Aldehydes & Ketones (AQA A Level Chemistry)

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.

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. 

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.

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.

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.

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

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

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.

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

Figure 1

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.

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.

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

Draw 3 dimensional structures of these stereoisomers.

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.

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.

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.

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). 

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.

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

Figure 1

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

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.

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

The density of propanal is 0.810 g cm⁻³. 

Calculate the percentage yield for this reaction.

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.

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

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

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

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) State 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.

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

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

Table 1

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.

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

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

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

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

Identify the nucleophile in this reaction.

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.

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

Outline the mechanism for the formation of compound B.

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.

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

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

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

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

Draw the skeletal formula of these isomers.