Aldehydes & Ketones — Structured Questions (Cambridge International A-Level Chemistry): Exam Questions

State the class of alcohol that can be oxidised to give:

i) an aldehyde

ii) a ketone

State the oxidation product of:

i) an aldehyde

ii) a ketone

Describe how 2,4-dinitrophenylhydrazine (2,4-DNPH) and melting point data can be used to determine the identity of an unknown carbonyl compound.

Butanone reacts with potassium cyanide, KCN, in the presence of a dilute acid catalyst to produce a hydroxynitrile by nucleophilic addition.

Complete the mechanism for this reaction.

Fig. 2.1

Extract:

Butanone reacts with potassium cyanide, KCN, in the presence of a dilute acid catalyst to produce a hydroxynitrile by nucleophilic addition.

State the systematic name of the organic product formed in part (a).

Explain why a number is not required to indicate the position of the carbonyl group in the name butanone.

Complete Table 3.1 to show the observations when an aldehyde and a ketone are each tested with Tollens' reagent.

Table 3.1

Complete Table 3.2 to show the observations when an aldehyde and a ketone are each warmed separately with Fehling's solution.

Table 3.2

Ethanal and propanal are each warmed separately with alkaline iodine solution, I₂(aq).

i) State which compound gives a yellow precipitate in this reaction.

ii) Explain your answer to part (c)(i).

Calcium and its compounds have a large variety of applications.

Calcium metal reacts readily with most acids.

When calcium metal is placed in dilute sulfuric acid, it reacts vigorously at first.

After a short time, a layer of calcium sulfate forms on the calcium metal and the reaction stops. Some of the calcium metal and dilute sulfuric acid remain unreacted.

Suggest an explanation for these observations.

Calcium ethanedioate is formed when calcium reacts with ethanedioic acid, HOOCCOOH.

Calcium ethanedioate contains one cation and one anion.

i) State the full electronic configuration of the cation in calcium ethanedioate.

ii) Deduce the charge on the cation.

iii) Draw the fully displayed formula of ethanedioic acid.

Calcium chlorate(I), Ca(ClO)₂, is used as an alternative to sodium chlorate(I), NaClO, in some household products.

i) The chlorate(I) ion is formed when cold aqueous sodium hydroxide reacts with chlorine.

Write an ionic equation for this reaction. State symbols are not required.

ii) The chlorate(I) ion is unstable and decomposes when heated as shown.

3ClO^– → 2Cl^– + ClO₃^–

This reaction can be described as a disproportionation reaction. Describe what is meant by disproportionation reaction.

iii) Deduce the oxidation number of chlorine in each species for the equation in (c)(ii).

Complete the boxes.

Calcium carbonate reacts with 2-hydroxypropanoic acid to form product Y.

Fig. 1.1

i) Identify the two other products of the reaction of 2-hydroxypropanoic acid with calcium carbonate.

Two possible methods of making 2-hydroxypropanoic acid are shown in Fig. 1.2.

Fig. 1.2

ii) State suitable reagents and conditions for reactions 1 and 3.

reaction 1 .......................................................................

reaction 3........................................................................

iii) Deduce the type of reaction that occurs in reaction 2.

iv) The reagent for reaction 4 is NaBH₄.

Identify the role of NaBH₄ in this reaction.

v) 2-hydroxypropanoic acid has a chiral centre.

State what is meant by chiral centre.

Compound W can be converted into three different organic compounds as shown by the reaction scheme in Fig. 2.1.

Fig. 2.1

Construct an equation for the formation of X, using [H] to represent one atom of hydrogen from the reducing agent.

Extract:

Compound W can be converted into three different organic compounds as shown by the reaction scheme in Fig. 2.1.

{align=center} Fig. 2.1

Product Y is formed from compound W as shown in Fig. 2.1.

i) State the specific type of stereoisomerism shown by product Y.

ii) Draw the mechanism for the nucleophilic addition of HCN to compound W. Show all relevant curly arrows, dipoles, and lone pairs.

iii) Explain how the mechanism leads to the formation of optical 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. Show your working.

A different carbonyl compound, Q, has an M_r of 72. Compound Q reacts with 2,4-dinitrophenylhydrazine but not with Tollens' reagent.

i) State the observation when Q reacts with 2,4-dinitrophenylhydrazine.

ii) State the functional group present in Q.

iii) Deduce the name or structural formula of compound Q.

Extract:

A different carbonyl compound, Q, has an Mr of 72. Compound Q reacts with 2,4-dinitrophenylhydrazine but not with Tollens' reagent.

ii) The functional group present in Q:

• Ketone

iii) Compound Q is:

• Butanone / CH3COCH2CH3

Compound Q can be reduced to compound R.

i) State a suitable reducing agent for this reaction.

ii) State the functional group present in compound R.

iii) Draw the structural formula of compound R.

2-hydroxybutanoic acid can be synthesised from propanal via a reaction with hydrogen cyanide and subsequent hydrolysis of 2-hydroxybutanenitrile. A possible pathway is shown in Fig. 3.1. The incomplete mechanism for reaction 2 is shown in Fig. 3.2.

compound A $\overset{\mathrm{reaction}1}{\rightarrow}$ propanal $\overset{\mathrm{reaction}2}{\rightarrow}$ 2-hydroxybutanenitrile $\overset{\mathrm{reaction}3}{\rightarrow}$ 2-hydroxybutanoic acid

Fig. 3.1

Fig. 3.2

i) State the reagents and conditions required for reaction 2.

ii) Complete the mechanism for this reaction, showing all relevant curly arrows, lone pairs, and partial charges.

2-hydroxybutanenitrile contains a chiral centre. Draw the three-dimensional structures of the two optical isomers.

Extract:

2-hydroxybutanoic acid can be synthesised from propanal via a reaction with hydrogen cyanide and subsequent hydrolysis of 2-hydroxybutanenitrile. A possible pathway is shown in Fig. 3.1.

compound A $$\overset{reaction 1}{\rightarrow}$$ propanal $$\overset{reaction 2}{\rightarrow}$$ 2-hydroxybutanenitrile $$\overset{reaction 3}{\rightarrow}$$ 2-hydroxybutanoic acid

State the reagent(s) for reaction 3.

Compound A forms propanal when oxidised with acidified potassium dichromate(VI) under distillation conditions.

Draw the structural formula of compound A.

Compound A (propan-1-ol) and propanal are each tested with the reagents shown in Table 3.1.

Complete Table 3.1 by identifying which compound(s) give a positive result for each test. If neither gives a positive result, write 'neither'.

Table 3.1

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

State the systematic names of two isomers with the formula C₃H₆O that cannot be reduced by aqueous NaBH₄.

Extract:

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

State the systematic names of the two isomers with the formula C₃H₆O that can be reduced by aqueous NaBH₄.

When NaBH₄ is used as a reducing agent, carbonyl compounds are reduced to alcohols.

i) State the class of alcohol formed when each of the following is reduced by NaBH₄:

an aldehyde (e.g. propanal)

a ketone (e.g. butanone)

ii) Explain why the reduction product of butanone can exhibit optical isomerism, but the reduction product of propanal cannot.

Extract:

When NaBH4 is used as a reducing agent, carbonyl compounds are reduced to alcohols.

Classes of alcohol formed:

• An aldehyde (e.g. propanal) is reduced to a primary alcohol
• A ketone (e.g. butanone) is reduced to a secondary alcohol

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

This question is about the reactions of an organic compound R.

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 R. Show your working.

ii) The relative molecular mass, M_r, of compound R is 86.0. Deduce the molecular formula of compound R.

Extract:

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.

ii) The relative molecular mass, Mr, of compound R is 86.0. Deduce the molecular formula of compound R.

Molecular formula = C5H10O

Compound R is a straight chain organic compound. Compound R reacts with HCN, followed by dilute acid, to form compound S, which shows optical isomerism.

Compounds R and S were tested with acidified potassium dichromate(VI) and Tollens' reagent. The results are shown in Table 2.1.

Table 2.1

Using this information and your answer from part (a), deduce the identities of compounds R and S. Justify your answer.

Extract:

Compound R is a straight chain organic compound. Compound R reacts with HCN, followed by dilute acid, to form compound S, which shows optical isomerism.

Compound S is 2-hydroxyhexanenitrile.

Draw the three-dimensional structures of the two optical isomers of compound S.

Extract:

Compound R is a straight chain organic compound. Compound R reacts with HCN, followed by dilute acid, to form compound S, which shows optical isomerism.

Compound S is formed from compound R (pentanal) by reaction with HCN and compound S is 2-hydroxyhexanenitrile.

Draw the skeletal formulae of two straight-chain positional isomers of compound S that do not exhibit optical isomerism.