Arenes (Cambridge (CIE) A Level Chemistry): Exam Questions

4-nitromethylbenzene can be prepared via an electrophilic substitution reaction as shown in Fig. 1.1.

Fig. 1.1

i) This reaction also forms an isomer of 4-nitromethylbenzene as a by-product. Draw the structure of this by-product.

[1]

ii) Write an equation for the reaction between HNO₃ and H₂SO₄ that forms the electrophile for this reaction.

 [1]

iii) Describe how the structure and bonding of the six-membered ring in intermediate T differ from those in methylbenzene. In your answer refer to the hybridisation, the π bonding and the bond angles in the ring system.

[3]

Benzocaine is used as a local anaesthetic. It can be synthesised from 4-nitromethylbenzene by the route shown in Fig. 1.2.

Fig. 1.2

i) Give the systematic name of compound W.

[1]

ii) Suggest the reagents and conditions for step 1. step 1 .....................................................................................

[2]

iii) Suggest the reagent for step 2. step 2 ...................................................................................................

[1]

iv) Suggest the reagents and conditions for step 3 and step 4. step 3 ........................................................................................................ step 4 ........................................................................................................

[2]

A sample of benzocaine was analysed by carbon-13 NMR and proton NMR spectroscopy.

i) Predict the number of peaks in the carbon-13 NMR spectrum of benzocaine.

[1]

Benzocaine was dissolved in CDCl₃ and the proton NMR spectrum of this solution was recorded as shown in Fig. 1.3.

Fig. 1.3

ii) The data in Table 1.2 should be used in answering this question. Complete Table 1.1 for the chemical shifts δ 1.2 ppm, 3.5 ppm and 5.5 ppm.

Table 1.1

[3]

iii) Explain the splitting pattern for the absorption at δ 1.2 ppm.

[1]

Table 1.2

Benzocaine can also be used to synthesise the azo compound S by the following route.

Fig. 1.4

i) Suggest the reagent(s) used for step 1.

 [1]

ii) Suggest structures for compounds R and S and draw them in the boxes in Fig. 1.4.

[2]

Benzene can undergo electrophilic substitution with ethanoyl chloride in the presence of aluminium chloride.

i) Write an equation to show the formation of the electrophile.

[1]

ii) Draw the mechanism for the reaction.

[3]

The organic product from part (a) can be reduced to form an alcohol.

Name a suitable reducing agent and write a chemical equation to show this reduction, using [H] to represent the reducing agent.

Outline the mechanism for the reaction of the ethanoyl chloride with aluminium chloride to form the acylium ion needed for electrophilic substitution.

Explain how the catalyst reforms.

Compound A can be formed from ethylbenzene by the following route in Fig. 3.1.

Fig. 3.1 

Suggest reagents and conditions for the following steps.

Step 1 ......................................................................................................

Step 2 .......................................................................................................

Step 3 .......................................................................................................

Suggest the structures of the organic products of the reactions between each of the compound A and the following reagents. If no reaction occurs write ‘no reaction’.

Reagent  Product with compound A Na H⁺ / KMnO₄

Explain why ethyl-2-nitrobenzene is formed rather than ethyl-3-nitrobenzene.

Using your answer to (a), write the equation for the formation of the electrophile in step 1.

Methylbenzene can undergo different reactions to form the products shown in Fig. 4.1.

Fig. 4.1

Give the reagents and conditions for these two reactions.

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

reaction 2 ........................................................................................

i) Name the mechanism of reaction 1 in part (a)

[1]

ii) Draw the structure of the product obtained if reaction 1 is carried out using an excess of bromine.

[1]

Draw the reaction mechanism for reaction 2.

Nitrobenzene reacts in the same conditions as reaction 2 in part (a). Draw and name the product of this reaction.

Benzene reacts with concentrated nitric acid in the presence of concentrated sulfuric acid.

Describe the structure and bonding of a benzene molecule, C₆​H₆​. In your answer, refer to:

• the hybridisation of the carbon atoms

• the bond angles within the ring

• how the σ and π bonds are formed.

i) Write an equation to show the generation of the electrophile.

[1]

ii) Name and draw the fully labelled mechanism for the formation of nitrobenzene.

[4]

i) State the relative order of increasing acidity for ethanol, phenol, and ethanoic acid.

[1]

ii) Identify the electronic effect of the alkyl group that makes the ethoxide ion less stable than the phenoxide ion.

[1]

iii) Describe the distribution of the negative charge in the ethanoate ion that makes it the most stable of these three conjugate bases.

[1]

Predict the major product formed when phenol reacts with dilute nitric acid.

Benzene can be converted into 3-chlorobenzoic acid in a multi-step synthesis.

Suggest a three-step synthetic route for this conversion. In your answer, identify the reagents and conditions for each step and draw the structures of the intermediate compounds formed.

The nitration of benzene is the first important step in manufacturing dyes and explosives. 

i)

Write the equation for the generation of the electrophile.

[1]

ii)

State which reactant acting as a Brønsted-Lowry base and justify your answer.

[1]

iii)

Identify the conjugate acid in the reaction.

[1]

Compound B is produced in two steps as outlined in Fig. 1.1.

Fig. 1.1

i)

Outline the reagents and conditions required for the production of compound A drawn in Fig. 1.1.

[2]

ii)

Using curly arrows, describe the mechanism for step 1.

[3]

Draw the dot-and-cross diagram for the structure of the catalyst, once the electrophile has been generated in part (b).

Explain why benzene can generally only undergo substitution reactions.

Fig. 2.1 shows the formation of a ketone followed by an alcohol. 

Fig. 2.1

Draw the mechanism to show the formation of the catalyst responsible for step 1.

Draw the mechanism for the formation of the ketone, including the regeneration of the catalyst required.

State the reagents required for step 2.

Benzene can be converted into cyclohexane as shown in Fig. 3.1.

Fig. 3.1

i) For this reaction, name the type of reaction and identify the reagent and conditions needed.

type of reaction ................................................................................

reagent and conditions ................................................................................

[2]

 ii) State the bond angles in benzene and cyclohexane.

bond angle in benzene ..............................

bond angle in cyclohexane ..............................

Explain your answers.

[2]

When benzene reacts with SO₃, as shown in Fig. 3.2, benzenesulfonic acid is produced.

Fig. 3.2

The mechanism of this reaction is similar to that of the nitration of benzene. Concentrated H₂SO₄ is used in an initial step to generate the SO₃H⁺ electrophile as shown.

SO₃ + H₂SO₄ → SO₃H⁺ + HSO₄^– 

i) Draw a mechanism for the reaction of benzene with SO₃H⁺ ions in Fig. 3.3. Include all necessary curly arrows and charges.

Fig. 3.3

[3]

 ii) Write an equation to show how the H₂SO₄ catalyst is reformed.

 [1]

3-aminobenzoic acid can be synthesised from methylbenzene in three steps as shown in Fig. 3.4.

Fig. 3.4

i) Draw the structures of M and N in the boxes.

[2]

ii) Suggest reagents and conditions for each step of the synthesis.

step 1 ................................................................................

step 2 ................................................................................

step 3 ................................................................................

[3]