemistry for Hong Kong A-Level 3B43
33.4 Oxidation of Alcohols (SB p.58)
1. Oxidation of Primary Alcohols to Alde1. Oxidation of Primary Alcohols to Aldehydeshydes
A typical laboratory set-up for the oxidation of ethanol to ethanal
New Way Chemistry for Hong Kong A-Level 3B44
33.4 Oxidation of Alcohols (SB p.58)
2. Oxidation of Primary Alcohols to Carb2. Oxidation of Primary Alcohols to Carboxylic Acidsoxylic 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
New Way Chemistry for Hong Kong A-Level 3B45
33.4 Oxidation of Alcohols (SB p.58)
2. Oxidation of Primary Alcohols to Carb2. Oxidation of Primary Alcohols to Carboxylic Acidsoxylic Acids• e.g.
Ethanol can be oxidized to ethanoic acid by acidified KMnO4
Ethanol Ethanoic acid
New Way Chemistry for Hong Kong A-Level 3B46
33.4 Oxidation of Alcohols (SB p.59)
2. Oxidation of Primary Alcohols to Carb2. Oxidation of Primary Alcohols to Carboxylic Acidsoxylic Acids
A reflux apparatus used for the oxidation of ethanol to ethanoic acid
New Way Chemistry for Hong Kong A-Level 3B47
33.4 Oxidation of Alcohols (SB p.59)
2. Oxidation of Primary Alcohols to Carb2. Oxidation of Primary Alcohols to Carboxylic Acidsoxylic Acids
A distillation apparatus used for the separation of ethanoic acid from the reaction mixture
New Way Chemistry for Hong Kong A-Level 3B48
33.4 Oxidation of Alcohols (SB p.59)
2. Oxidation of Primary Alcohols to Carb2. Oxidation of Primary Alcohols to Carboxylic Acidsoxylic 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
New Way Chemistry for Hong Kong A-Level 3B49
33.4 Oxidation of Alcohols (SB p.59)
2. Oxidation of Primary Alcohols to Carb2. Oxidation of Primary Alcohols to Carboxylic Acidsoxylic Acids
• When the drunken driver blows into the bag
the ethanol molecules reduce the orange Cr2O7
2- 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”
New Way Chemistry for Hong Kong A-Level 3B50
33.4 Oxidation of Alcohols (SB p.59)
2. Oxidation of Primary Alcohols to Carb2. Oxidation of Primary Alcohols to Carboxylic Acidsoxylic Acids
Demonstration of the use of the breathalyser
New Way Chemistry for Hong Kong A-Level 3B51
33.4 Oxidation of Alcohols (SB p.59)
Oxidation of Secondary Oxidation of Secondary AlcoholsAlcohols
• Secondary alcohols can be oxidized to ketones by either acidified K2Cr2O7 or acidified KMnO4
New Way Chemistry for Hong Kong A-Level 3B52
33.4 Oxidation of Alcohols (SB p.59)
Oxidation of Secondary Oxidation of Secondary AlcoholsAlcohols
• The reaction usually stops at the ketone stage
further oxidation requires the breaking of a carbon-carbon bond
difficult to proceed
New Way Chemistry for Hong Kong A-Level 3B53
33.4 Oxidation of Alcohols (SB p.60)
Oxidation of Secondary Oxidation of Secondary AlcoholsAlcohols• e.g.
Propan-2-ol can be oxidized to form propanone
New Way Chemistry for Hong Kong A-Level 3B54
33.4 Oxidation of Alcohols (SB p.60)
Oxidation of Tertiary AlcoholsOxidation 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
New Way Chemistry for Hong Kong A-Level 3B55
33.4 Oxidation of Alcohols (SB p.60)
Oxidation of Tertiary AlcoholsOxidation 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
New Way Chemistry for Hong Kong A-Level 3B56
33.4 Oxidation of Alcohols (SB p.60)
Oxidation of Tertiary AlcoholsOxidation of Tertiary Alcohols
• e.g.
2-Methylbutan-2-ol Propanone Ethanoic acid
heat
Check Point 33-4Check Point 33-4
New Way Chemistry for Hong Kong A-Level 3B57
33.533.5Redox Redox
Reactions of Reactions of Aldehydes Aldehydes
and Ketonesand Ketones
New Way Chemistry for Hong Kong A-Level 3B58
33.5 Redox Reactions of Aldehydes and Ketones (SB p.62) Aldehydes and Aldehydes and
KetonesKetones• Carbonyl compounds that contain the
carbonyl group
New Way Chemistry for Hong Kong A-Level 3B59
33.5 Redox Reactions of Aldehydes and Ketones (SB p.62)
Oxidation of Carbonyl Oxidation of Carbonyl CompoundsCompounds
• Aldehydes are readily oxidized by acidified KMnO4 or K2Cr2O7 to form carboxylic acids
New Way Chemistry for Hong Kong A-Level 3B60
33.5 Redox Reactions of Aldehydes and Ketones (SB p.62)
Oxidation of Carbonyl Oxidation of Carbonyl CompoundsCompounds
• Ketones do not undergo oxidations readily
their oxidation involves the cleavage of the strong CC bond
more severe conditions are required to bring about the oxidation
New Way Chemistry for Hong Kong A-Level 3B61
33.5 Redox Reactions of Aldehydes and Ketones (SB p.62)
Oxidation of Carbonyl Oxidation of Carbonyl CompoundsCompounds
• With the action of hot acidified KMnO4,
the CC bonds in ketones would be broken
a mixture of carboxylic acids would be formed
New Way Chemistry for Hong Kong A-Level 3B62
33.5 Redox Reactions of Aldehydes and Ketones (SB p.63) Reduction of Carbonyl Reduction of Carbonyl
CompoundsCompounds• Both aldehydes and ketones undergo
reduction reactions readily
forming 1o and 2o alcohols respectively
• Reducing agents:
lithium tetrahydridoaluminate (LiAlH4)
sodium tetrahydridoborate (NaBH4)
New Way Chemistry for Hong Kong A-Level 3B63
33.5 Redox Reactions of Aldehydes and Ketones (SB p.63) Reduction of Carbonyl Reduction of Carbonyl
CompoundsCompounds• 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
New Way Chemistry for Hong Kong A-Level 3B64
33.5 Redox Reactions of Aldehydes and Ketones (SB p.63) Reduction of Carbonyl Reduction of Carbonyl
CompoundsCompounds
New Way Chemistry for Hong Kong A-Level 3B65
33.5 Redox Reactions of Aldehydes and Ketones (SB p.63) Reduction of Carbonyl Reduction of Carbonyl
CompoundsCompounds• 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
New Way Chemistry for Hong Kong A-Level 3B66
33.5 Redox Reactions of Aldehydes and Ketones (SB p.63) Reduction of Carbonyl Reduction of Carbonyl
CompoundsCompounds
Check Point 33-5Check Point 33-5
New Way Chemistry for Hong Kong A-Level 3B67
33.633.6Redox Redox
Reactions of Reactions of Carboxylic Carboxylic
AcidsAcids
New Way Chemistry for Hong Kong A-Level 3B68
33.6 Redox Reactions of Carboxylic Acids (SB p.64)
Carboxylic Carboxylic AcidsAcids• A group of organic compounds containing
the carboxyl group
• Examples:
New Way Chemistry for Hong Kong A-Level 3B69
33.6 Redox Reactions of Carboxylic Acids (SB p.64)
Reduction of Carboxylic Reduction of Carboxylic AcidsAcids• 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 33-6Check Point 33-6
New Way Chemistry for Hong Kong A-Level 3B70
33.733.7Redox Redox
Reactions of Reactions of AlkenesAlkenes
New Way Chemistry for Hong Kong A-Level 3B71
33.7 Redox Reactions of Alkenes (SB p.65)
AlkeneAlkeness• Alkenes are unsaturated hydrocarbons
containing C = C bonds
• The C = C bonds are readily oxidized
alkenes are able to undergo oxidation reactions
New Way Chemistry for Hong Kong A-Level 3B72
33.7 Redox Reactions of Alkenes (SB p.65)
AlkeneAlkeness
• Alkenes can accept hydrogen to form alkanes
alkenes are also able to undergo reduction reactions
New Way Chemistry for Hong Kong A-Level 3B73
33.7 Redox Reactions of Alkenes (SB p.65)
Oxidation of Alkenes by Oxidation of Alkenes by Potassium Manganate(VII)Potassium Manganate(VII)
• Alkenes react with alkaline KMnO4
form 1,2-diols called glycols
New Way Chemistry for Hong Kong A-Level 3B74
33.7 Redox Reactions of Alkenes (SB p.65)
Oxidation of Alkenes by Oxidation of Alkenes by Potassium Manganate(VII)Potassium Manganate(VII)
• Ethene is oxidized to ethane-1,2-diol
• The manganate(VII) ions are reduced to manganese(IV) oxide
New Way Chemistry for Hong Kong A-Level 3B75
33.7 Redox Reactions of Alkenes (SB p.65)
Oxidation of Alkenes by Oxidation of Alkenes by Potassium Manganate(VII)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
New Way Chemistry for Hong Kong A-Level 3B76
33.7 Redox Reactions of Alkenes (SB p.66)
Oxidation of Alkenes by Oxidation of Alkenes by OzoneOzone• Alkenes react rapidly and quantitatively wit
h 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+)
New Way Chemistry for Hong Kong A-Level 3B77
33.7 Redox Reactions of Alkenes (SB p.66)
Oxidation of Alkenes by Oxidation of Alkenes by OzoneOzone• The reduction produces carbonyl compounds
can be safely isolated and identified
New Way Chemistry for Hong Kong A-Level 3B78
33.7 Redox Reactions of Alkenes (SB p.66)
Oxidation of Alkenes by Oxidation of Alkenes by OzoneOzone
• 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
New Way Chemistry for Hong Kong A-Level 3B79
33.7 Redox Reactions of Alkenes (SB p.66)
Oxidation of Alkenes by Oxidation of Alkenes by OzoneOzone• e.g.
Ozonolysis of but-1-ene gives two different aldehydes
New Way Chemistry for Hong Kong A-Level 3B80
33.7 Redox Reactions of Alkenes (SB p.66)
Oxidation of Alkenes by Oxidation of Alkenes by OzoneOzone
• e.g.
Ozonolysis of but-2-ene gives one aldehyde
Example 33-7Example 33-7
New Way Chemistry for Hong Kong A-Level 3B81
33.7 Redox Reactions of Alkenes (SB p.68)
Reduction of Alkenes Reduction of Alkenes (Hydrogenation of (Hydrogenation of Alkenes)Alkenes)• Alkenes react with hydrogen in the
presence of metal catalysts (Ni, Pd and Pt)
form alkanes
New Way Chemistry for Hong Kong A-Level 3B82
33.7 Redox Reactions of Alkenes (SB p.68)Reduction of Alkenes Reduction of Alkenes (Hydrogenation of (Hydrogenation of Alkenes)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
New Way Chemistry for Hong Kong A-Level 3B83
33.7 Redox Reactions of Alkenes (SB p.68)Reduction of Alkenes Reduction of Alkenes (Hydrogenation of (Hydrogenation of Alkenes)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 present in an unsaturated
hydrocarbon molecule can be deduced
Check Point 33-7Check Point 33-7
New Way Chemistry for Hong Kong A-Level 3B84
33.833.8AutooxidatioAutooxidatio
n of Fats n of Fats and Oilsand Oils
New Way Chemistry for Hong Kong A-Level 3B85
33.8 Autooxidation of Fats and Oils (SB p.69)
Oxidation of Fats and Oxidation of Fats and OilsOils• Fats and oils are esters of propane-1,2,3-
triol and carboxylic acids of fairly long carbon chains
• Some of these acids may contain one or more C = C bonds in them
known as unsaturated carboxylic acids
New Way Chemistry for Hong Kong A-Level 3B86
33.8 Autooxidation of Fats and Oils (SB p.69)
Oxidation of Fats and Oxidation of Fats and OilsOils• When fats and oils are exposed to air,
the C = C bonds will be oxidized
the fats and oils will develop an “off ” odour and unpleasant flavour
New Way Chemistry for Hong Kong A-Level 3B87
33.8 Autooxidation of Fats and Oils (SB p.70)
Oxidation of Fats and Oxidation of Fats and OilsOils• Fats and oils having a high degree of
unsaturation are more susceptible to oxidation
• The oxidation follows a free radical mechanism
accelerated by trace metals, light and free radical initiators
New Way Chemistry for Hong Kong A-Level 3B88
33.8 Autooxidation of Fats and Oils (SB p.70)
Oxidation of Fats and Oxidation of Fats and OilsOils
• The hydroperoxides produced are flavourless and odourless
decompose readily to form highly reactive hydroperoxide free radicals
New Way Chemistry for Hong Kong A-Level 3B89
33.8 Autooxidation of Fats and Oils (SB p.70)
Oxidation of Fats and Oxidation of Fats and OilsOils• These radicals set up a chain reaction
produce volatile, flavoured compounds of aldehydes, ketones and carboxylic acids
responsible for their rancid flavour
• This process is called autooxidation
New Way Chemistry for Hong Kong A-Level 3B90
33.8 Autooxidation of Fats and Oils (SB p.70)
Oxidation of Fats and Oxidation of Fats and OilsOils• Autooxidation can be controlled but cannot
be stopped
• The addition of antioxidants (e.g. BHA and BHT) can slow down the oxidative spoilage of fats and oils
• Many vegetable oils contain natural antioxidants (e.g. vitamin C)
can withstand autooxidation for a longer time
New Way Chemistry for Hong Kong A-Level 3B91
33.8 Autooxidation of Fats and Oils (SB p.70)
Principle of BHA/BHT as Principle of BHA/BHT as AntioxidantsAntioxidants
• BHA and BHT are common antioxidants used in food
retard the development of oxidative rancidity in unsaturated fats and oils
New Way Chemistry for Hong Kong A-Level 3B92
33.8 Autooxidation of Fats and Oils (SB p.70)
Principle of BHA/BHT as Principle of BHA/BHT as AntioxidantsAntioxidants
• BHA and BHT work by:
donating the hydrogen atom of the OH group to the free
hydroperoxide radical (ROO •) involved in the autooxidation of fats and oils
stopping the chain reactions in oxidative spoilage:
AH + ROO • ROOH + A •
New Way Chemistry for Hong Kong A-Level 3B93
33.8 Autooxidation of Fats and Oils (SB p.70)
Principle of BHA/BHT as Principle of BHA/BHT as AntioxidantsAntioxidants
AH + ROO • ROOH + A •
where AH represents the antioxidant, and A • is a radical derived from the antioxidant
e.g.
New Way Chemistry for Hong Kong A-Level 3B94
33.8 Autooxidation of Fats and Oils (SB p.71)
Principle of BHA/BHT as Principle of BHA/BHT as AntioxidantsAntioxidants
Foods containing BHA and BHT
Check Point 33-8Check Point 33-8
New Way Chemistry for Hong Kong A-Level 3B96
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
New Way Chemistry for Hong Kong A-Level 3B97
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
New Way Chemistry for Hong Kong A-Level 3B98
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
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New Way Chemistry for Hong Kong A-Level 3B99
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)
New Way Chemistry for Hong Kong A-Level 3B100
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)
New Way Chemistry for Hong Kong A-Level 3B101
33.2 Redox Reactions (SB p.54)
(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
Answer(c) (i) Oxidation (ii)
Oxidation
(iii) Reduction (iv)
Reduction
(v) Oxidation
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New Way Chemistry for Hong Kong A-Level 3B102
33.3 Oxidation of Alkylbenzenes (SB p.56)
Why is tert-butylbenzene resistant to side-chainoxidation?
Answertert-Butylbenzene does not have a
benzylic hydrogen atom.
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New Way Chemistry for Hong Kong A-Level 3B103
33.3 Oxidation of Alkylbenzenes (SB p.56)
State the conditions under which ethylbenzene can be converted to benzoic acid in the laboratory. Answer
Reagents: 1. potassium
manganate(VII),
sodium hydroxide
2. dilute sulphuric acid
Condition: heating under reflux
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New Way Chemistry for Hong Kong A-Level 3B104
Draw the structural formulae for the major organic products in the following reactions:
(a) Propan-1-ol
(b) Propan-2-ol
K2Cr2O7/H+reflux
K2Cr2O7/H+reflux
33.4 Oxidation of Alcohols (SB p.60)
Answer(a) (b)
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New Way Chemistry for Hong Kong A-Level 3B105
33.5 Redox Reactions of Aldehydes and Ketones (SB p.63)
What is the species responsible for the reducingproperty of LiAlH4 and NaBH4?
AnswerHydride ion, H–
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New Way Chemistry for Hong Kong A-Level 3B106
Give the structural formulae for the major organic products of the following reactions:
(a) (b)
(c) (d)
33.5 Redox Reactions of Aldehydes and Ketones (SB p.64)
Answer
New Way Chemistry for Hong Kong A-Level 3B107
33.5 Redox Reactions of Aldehydes and Ketones (SB p.64)
(a) (b)
(c) CH3CH2OH (d)
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New Way Chemistry for Hong Kong A-Level 3B108
Give the structural formulae for the major organic products, if any, in the following reactions:
(a)
(b)
(c)
33.6 Redox Reactions of Carboxylic Acids (SB p.65)
Answer
(a)
(b)
(c) No reaction
New Way Chemistry for Hong Kong A-Level 3B109
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:
(d)
(e)
(f)
Answer
(d)
(e)
(f)
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New Way Chemistry for Hong Kong A-Level 3B110
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:
Answer
Hydrocarbon
Molecular formula
Products after ozonolysis
A C3H6
B C6H10
C C10H16
New Way Chemistry for Hong Kong A-Level 3B111
33.7 Redox Reactions of Alkenes (SB p.67)
A: As C3H6 can be expressed as CnH2n, the hydrocarbon is a compo
und with one C = C double bond. When A undergoes
ozonolysis, and are formed.
∴ The possible structure of A is CH3CH = CH2.
New Way Chemistry for Hong Kong A-Level 3B112
33.7 Redox Reactions of Alkenes (SB p.67)
B: As C6H10 can be expressed as CnH2n–2 and only one dicarbonyl co
mpound is formed on ozonolysis, the hydrocarbon is an alicyclic c
ompound with one C = C double bond.
∴ The possible structure of B is .
New Way Chemistry for Hong Kong A-Level 3B113
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 hydrocar
bon is an acyclic compound with three C = C double bonds.
∴ The possible structure of C is
CH3CH = CHCH2CH = CHCH2CH = CHCH3.
ozonolysis
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New Way Chemistry for Hong Kong A-Level 3B114
Give the structural formulae for the major organic products, if any, in the following reactions:
(a)
(b)
(c)
33.7 Redox Reactions of Alkenes (SB p.68)
Answer
New Way Chemistry for Hong Kong A-Level 3B115
33.7 Redox Reactions of Alkenes (SB p.68)
(a)
(b)
(c)
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New Way Chemistry for Hong Kong A-Level 3B116
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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