Benzene & Aromatic Compounds (OCR 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.

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.

Explain why a molecule of benzene, C₆H₆ won’t tend to react with bromine whereas hexene, C₆H₁₂, will react to form dibromohexane, C₆H₁₂Br₂. 

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.

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.

State the reagents and the name of the mechanism used to convert benzene into nitrobenzene.

Using your answer to (a), formulate the equation for the formation of the nitronium ion.

Using curly arrows to indicate the movement of electron pairs, explain the mechanism for the nitration of benzene.

Nitrobenzene can be converted into aniline in a two-step process. State the reagents for this conversion.

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 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.

A student reacted phenol with bromine at room temperature forming a white precipitate. Draw the structure for the organic product that forms the white precipitate.

When phenol reacts with sodium hydroxide it acts as a weak acid. Explain why phenol can act as a weak acid.

A student investigated two reactions of phenylethene, C₆H₅CHCH₂. First she reacted phenylethene with excess bromine at room temperature to form Compound A. She then added aluminium bromide, AlBr₃ to the reaction mixture to form Compound B. 

Draw the structures of Compound A and B. 

Name and outline the mechanism for the formation of Compound B from Compound A.

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

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 involves the formation of a nitronium ion for the nitration of Toluene,
as shown in the following equation:

HNO₃ + 2H₂SO₄ → NO₂⁺ + 2HSO₄^- + H₃O⁺

This question is about the reactions of phenol, cyclohexene and benzene. 

Explain the difference in bromination of phenol and benzene. In your answer, include structures of any organic products formed in the reaction. 

Explain why cyclohexene does not require aluminium bromide when it undergoes bromination. 

A mixture of 2-nitrophenol and 4-nitrophenol can be formed via the pathway shown in Figure 1.

Figure 1

[4]

Phenol reacts with sodium to form a salt and hydrogen gas in an acid base reaction.

Explain why phenol behaves as a weak acid.

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

Outline the generation of the electrophile for the nitration of benzene by writing and indicating in your equation 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.

Draw the Lewis structure for the structure of the catalyst once the electrophile has been generated in part b).

Explain why benzene can only undergo substitution reactions. 

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.

Figure 1 shows the formation of a ketone followed by an alcohol. 

Using curly arrows, describe a mechanism to show the formation of the catalyst responsible for step 1.

Using curly arrows, describe the mechanism for the formation of the ketone, including the regeneration of the catalyst required.

State the reagents required for step 2.

Nitration of a benzene ring forms nitrobenzene. 

The structures of the products have the same molecular formula. Explain why these products are different. In your answer include the structures of compound X and Y and reagents for reactions A and B. 

Using curly arrows, describe the mechanism for reaction A. 

A student suggests that both 4-bromophenylamine and compound Y will have the following peaks in the 13C NMR spectrum

• Between 20 and 50 ppm for the C-N bond
• Between 30 and 70 ppm for the C-Br bond

Evaluate the student’s suggestion

State the IUPAC name of Compound Z and draw the structure.