4.2.1 (b,c) Combustion and Oxidation of Alcohols

Syllabus

(b) Combustion of Alcohols

(i) the Oxidation of Primary Alcohols to form Aldehydes and Carboxylic acids; the control of the oxidation product using different reaction conditions

(ii) the Oxidation of Secondary Alcohols to form Ketones

(iii) the resistance to Oxidation of Tertiary Alcohols

{Equations should use [O] to represent the Oxidising Agent.}

What does this mean?

Combustion

All Alcohols can burn.

But as with Alkanes they need to be vapours.

So since short Alkanes (methanol, ethanol) evaporate easily they will burn easily.

They also need less Oxygen per mole so they are likely to burn completely and therefore without making soot or Carbon Monoxide.

They can both be produced from agricultural produce so could be biofuel.

The longer the Alcohol the less volatile it is and so the less easily flammable.

And the more Oxygen is required to burn one mole so the more likely they are to burn with sooty flames (and make Carbon Monoxide).

However, they would also provide more energy per mole.

You would be most likely to be asked to write an equation for the combustion of a named Alcohol.

This requires you to;

Be able to work out the formula of the Alcohol (from the general formula C_nH_2n+2O or by drawing it and counting atoms)
Recall that it should make nothing but Carbon Dioxide and Water.
Remember that (unlike Alkanes) there is an Oxygen atom in the Alcohol molecule when balancing.

The most commonly asked has always been Ethanol since this is the bio-fuel produced in highest quantities.

1- Ethanol is C₂H₅OH

2 - C₂H₅OH + O₂ → CO₂ + H₂O

3 - C₂H₅OH + O₂ → 2CO₂ + 3H₂O

4 - C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O

Oxidising Agent

To Oxidise an Alcohol requires an Oxidising Agent - a substance that can take away electrons by being reduced.

The usual (but not only) Oxidising Agent is Acidified Potassium Dichromate.

If asked this then you must include the word Acidified as Potassium Dichromate won't oxidise anything without a lot of concentrated acid.

You should also be able to say what you would observe during the reaction.

Potassium Dichromate is Orange.

When the reaction is over it will appear to be Green.

You will not be asked to write a full equation for the reaction at this stage.

But you should know that the Dichromate (VI) ion is Cr₂O₇^2-.

You should probably also know that the green colour is the Cr³⁺ ions they turn into.

You will be asked to write an equation for the Oxidation of an Alcohol using [O] to represent the Oxidising Agent.

You should treat this as an Oxygen atom for the purposes of balancing the equation you are writing.

But before you can start you need to know how Alcohols behave when Oxidised.

Oxidising Primary Alcohols

Primary Alcohols are first oxidised to Aldehydes.

And then are re-oxidised to Carboxylic Acids.

This can be done in two steps or all at once.

The Aldehyde functional group is a C=O bond at the end of a Carbon chain.

Although you shouldn't confuse this with the Carboxylic Acid functional group.

If you look at Equation 1 (left) you can see that no extra Oxygen atoms are needed to make Ethanal from Ethanol but an Oxygen atom is needed to remove two Hydrogen atoms, forming water.

So the balanced equation needs one [O] to represent this additional Oxygen atom.

Equation 2 is actually adding and Oxygen atom to the Ethanal to form Ethanoic acid.

No Hydrogen atoms are removed this time so only one extra [O] is needed.

If both reactions are carried out at the same time the overall equation would be:

CH₃CH₂OH + 2[O] → CH₃COOH + H₂O

Oxidising Secondary Alcohols

Primary Alcohols are oxidised to Ketones.

Unlike Aldehydes, Ketones cannot then be oxidised any further.

The Ketone functional group is a C=O bond in the middle of a Carbon chain.

Both Aldehydes and Ketones are sometimes referred to as Carbonyls.

They share some but not all their Chemistry.

Again, turning from an Alcohol to a Ketones doesn't involve adding any extra Oxygen atoms to the Carbon chain.

But it still involves removing Hydrogen atoms so an [O] is needed to form water.

The equation for Propan-2-ol would be:

CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O

Tertiary Alcohols

Tertiary Alcohols can't be Oxidised at all.

That's not particularly interesting but examiners like to make use of this fact.

A typical question might include a number of Alcohols being heated with a H⁺/Cr₂O₇^2- mixture.

You would be expected to work out that only those which do not turn green can be Tertiary.

Reflux and Distillation

It doesn't matter what you do to Tertiary Alcohols - nothing will change.

But to make sure that Secondary and Primary Alcohols are Oxidised by the Dichromate heat is needed.

One of the problems with this is that Aldehydes and Ketones can't Hydrogen bond and so are quite volatile.

So heating the mixture could evaporate the desired product.

To avoid this we set up a reflux - a condensor above the reaction vessel that condenses any evaporating product (or unreacted reactant) and returns it to the flask.

Unless you over-heat and destroy the product this means that every Alcohol molecule gets multiple chances to react, ensuring the maximum amount of product.

But it does mean that the product is mixed with remaining reactants.

This is the ideal method if we want to make a Ketone from a Secondary Alcohol or a Carboxylic Acid from a Primary Alcohol.

But if you wanted to make an Aldehyde from a Primary Alcohol this method would re-oxidise some/most of your product to a Carboxylic Acid.

We need an alternative method that will stop the reaction at the Aldehyde stage.

This time we use the condenser in the more usual orientation.

We don't even heat the mixture since the reaction is exothermic.

The heat generated is enough to evaporate the Aldehyde through the condenser and into the flask where it is immediately cooled to stop it disappearing into the air.

But the temperature is not high enough for the Alcohol to make it out of the flask because the Hydrogen bonds in Alcohols make the boiling temps higher.

And if any Carboxylic Acid was made it would also Hydrogen bond and would not be able to evaporate either.

Video

Exam-style Questions

1.(a) Ethanol can be oxidised to an Aldehyde and to a Carboxylic acid.

(i) Draw the structure of this Aldehyde and of this Carboxylic acid.

Structure of Aldehyde Structure of Carboxylic acid

(ii) Give a suitable reagent and reaction conditions for the oxidation of Ethanol to form the Carboxylic acid as the major product.

Reagent ...............................................................................................................................................................

Conditions ............................................................................................................................................................

........................................................................................................................................................................(5)

(ii) Draw the structure of an Alcohol containing four carbon atoms which can be oxidised to a Ketone.

(2)

(d) In the presence of a catalyst, Ethanol can be dehydrated to Ethene.

Give a suitable catalyst for use in this reaction.

.........................................................................................................................................................................(1)

2. (a) (i) Give a suitable reagent and state the necessary conditions for the conversion of Propan-2-ol into Propanone. Name the type of reaction.

Reagent……………………………………………………………................................................................................................……….…..

Conditions……………………………….…………………………………...............................................................................................…..

Type of reaction ….…………………………………………............................................................................................………………(3)

(b) Propanal is an isomer of Propanone.

(i) Draw the structure of Propanal.

(ii) A chemical test can be used to distinguish between separate samples of Propanone and Propanal. Give a suitable reagent for the test and describe what you would observe with Propanone and with Propanal.

Test reagent ……..………………………………………….............................................................................................…………………..

Observation with Propanone …….………......................................................................................…………………………………….

Observation with Propanone …….……………………………………………....................................................................................(4)

(Total 7 marks)

3. Four isomers with the formula C₄H₉OH are given below.

Isomer

CH₃CH₂CH₂CH₂OH

Name

butan-1-ol

2-methylpropan-2-ol

(i) Complete the naming of the isomers in the table above.

(ii) Name the type of isomerism shown by these four isomers.

.....................................................................................................................................

(Total 3 marks)

(b) One of the isomers in part (a) is resistant to oxidation by acidified potassium dichromate(VI).

(i) Identify this isomer.

...........................................................................................................................

(ii) This isomer can be dehydrated. Give a suitable dehydrating agent and write an equation for this dehydration reaction.

Dehydrating agent.............................................................................................

Equation ...........................................................................................................(3)

(b) (i) Identify the isomer in part (a) which can be oxidised to a ketone. Give the structure of the ketone formed.

Isomer ...............................................................................................................

Structure of the ketone

(ii) Identify one of the isomers in part (a) which can be oxidised to an aldehyde. Give the structure of the aldehyde formed.

Isomer ...............................................................................................................

Structure of the aldehyde

(c) Butan-1-ol can be oxidised to form a carboxylic acid. Using [O] to represent the oxidising agent, write an equation for this reaction and name the product.

Equation .....................................................................................................................

Name of product .........................................................................................................(2)

Answers

1(a) (i)

Structure of Aldehyde

Structure of Carboxylic acid

(1)

NOT CH3CHO

(1)

NOT CH3COOH

Penalise incorrect R group once

(ii) Reagent: Sodium (Potassium) dichromate (VI)

(VI not essential) (1) M1

Conditions: acidified or sulphuric acid (1) Can be with reagent M2

(heat under reflux) (1) M3

Or correct formula for M1 and M2

M2 depends on M1 (but M2 correct from Cr₂O₇^2–, K₂Cr₂O₇– etc

M3 mark independent

Credit KMnO₄ for M1

Ignore T and P for M2

(ii) (1) 2

(d) (i) Al₂O₃ or H₂SO₄ or H₃PO₄ (1)

Name or formula

2. (a) (i) Potassium (OR sodium) dichromate(VI) OR correct formula OR potassium manganate(VII) 1

(Oxidation state not needed, but must be correct if included)

(Penalise errors in the formula or oxidation state, but mark conditions)

Acidified OR H₂SO₄ / HCl (NOT with KMnO₄) / H₃PO₄ / HNO₃ 1

(Ignore heat or reflux)

(Credit “acidified” as part of reagent)

Oxidation or redox 1

(b) (i)

(Structure must show aldehyde structure)

(Credit C₂H₅ as alternative to CH₃CH₂)

(ii)

stays orange 1

(Provided reagent is correct, credit “no reaction”, “no change”, “nothing”, “no observation” for M2)

3. (i) 2

Allow e in the names

(ii) Structural (1) 3

OR chain and position(al)

[3]

(b) (i) 2-methylpropan-2-ol (1) OR the second one

(ii) Dehydrating agent: Conc Sulphuric/Phosphoric or AluminiumOxide(1) 3

Equation:

Allow C₄H₉OH in equation provided RHS is correct

if b(i) is blank, b(ii) equation must be full for credit

i.e. NOT C₄H₉OH

Mark consequential on b(i)

(b) (i) Isomer: butan-2-ol OR the fourth one

[look at name in table]

wrong isomer = CE

Structure of the ketone:

(ii) Isomer: butan-1-ol OR the first one

OR 2-methylpropan-1-ol OR the third one

[look at name in table]

Structure of the aldehyde:

Either

(or C₃H₇COOH) + H₂O (1)

Name of product: butanoic acid (1) 2

Accept butaneoic acid

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