AQA A Level Chemistry

Topic Questions

7.2 Aldehydes & Ketones (A-level only)

1a3 marks

Compare the structures of propanal and propanone shown in Figure 1.

Figure 1

1-3

1b2 marks

State the reactants that can be oxidised to give an aldehyde and a ketone as products.

1c2 marks

State the oxidation products, where appropriate, of an aldehyde and a ketone.

1d1 mark

Name the oxidising agent that gives a colour change of orange to green when:

  • Primary alcohols are oxidised to aldehydes
  • Secondary alcohols are oxidised to ketones
  • Aldehydes are oxidised to carboxylic acids (under reflux)

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2a2 marks

State two chemical reagents that can be used as definitive tests to distinguish between aldehydes and ketones.

You are not expected to describe the results of these tests.

2b4 marks

Table 1 shows the initial and final observations for the reactions of the reagents in part (a) with aldehydes and ketones.

The initial and final observations for ketones have been completed.

Table 1

Test

Ketone

Aldehyde

Initial observation

Final observation

Initial observation

Final observation

 

Blue solution

Blue solution

Blue solution

 
 

Colourless solution

Colourless solution

Colourless solution

 


Identify the correct chemical tests and complete the final observations for aldehydes.

2c1 mark

State the formula of the linear silver complex in Tollens' reagent, that is also known as ammoniacal silver nitrate. 

2d2 marks

Name another chemical reagent that can be used to distinguish between aldehydes and ketones.

State the extra condition for this reagent to be used as a test for aldehydes and ketones.

This reagent is not specific for only aldehydes and ketones.

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3a1 mark

Identify the type of reaction that converts aldehydes and ketones to their corresponding parent alcohol.

3b1 mark

Name a reducing agents that can convert aldehydes and ketones into their corresponding alcohols.

3c1 mark

Name the organic product formed by the reduction of methylpropanal, Figure 1, with lithium tetrahydridoaluminate.

Figure 1

2-2

3d4 marks

Name and outline the mechanism for the reduction of propanone by sodium tetrahydridoborate, NaBH4.

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4a2 marks

Explain why the carbonyl bond of an aldehyde or ketone is polar.

4b2 marks

Describe the structure of a carbonyl compound that allows it to sometimes produce optical isomers when reacted with chemicals such as hydrogen cyanide.

4c2 marks

Draw the optical isomers formed by the reaction of propanal with hydrogen cyanide.

4d1 mark

Name the product of the reaction between ethanal and hydrogen cyanide, Figure 1.

Figure 1

3-2

4e2 marks

Explain why the reaction of propanone with hydrogen cyanide does not form optical isomers.

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5a1 mark

Draw the displayed formula of the functional group isomer of butanone.

5b1 mark

Explain why butanone has no positional isomers.

5c2 marks

Draw the organic product, where appropriate, for the reaction of butanal with:

i)
Sodium tetrahydridoborate, NaBH4.

ii)
Ammoniacal silver nitrate solution.
5d2 marks

Name the organic product, where appropriate, for the reaction of butanone with:

i)
An excess of acidified potassium dichromate(VI) solution, K2Cr2O7, under reflux.

ii)
Lithium tetrahydridoaluminate, LiAlH4.

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1a2 marks

Aqueous sodium tetrahydridoborate, NaBH4, is a common reducing agent.

Identify two isomers with the formula C3H6O that cannot be reduced by aqueous NaBH4.

1b2 marks

Identify the two isomers with the formula C3H6O that can be reduced by aqueous NaBH4.

1c4 marks

When NaBH4 is used as a reducing agent followed by the addition of acid, the reduction products of ketones can exhibit optical isomerism, while the reduction products of aldehydes cannot.

i)
Classify the reduction products of aldehydes and ketones.
 

ii)
Explain why the reduction products of ketones can exhibit optical isomerism, while the reduction products of aldehydes cannot.
1d2 marks

Explain why the reduction product of a carbonyl compound, by NaBH4 and acid, cannot be a tertiary alcohol.

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2a3 marks

The boiling points of three organic compounds are shown in Table 1.

Table 1

Chemical

Boiling point / OC

Butane 

-1

Propanal 

49

Propan-1-ol 

97

 

i)
Suggest why the three organic compounds might be expected to have similar boiling points.

ii)
Explain why the three organic compounds do not have similar boiling points.

2b2 marks

Suggest a value for the boiling point of propanone. Justify your answer.

2c3 marks

Explain why there are no hydrogen bonds between propanone molecules, even though propanone forms hydrogen bonds in water.

2d2 marks

Describe one test tube reaction that could be used to distinguish between samples of propanal and propanone.

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3a1 mark

An organic compound, A, has the molecular formula C5H8O and undergoes nucleophilic addition with sodium tetrahydridoborate. 

Identify the nucleophile in this reaction.

3b5 marks

The organic compound, A, reacts with sodium tetrahydridoborate, NaBH4, to form a symmetrical, secondary alcohol, B, as shown.

A + 2[H] → B


Identify the organic compounds A and B. Justify your answer.

3c2 marks

Suggest one chemical test, including the expected result, that helps to prove your structure of compound B in part (b).

3d2 marks

Outline the mechanism for the formation of compound B.

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4a4 marks

Analysis of 15.0 g of an organic compound, R, showed it to contain 69.8% carbon,2.79 g of oxygen and the remaining mass was hydrogen.

i)
Calculate the empirical formula of the organic compound R.

ii)
Compound R has a molecular mass of 86.0 g mol-1, deduce the molecular formula of compound R.
4b4 marks

Compound R is a straight chain organic compound. One isomer of compound R produces a racemic mixture of compound S when reacted with potassium cyanide followed by dilute acid.

Compounds R and S were tested with acidified potassium dichromate(VI) and Fehling’s solution. The results are shown in Table 1.

Table 1

 

R

S

Acidified potassium dichromate(VI) 

Green solution

Green solution

Fehling’s solution 

Brick-red precipitate

No visible change

Using this information and your answer from part (a), identify compounds R and S.
Justify your answer.

4c1 mark

Draw 3D representations of the two isomers responsible for the racemic mixture of compound S.

4d1 mark

There are some isomers of compound S that do not display optical isomerism.

Draw the skeletal formula of these isomers.

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1a6 marks

Acidified potassium dichromate(VI) solution and Tollens’ reagent are commonly used to distinguish between organic compounds with different functional groups.

 
i)
State how acidified potassium dichromate(VI) solution could be used to distinguish between propanal, propanone and propan-1-ol through a series of test tube reactions.
You should include any issues that may arise.
 
ii)
Write chemical equations for any identified reactions in part (i), using [O] to represent the acidified potassium dichromate(VI) solution.
1b2 marks

After testing propanal, propanone and propan-1-ol with acidified potassium dichromate(VI) solution as described in part (a), a second set of test-tube reactions was performed.  

Suggest how the organic compounds, propanal, propanone and propan-1-ol were further identified using Tollens’ reagent. 

1c3 marks

In separate test tubes, 2,4-dinitrophenylhydrazine was used to test the organic  compounds, propanal, propanone and propan-1-ol. When 2,4-dinitrophenylhydrazine is added to an organic compound, the observation if a carbonyl group is present is a yellow-orange precipitate. 

Explain why 2,4-dinitrophenylhydrazine does not confirm the presence of propanal and suggest an alternative test tube reaction, that has not been performed yet, to prove that one of the organic compounds is propanal.

1d1 mark

Acidified potassium dichromate(VI) solution is less expensive than Tollen’s reagent. 

Suggest why Tollens’ reagent is still used as the oxidising agent in the specific test for aldehydes.

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2a1 mark

NaBH4 is a common reducing agent used in Organic Chemistry.

Identify the organic compound that would undergo reduction, using NaBH4, to produce butan-2-ol.

2b5 marks

Propanone can also be converted into a secondary alcohol.
Name and outline the mechanism for this reaction.

2c3 marks

Explain why the secondary alcohol produced in part (b) demonstrates no optical activity. 

2d1 mark

A racemic mixture was formed from the reaction of the aldehyde pentanal with KCN, followed by dilute acid. 

Draw the displayed formulae of the organic products responsible for the racemic mixture.

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3a2 marks

Propanal can be converted into 2-hydroxybutanoic acid by a two-step synthesis as outlined in Figure 1

Figure 1

2-3

Hydrogen cyanide is a weak acid that can be used to extend the length of  a carbon chain in a controlled fashion. Potassium cyanide is often used as an alternative due to the well documented poisonous effects of hydrogen cyanide. 

Use your knowledge of rates of reaction to explain another reason why potassium cyanide would be the preferred reagent to perform Step 1.

3b6 marks

In Step 1 of the reaction sequence shown in part (a), propanal is converted into an intermediate. 

i)
Name and outline the mechanism to form this intermediate.

ii)
Give the IUPAC name of the intermediate product formed.

3c2 marks

The products formed when propanal reacts with KCN exhibit optical isomerism.

Draw 3 dimensional structures of these stereoisomers.

3d2 marks

An isomer of propanal undergoes the Step 1 reaction as outlined in Figure 1 from part (a),  but the product of this reaction has no effect on plane polarised light.

Draw the product and explain why there is no effect on plane polarised light.

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4a4 marks

Three compounds, X, Y and Z, were found to be structural isomers with a relative molecular mass of 72.

Analysis shows them to contain the following percentages by mass: 

C - 66.7%    H - 11.1%     O - 22.2%

Calculate the molecular formula of the isomers.

4b4 marks

All three isomers, X, Y and Z, show a sharp IR absorption peak at around 1750 cm-1.  

Use this information and your answer to part (a) to suggest possible structural formulae for the isomers, X, Y and Z.

4c7 marks

Certain reagents can be used to oxidise two of the three isomers, X, Y and Z.  

i)
Name two reagents that will only react with two of the three isomers and state the expected observation for each.

ii)
Write equations using structural formulae to show the oxidation reactions which you have identified in part (i). 
4d3 marks

Spectroscopic methods are commonly used to identify chemicals.

i)
State why IR spectroscopy alone is not enough to distinguish between X, Y and Z. 

ii)
Explain what you would expect to see on the 13C NMR spectra of X, Y and Z.
Your answer does not need to include specific 13C NMR chemical shifts from the Data Table.

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5a1 mark

Compound W can be converted into three different organic compounds as shown by the reaction scheme in Figure 1.

Figure 1

6-2

Write an equation for the formation of X. Use [H] to represent the reagent in the equation.

5b5 marks

Identify the specific type of isomerism shown by the product Y, in Figure 1

Outline the mechanism to show the formation Y and explain how this leads to the isomers of Y.

5c3 marks

When 5.00 cm3 of propanal (Mr = 58.0) were reacted with an excess of acidified potassium dichromate (VI) solution, 4.25 g of propanoic acid (Mr = 74.0) were obtained.

The density of propanal is 0.810 g cm−3

Calculate the percentage yield for this reaction.

5d4 marks

In another experiment, a student made propanoic acid from propanal, however they did not see the orange to green colour change that they were expecting. 

The student did not see the initial colour of the mixture but at the end of their experiment they described their reaction mixture as a dull and murky orange colour. 

i)
Suggest the reagent that the student had used and explain the colour change that they observed.

ii)
Suggest another reagent that the student could not have used and explain why.

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