Electrophilic substitution

This question is about aromatic compounds.

The diagrams show the Kekulé and delocalised models of benzene.

Kekulé Delocalised

One piece of evidence for the delocalised model is that benzene does not decolourise aqueous bromine.

Suggest two other pieces of evidence which support the delocalised model of benzene.

A student investigates the reactions of benzene and phenol with excess bromine.

The student’s observations are given in Table 20.1.

Table 20.1

Explain the difference in the relative reactivities of benzene and phenol with bromine.

The white precipitate has a relative molecular mass of 330.7.

Construct an equation for the reaction of phenol with excess bromine shown in Table 20.1.

Show the structure of the organic species.

Compound G is an organic compound used in the production of pesticides.

Compound G can be synthesised from benzene in the two-stage process shown.

Synthesis 1

Benzene
Methylbenzene
Compound G

Suggest a reagent and halogen carrier that could be used in Stage 1 of Synthesis 1.

Stage 2 of Synthesis 1 is the conversion of methylbenzene to compound G.

Outline the mechanism for the reaction in Stage 2.

Show the role of H₂SO₄ as a catalyst.

A student suggested an alternative two-stage synthesis of compound G from benzene.

Synthesis 2

Benzene
Nitrobenzene
Compound G

When Synthesis 2 is attempted, the synthesis is not successful and a different organic compound from compound G is produced.

• Suggest why Synthesis 1 is successful but Synthesis 2 is not successful.

• Suggest the structure of the different organic compound that is formed in Synthesis 2.

In the production of a pesticide, compound G is reduced to amine H.

Suggest suitable reagents and complete the equation for this reduction.

Reagents........

Equation:

Compound G
Amine H

This question is about aromatic compounds.

Ethylbenzene, C₆H₅CH₂CH₃, can be prepared by reacting benzene with chloroethane, CH₃CH₂Cl, in the presence of Al Cl₃. The Al Cl₃ acts as a halogen carrier.

Ethylbenzene

In the mechanism, chloroethane reacts with the halogen carrier to form a carbocation, which acts as the electrophile.

What is meant by the term electrophile ?

Outline the mechanism for this reaction, including the role of Al Cl₃ as a halogen carrier.

The table shows directing effects for different groups in the electrophilic substitution of aromatic compounds.

The reactions of C₆H₅NH₂ are similar to the reactions of phenol.

Write an equation for the tri-substitution of C₆H₅NH₂ with chlorine.

Explain why chlorine reacts much more readily with C₆H₅NH₂ than with benzene.

Outline the mechanism for this reaction, including the role of H₂SO₄ as a catalyst.

Determine the percentage yield of 1,3-dinitrobenzene.

Give your answer to 3 significant figures.

Describe how to purify the impure crystals in Step 3.

This question is about the chemistry of aromatic compounds.

What chemical test(s) could be used to confirm the presence of the phenol group in compounds K and L ?

A student thought that¹³C NMR spectroscopy could be used to distinguish between compounds J, K and L.

Explain, with reasoning, whether the student is correct.

Compound J is substituted at the 2-and 4-positions by chlorine in the presence of a catalyst.

Outline the mechanism for the 4 substitution of compound J by chlorine in the presence of a catalyst.

Show the role of the catalyst.

Compounds K and L react with chlorine much more readily than compound J.

Explain why.

Compound J, C₆H₅CH₂CH₂OH, is reacted with acidified potassium dichromate(VI) under reflux to form organic product M.

Write an equation for this reaction.

Use [O] to represent the oxidising agent and show the structure of M.

A two-stage synthesis of an amine from compound J is shown below.

Add the reagents for each stage of this synthesis.

Compound J

This question is about reactions of phenol.

Complete the mechanism below for Stage 1 and Stage 2.

Show curly arrows, the structure of the intermediate and the missing formulae on the dotted lines.

Two molecules of salicylic acid can react together in the presence of an acid catalyst to form compound B.

Compound B has three rings and a molecular formula of C₁₄H₈O₄.

Write the equation for this reaction showing the structures of organic compounds.

A student reacts phenol with nitric acid and sulfuric acid at 100 °C to form impure crystals of an organic compound, C. The student purifies the crystals by recrystallisation.

Describe how the student could recrystallise the impure crystals to obtain a pure sample of C.

The pure sample of C is analysed to give the following results.

Percentage composition by mass: C, 31.44%; H, 1.31%; N, 18.34%; O, 48.91%.

The mass spectrum of C shows a molecular ion peak at m / z = 229.0

The¹³C NMR spectrum of C is shown below.

The table shows directing effects for different groups in the electrophilic substitution of aromatic compounds.

Analyse all the information to suggest the structure for C.

Show all your reasoning.

compound C

Which compound(s) could be prepared by reacting benzene with an acyl chloride in the presence of a halogen carrier?

A 1, 2 and 3

B Only 1 and 2

C Only 2 and 3

D Only 1

Compounds B and C, shown below, are unsaturated hydrocarbons containing nine carbon atoms.

Compound B reacts with chlorine at room temperature, but compound C requires the presence of a halogen carrier.

In both reactions, the organic compound reacts with chlorine in a 1:1 molar ratio.

Draw the structures of the organic product of each reaction.

Outline the mechanism for the reaction of compound C with chlorine.

Show the role of the halogen carrier.

Compound C can be prepared by ‘trimerisation’ of propanone using concentrated sulfuric acid as a catalyst.

Suggest an equation for this reaction, using molecular formulae.

An organic chemist is investigating compound D for possible use as a medicine.

The chemist proposes a synthesis of compound D from compound C.

The chemist develops a three-stage synthesis of compound D from compound C.

Complete the flowchart. Show structures for organic compounds.

Phenol reacts with bromine.

Which is the least likely organic product?

This question is about benzene.

Over time, the Kekulé and delocalised models have been used to describe the bonding and structure of a benzene molecule.

Describe, in terms of orbital overlap, the similarities and differences between the bonding in the Kekulé model and the delocalised model of benzene.

Experimental evidence led to the general acceptance of the delocalised model over the Kekulé model.

Describe two pieces of evidence to support the delocalised model of benzene.

Benzene can be used as the starting material for the synthesis of compounds D and E, shown below.

In the diagrams C₆H₅ is a phenyl group.

Compounds D and E can be converted into polymers.

Draw two repeat units of these polymers.

Two repeat units of polymer formed from D

Two repeat units of polymer formed from E

State the type of polymer formed from compounds D and E.

From compound D........

From compound E........

In the synthesis of compounds D and E, benzene is first reacted with ethanoyl chloride, CH₃COCl, to form phenylethanone, shown below.

The reaction takes place in the presence of aluminium chloride, Al Cl₃, which acts as a catalyst.

In the mechanism for this reaction,

• ethanoyl chloride first reacts with aluminium chloride to form the CH₃–C⁺=O cation

• the CH₃–C⁺=O cation then behaves as an electrophile.

Complete the mechanism for the reaction.

Include equations to show the role of the Al Cl₃ catalyst, relevant curly arrows and the structure of the intermediate.

Formation of electrophile........

Regeneration of catalyst........

Complete the flowchart for the synthesis of compounds D and E from phenylethanone.

Benzene reacts with an organic reagent in the presence of a halogen carrier to form phenylethanone.

Which organic reagent is required?

A CH₃CH₂OH

B CH₃CHO

C CH₃COCl

D CH₃COOH

This question is about aromatic compounds.

Phenol undergoes nitration more readily than benzene.

A student carries out the nitration of phenol with dilute nitric acid to produce 2-nitrophenol and 4-nitrophenol.

A small amount of 3-nitrophenol is also produced.

2-nitrophenol 3-nitrophenol 4-nitrophenol

The student thought that¹³C NMR spectroscopy could be used to distinguish between these three nitrophenols.

Explain whether the student is correct.

Explain why phenol is nitrated more readily than benzene.

Methylbenzene reacts with sulfur trioxide, SO₃, to form D, shown below.

The electrophile in this reaction is SO₃.

Complete the mechanism for the formation of D. Show curly arrows and the structure of the intermediate.

This question is about organic reactions.

Give the systematic name for compound A.

What type of reaction has taken place?

A similar reaction takes place when propanone, (CH₃)₂CO, is mixed with OH^–(aq) ions.

Draw the structure of the organic product of this reaction.

* Many organic reactions use electrophiles as reagents.

Explain the role of electrophiles in organic chemistry.

Your answer should include one reaction of an aliphatic compound and one reaction of an

aromatic compound, including relevant mechanisms.

This question is about the chemistry of aromatic compounds.

Benzoic acid can be nitrated by concentrated nitric acid in the presence of concentrated sulfuric acid as a catalyst, as shown in Equation 17.1.

The organic product of this reaction is 3-nitrobenzoic acid.

Outline the mechanism for this nitration of benzoic acid.

Show how H₂SO₄ behaves as a catalyst.

* A chemist carries out the reaction in Equation 17.1 using 4.97 g of benzoic acid.

The chemist obtains 3-nitrobenzoic acid as an impure solid.

The chemist purifies the solid to obtain 4.85 g of 3-nitrobenzoic acid.

Describe a method to obtain a pure sample of 3-nitrobenzoic acid from the impure solid, determine the percentage yield and check its purity.

A student investigates the relative ease of nitration of phenol, benzene, and benzoic acid.

The student finds that the conditions required for the nitration of each compound are different, as shown in Table 17.1.

State the trend in the relative ease of nitration of phenol, benzene, and benzoic acid.

Apply your knowledge of the bonding in arenes to explain the trend in part (b)(i).

A student synthesises 3-bromophenylamine, shown below, starting from nitrobenzene.

Complete the flowchart showing the structure of the intermediate and the formulae the reagents for each stage.

Another student attempts the same synthesis but carries out reduction before bromination. The student was surprised to find that two structural isomers of 3-bromophenylamine had been formed instead of the desired organic product.

Explain this result and suggest the structures of the two isomers that formed.

Explanation........

Structures