Organic Chemistry: Arenes (Edexcel International A Level Chemistry)

Topic Questions

1a6 marks

Benzoic acid is a white crystalline solid with the structure shown.

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It is found in many plants as it is an important building block for the biosynthesis of a variety of compounds, such as plant hormones and attractants for pollinators.

The role of benzoic acid in the chemical industry is also widespread and approximately 500000 tonnes are produced annually. It is used in the synthesis of many compounds, including medicines, dyes and insect repellents.

Such synthetic dyes are often classified as aryl azo dyes. These dyes have a range of vivid colours and a wide range of uses in many industries, including food and textiles. Their synthesis involves the formation of a diazonium ion. This ion then reacts with a phenol in a coupling reaction, to form the dye. The relative simplicity of the reactions involved and ready availability of starting materials make azo dyes cheap to produce.

Salts of benzoic acid, such as calcium benzoate and sodium benzoate, are used in the food industry as preservatives.

Devise a reaction scheme to produce benzoic acid from benzene, via bromobenzene and then a Grignard reagent.

Include the reagents and essential conditions for each step and give the name or structure of each of the intermediate compounds.

Details of practical procedures and reaction mechanisms are not required.

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1b9 marks

Benzoic acid can be used in the synthesis of azo dyes.

In Step 1, benzoic acid reacts with concentrated nitric acid to form 3‐nitrobenzoic acid.

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Draw the mechanism for the reaction, using appropriate curly arrows.

Include equations showing the role of the catalyst and how it is regenerated.

(5)

ii)

In Step 2, the 3‐nitrobenzoic acid reacts to form 3‐aminobenzoic acid.

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State the reagents required for this reaction.

(1)

iii)

In Step 3, the 3‐aminobenzoic acid reacts with sodium nitrite and dilute hydrochloric acid, forming a diazonium ion.

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State a temperature at which this reaction should take place, giving one reason for your answer.

(2)

iv)

Draw the structure of the azo dye formed when the diazonium ion reacts with phenol.

(1)

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6 marks

Hydrated calcium benzoate is used as a preservative in soft drinks.
It has the formula Ca(C₆H₅COO)₂·xH₂O.

2.60 g of hydrated calcium benzoate was dissolved in deionised water.
Excess lead(II) nitrate solution was added, forming a precipitate of lead(II) benzoate, Pb(C₆H₅COO)₂(s). This precipitate was removed and dried.
The mass of the dry solid was 3.89 g.

Calculate the molar mass of hydrated calcium benzoate and hence deduce the value of x.

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1a2 marks

This question is about benzene, C₆H₆ , a colourless liquid first isolated in 1825, and some related compounds.

Three C₆H₆ structures proposed in the 1860s are shown.

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The delocalised model for the structure of benzene has been accepted since the 1930s following the study of its X‐ray diffraction pattern and the understanding of electron delocalisation in bonding theory.

The Dewar and Ladenburg structures have since been isolated as stable compounds but there is no compound with the Kekulé structure.

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Describe a chemical test, including the result, that could distinguish the Dewar structure from benzene.

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2 marks

State one similarity and one difference you would expect in the low resolution proton NMR spectrum of the Ladenburg structure and that of benzene.

You must include data from the Data Booklet to support your answer.

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1c2 marks

Explain how X‐ray diffraction shows that benzene has a delocalised structure and not a Kekulé structure.

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3 marks

The Ladenburg and Dewar structures both isomerise to benzene.
The enthalpy changes are –376 kJ mol^–1 and –297 kJ mol^–1 respectively.

Draw a labelled enthalpy level diagram showing the relative thermodynamic stability of the Ladenburg structure, the Dewar structure and benzene.
Include the enthalpy change values in kJ mol^–1.
Your diagram does not need to be to scale.

(2)

ii)

Give a possible reason why the isomerisation of the Dewar structure to benzene has a lower activation energy than that of the Ladenburg structure to benzene.

(1)

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2 marks

The enthalpy change of hydrogenation of hex‐3‐ene is –118 kJ mol^–1.

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The table shows the enthalpy changes of hydrogenation of two further alkenes containing six carbon atoms.

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Use your knowledge of benzene thermochemistry to suggest explanations for both of these enthalpy changes of hydrogenation in relation to the value for hex‐3‐ene.

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1f8 marks

Methylbenzene, C₆H₅CH₃ , reacts with ethanoyl chloride, CH₃COCl, in the presence of the catalyst aluminium chloride, AlCl₃ , to form a mixture of organic products with the formula CH₃COC₆H₄CH₃.

C₆H₅CH₃ + CH₃COCl → CH₃COC₆H₄CH₃ + HCl

Draw the skeletal formulae of three different arenes with the formula CH₃COC₆H₄CH₃ .

(2)

ii)

The ¹³C NMR spectrum of one of these arenes, compound X, is shown.

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Identify compound X.
Use the number of peaks on the ¹³C NMR spectrum to justify your answer.

(2)

iii)

Complete the diagram, including curly arrows, to show the mechanism for the formation of compound X in this reaction.
Include an equation for the regeneration of the catalyst.

(4)

CH₃COCl + AlCl₃ → CH₃CO⁺+ AlC $l_{4}^{-}$

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3a5 marks

Coffee Chemistry

There are over a thousand chemical compounds in coffee and their physiological effects are the subject of considerable speculation and research. The verdict on coffee is contradictory: some of the compounds have been identified as toxic and even carcinogenic but others are antioxidants associated with cancer prevention. Recent research has identified compounds in coffee that might be used in the treatment of prostate cancer.

By far the best known compound in coffee is caffeine, the most widely consumed psychoactive drug in the world. In small amounts it is a stimulant but doses in excess of 10 g per day are toxic. Caffeine contains amide and amine groups.

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Chlorogenic acid is responsible for the acidic taste of coffee. It is an antioxidant and has also been shown to slightly decrease blood pressure.

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Caffeic acid, quinic acid and acetoin are also present in coffee.

Another way of drawing the structure of caffeine is shown.

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The bonding represented by this diagram of caffeine differs from that given in the passage.

Explain what this diagram indicates about the bonding in caffeine, stating the effect on the structure of caffeine.

(3)

ii)

Suggest why caffeine is a much weaker base than a primary amine such as ethylamine, even though the right-hand ring has two amine groups.

(2)

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3b6 marks

A 200 cm³ cup of coffee contains approximately 85 mg of caffeine.

Calculate the concentration, in mol dm⁻³, of caffeine in this cup of coffee.

Give your answer to an appropriate number of significant figures.

(4)

ii)

The removal of caffeine from the body is a first order reaction with a half-life of between three and seven hours for an adult.

An adult drinks coffee containing a total of 160 mg of caffeine.

Calculate to the nearest hour the minimum time needed for the amount of caffeine in their body to drop to 20 mg.

(2)

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3c6 marks

Chlorogenic acid is an ester of caffeic acid, a compound that is present in all plants.

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A student suggested that caffeic acid could be synthesised by an electrophilic substitution of 1,2-dihydroxybenzene.

Draw the mechanism of this electrophilic substitution, including the formation of a suitable electrophile.

(5)

ii)

Deduce the structure of quinic acid which combines with caffeic acid to form chlorogenic acid.

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(1)

quinic acid

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3d3 marks

The structure of acetoin is shown with one of the proton environments labelled.

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Identify the other proton environments of acetoin on the structure and label them B, C etc.

(1)

ii)

Complete the table to show the splitting pattern in the high resolution proton NMR spectrum of acetoin.

(2)

Proton environment	Splitting pattern
A

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