new way chemistry for hong kong a-level 3b 1 redox reactions 33.1organic synthesis 33.2redox...

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


33.133.1Organic Organic

SynthesisSynthesis


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



Organic Organic SynthesisSynthesis• A synthesis usually involves more than

one step

Target molecule

1st Precursor

2nd Precursor

Starting Starting materialmaterial



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



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




• 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?




A B C D E

60 % conversion

60 % conversion

60 % conversion

60 % conversion

Yield of the desired product

= 60 % 60 % 60 % 60 %

= 12.96 %




• 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



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)



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


33.233.2Redox Redox

ReactionsReactions


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



OxidationOxidation

• Oxidation of an organic compound usually corresponds to:

an increase in oxygen content

a decrease in hydrogen content



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



OxidationOxidation• e.g.

Converting ethanol to ethanal is also an oxidation process

the hydrogen content of ethanal is lower than that of ethanol



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)



ReductionReduction

• Reduction of an organic compound usually corresponds to:

an increase in hydrogen content

a decrease in oxygen content



ReductionReduction• e.g.

Converting ethanoic acid to ethanal is a reduction

the oxygen content of ethanal is lower than that of ethanoic acid



ReductionReduction• e.g.

Converting ethanal to ethanol is also a reduction process

the hydrogen content of ethanol is higher than that of ethanal



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)



33.333.3Oxidation of Oxidation of AlkylbenzeneAlkylbenzene

ss


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



AlkylbenzenesAlkylbenzenes

• Examples of alkylbenzenes:



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



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




• All alkylbenzenes are oxidized to benzoic acid

except the alkylbenzenes with a tertiary alkyl group

they do not have a benzylic hydrogen atom



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



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



Oxidation of Oxidation of AlkylbenzenesAlkylbenzenes• e.g.


33.433.4Oxidation of Oxidation of

AlcoholsAlcohols


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



AlcoholAlcoholss• Examples of alcohols:



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



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



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



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



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



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



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



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



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



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




A distillation apparatus used for the separation of ethanoic acid from the reaction mixture



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




• 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”




Demonstration of the use of the breathalyser



Oxidation of Secondary Oxidation of Secondary AlcoholsAlcohols

• Secondary alcohols can be oxidized to ketones by either acidified K2Cr2O7 or acidified KMnO4



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



Oxidation of Secondary Oxidation of Secondary AlcoholsAlcohols• e.g.

Propan-2-ol can be oxidized to form propanone



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




• 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




• e.g.

2-Methylbutan-2-ol Propanone Ethanoic acid

heat



33.533.5Redox Redox

Reactions of Reactions of Aldehydes Aldehydes

and Ketonesand Ketones


33.5 Redox Reactions of Aldehydes and Ketones (SB p.62) Aldehydes and Aldehydes and

KetonesKetones• Carbonyl compounds that contain the

carbonyl group


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




• 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




• With the action of hot acidified KMnO4,

the CC bonds in ketones would be broken

a mixture of carboxylic acids would be formed


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)



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



CompoundsCompounds



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



CompoundsCompounds



33.633.6Redox Redox

Reactions of Reactions of Carboxylic Carboxylic

AcidsAcids


33.6 Redox Reactions of Carboxylic Acids (SB p.64)

Carboxylic Carboxylic AcidsAcids• A group of organic compounds containing

the carboxyl group

• Examples:



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



33.733.7Redox Redox

Reactions of Reactions of AlkenesAlkenes


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



AlkeneAlkeness

• Alkenes can accept hydrogen to form alkanes

alkenes are also able to undergo reduction reactions



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




• Ethene is oxidized to ethane-1,2-diol

• The manganate(VII) ions are reduced to manganese(IV) oxide




• 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



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+)



Oxidation of Alkenes by Oxidation of Alkenes by OzoneOzone• The reduction produces carbonyl compounds

can be safely isolated and identified



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



Oxidation of Alkenes by Oxidation of Alkenes by OzoneOzone• e.g.

Ozonolysis of but-1-ene gives two different aldehydes



Oxidation of Alkenes by Oxidation of Alkenes by OzoneOzone

• e.g.

Ozonolysis of but-2-ene gives one aldehyde

Example 33-7Example 33-7



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


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


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



33.833.8AutooxidatioAutooxidatio

n of Fats n of Fats and Oilsand Oils


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



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



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



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



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



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



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




• 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 •




AH + ROO • ROOH + A •

where AH represents the antioxidant, and A • is a radical derived from the antioxidant

e.g.




Foods containing BHA and BHT



The END



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



(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



(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



(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)



(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)



(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

Back



Why is tert-butylbenzene resistant to side-chainoxidation?

Answertert-Butylbenzene does not have a

benzylic hydrogen atom.

Back



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

Back


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


Answer(a) (b)

Back



What is the species responsible for the reducingproperty of LiAlH4 and NaBH4?

AnswerHydride ion, H–

Back


Give the structural formulae for the major organic products of the following reactions:

(a) (b)

(c) (d)


Answer



(a) (b)

(c) CH3CH2OH (d)

Back


Give the structural formulae for the major organic products, if any, in the following reactions:

(a)

(b)

(c)


Answer

(a)

(b)

(c) No reaction




(d)

(e)

(f)

Answer

(d)

(e)

(f)

Back



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



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.



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 .



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

Back



(a)

(b)

(c)


Answer



(a)

(b)

(c)

Back



(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.

Back

new way chemistry for hong kong a-level 3b 1 redox reactions 33.1organic synthesis 33.2redox...

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