Aromatic Chemistry (AQA A Level Chemistry)

Describe the structure of cyclohexene. Explain why the molecule is not planar. Your answer should include any necessary bond angles.

The following information is known about the hydrogenation of cyclohexene: 

   cyclohexene + hydrogen → cyclohexane                  ΔH^Θ = -120 kJ mol^-1 

Use this data to predict a value for the hydrogenation of the theoretical compound, cyclohexa-1,3,5-triene. 

State and explain the stability of benzene compared to the theoretical cyclohexa-1,3,5-triene compound.

Many compounds in Chemistry contain delocalised electrons.

Use your knowledge of structure and bonding to explain which of the following compounds does not contain delocalised electrons:

• Graphite

• Poly(propene)

• Sodium

Benzene and three of its chemical derivatives, chlorobenzene, methylbenzene and phenol, are aromatic compounds which have a variety of industrial applications. 

Benzene is mainly used in the manufacture of other more complex chemicals, such as ethylbenzene and cumene. Billions of kilograms of ethylbenzene and cumene are produced annually.

Like benzene, chlorobenzene is used to make several other chemicals. It is also used to degrease car parts and in some pesticide formulations. 

In industry, methylbenzene is known by its common name, toluene. It is used as a solvent in paint thinner, permanent markers, contact cement and glue.

Phenol is a benzene ring with an -OH group attached in place of one of the hydrogen atoms, and its chemical derivatives are used in the manufacture of various plastics, nylon, detergents, herbicides and various pharmaceutical drugs.

Compare the relative reactivities of benzene, chlorobenzene, methylbenzene and phenol in terms of electrophilic substitution reactions. 

Benzene can undergo electrophilic substitution with ethanoyl chloride in the presence of a suitable catalyst.

Name the catalyst and write an equation to show the formation of the electrophile.

Outline the mechanism for this reaction

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.

The alcohol produced in part (b) is a mixture of isomers.

Draw the 3-dimensional representation of these isomers. Assuming that a single pure isomer was produced, describe a test which could be used to prove that the product was pure.

Propanoyl chloride can be reacted with the alcohol formed in part (b) to produce an ester.

Explain the expected observations during the reaction and draw the ester produced.

Ethylbenzene, C₆H₅CH₂CH₃, is an important chemical intermediate in the manufacture of polystyrene, pesticides, rubber, paint and inks.

The synthesis of ethylbenzene can occur via Friedel-Crafts alkylation using ethene, hydrogen chloride, benzene and an aluminium chloride, AlCl₃, catalyst.

Name the mechanism for the reaction of ethene and hydrogen chloride.

Write an equation for the subsequent formation of the carbocation that reacts with benzene to produce ethylbenzene.

Outline the mechanism for the reaction of benzene with the carbocation that was formed in part (a).

Ethylbenzene can also be formed by reacting ethanoyl chloride with benzene, in the presence of an AlCl₃ catalyst.

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

Aluminium chloride is used as the catalyst in the reaction between benzene and ethanoyl chloride.

Explain how the catalyst reforms.

Paracetamol is a common painkiller that is often manufactured in a multi-step process using 4-aminophenol. Phenol is a common organic chemical, comprised of a benzene ring with one hydroxyl group attached.    

Suggest a two-step synthetic route to form 4-aminophenol from phenol. 

Step 1 of your route should include reagents, conditions, equations to form the electrophile and the identity of the product.

Step 2 of your synthetic route should include the reagents to form 4-aminophenol from your Step 1 product.

The uncontrolled nitration of phenol can lead to the formation of 2,4,6-trinitrophenol, commonly known as picric acid.

This reaction would require formation of a dinitrophenol intermediate. Draw the possible isomers of this intermediate and name the type of isomerism displayed.

Paracetamol, figure 1, can be manufactured by the reaction of 4-aminophenol with ethanoyl chloride.

Figure 1

Name and outline the mechanism for this reaction.

A student started with 3.96 g of ethanoyl chloride (Mr = 78.5).

After preparation of impure paracetamol followed by recrystallisation, the student actually obtained 6.11 g of pure paracetamol (Mr = 151).

Calculate the percentage yield of pure paracetamol from this experiment.

2,4,6-trinitrotoluene (TNT) can be manufactured from benzene as shown in Figure 1.

Figure 1

Step 1 involves the Friedel-Crafts alkylation of benzene to produce methylbenzene, commonly known as toluene.

Calculate the percentage atom economy for Step 1.

Step 2 of Figure 1 involves the formation of a nitronium ion for the nitration of toluene, as shown in the following equation: 

HNO₃ + 2H₂SO₄ → NO₂⁺ + 2HSO₄^- + H₃O⁺ 

i) Explain the role of the nitric acid in the formation of the electrophile.

ii) Explain the role of the sulphuric acid in the overall nitration reaction.

Outline the mechanism, show in Step 2 of Figure 1, for the formation of 4-nitrotoluene (Compound A) from toluene.

The nitration steps can be hard to control and lead to the runaway formation of 2,4,6-trinitrotoluene, otherwise known as TNT.

A sample was taken during the nitration steps. Analysis of the sample showed the compound to have 7 peaks on its ¹³C-NMR spectrum.

State which of the nitro compounds, A, B or C, is in the sample tested. Justify your answer. 

Benzene is an important industrial chemical used in a variety of manufacturing processes. Cyclohexane is a cyclic hydrocarbon with the formula C₆H₁₂. 

i) Outline a reaction for the conversion of benzene into cyclohexane and include suitable reagents and conditions for this reaction. 

ii) In 1885 Kekule proposed a model for the structure and bonding for benzene, however there is evidence to suggest that this model may not be correct and there are alternative structures for benzene proposed. Explain why scientists doubted Kekule’s structure.

Two different structures when representing benzene are often shown in textbooks, both of which are shown in Figure 1.

Figure 1

 By using diagrams to help, describe the difference in orbital overlap the difference in bonding structures X and Y represent.

The theoretical molecule cyclohexa-1,3,5-triene reacts differently with bromine than 

i) Benzene will react with bromine in the presence of aluminium bromide. Draw the products of this reaction.

ii) Draw the products produced from the reaction of cyclohexa-1,3,5-triene with bromine.

Draw a diagram to show the resonance structure in a molecule of benzene.

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

i) Outline the generation of the electrophile for the nitration of benzene by writing an equation.

ii) In your equation, identify which reactant is acting as a Brønsted Lowry base.  Explain your answer and identify the conjugate acid and base pairs in the reaction. 

Compound B is produced in two steps. 

Figure 1

i) Outline the reagents and conditions required in for the production of Compound A drawn in Figure 1.

ii) Outline the mechanism for step 1.

Draw a Lewis structure for the structure of the catalyst once the electrophile has been generated. 

Explain why benzene can only undergo substitution reactions.

Benzene undergoes a substitution reaction to form compound X. Give the formulas of the reagents required for this conversion in the presence of a catalyst.

Figure 1

Compound X undergoes nitration on the benzene ring.

Outline a mechanism for this reaction.

N-phenylethanamide can be produced in two steps from nitrobenzene. Give the reagents required for both steps and draw the structures for the compounds formed.

State the name of the mechanism required for the second step of the reaction and describe the first step of the mechanism to produce the intermediate.  

Propene will react with benzene in the presence of aluminium chloride and hydrogen chloride.

Outline the mechanism that occurs in order to generate the positively charged carbon species in this reaction. 

Outline the mechanism for the formation of the final products for the reaction outlined in part (a).

State the type of reaction that occurs in part (b). 

Explain why a molecule of benzene won’t tend to react with ammonia to form phenylamine.  

The energy change for hydrogenation of cyclohexene is -120 kJ mol-1. However, when benzene undergoes hydrogenation, the energy change is 152 kJ mol-1 less than expected. 

Use this data to explain the relative stabilities of benzene and the theoretical cyclohexa-1,3,5-triene molecule.

When cyclohexa-1,3-diene undergoes hydrogenation it will have a different value than expected. 

Using the data given in part (a) explain why cyclohexa-1,3-diene has a different value than expected and suggest a value for the energy change for this reaction. 

Explain why the energy change for the hydrogenation of cyclohexa-1,4-diene will be different from the value for the hydrogenation of cyclohexa-1,3-diene.

Ibuprofen is an anti-inflammatory drug that is readily available. Its structure is shown in Figure 1.

Identify both an sp² and sp³ hybridised carbon atoms present in the ibuprofen structure.  Justify your answer. 

Figure 1