Benzene & aromatic chemistry

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 two metal hydrides, lithium aluminium hydride, LiAl H₄, and sodium borohydride, NaBH₄. Both hydrides are used as reducing agents in organic chemistry.

What is the systematic name of the alcohol C₆H₅CH₂OH?

Calculate the mass of LiAl H₄ that the student should use in Step 2 and the percentage yield of pure C₆H₅CH₂OH obtained in Step 4.

Give the mass of LiAl H₄ and the percentage yield of pure C₆H₅CH₂OH to 3 significant figures.

Percentage yield of pure C₆H₅CH₂OH =........ %

In the preparation, LiAl H₄ behaves as a nucleophile.

What is meant by the term nucleophile ?

The mechanism for Step 2 in the reduction of C₆H₅COOC₂H₅ is shown below. In the mechanism, LiAl H₄ can be represented as H^–.

Complete the mechanism by adding curly arrows to the three boxes.

Suggest why the student heated the mixture from Step 2 with dilute aqueous acid in Step 3.

NaBH₄ contains Na⁺ and BH₄^– ions.

Draw a ‘dot‑and‑cross’ diagram for NaBH₄.

Use a different symbol for the electrons from each element.

Show outer electrons only.

NaBH₄/ O₂ cells are being investigated as potential fuel cells.

• The positive electrode contains O₂.

• The negative electrode contains NaBH₄.

The reaction at the positive electrode and the overall cell reaction for a NaBH₄/ O₂ fuel cell are shown below.

Overall cell reaction: NaBH₄ + 2O₂ NaBO₂ + 2H₂O E = 1.64 V

What is the standard electrode potential of the negative electrode in this cell?

Construct the equation for the reaction at the negative electrode of this cell.

Which hydrocarbons are unsaturated?

Which hydrocarbons are alicyclic?

Which hydrocarbons have the general formula C_nH_2n?

What is the systematic name of hydrocarbon C ?

Explain why hydrocarbon D is a Z-stereoisomer.

This question is about the chemistry of compounds containing phosphorus.

Phosphorus forms several acids including H₃PO₄ and H₃PO₃.

H₃PO₄ is a tribasic acid. The equilibria for the dissociations are shown below.

During the equilibria, H₂PO₄^– behaves both as an acid and as a base.

Explain this statement, using the equilibria 1, 2 and 3, as required.

In a H₃PO₃ molecule, the O atoms are covalently bonded to the P atom. The H atoms are bonded to the O atoms.

Draw the structure of a H₃PO₃ molecule, showing all the bonds.

On your diagram, add the values for the O–P–O and P–O–H bond angles.

The systematic name of H₃PO₄ is phosphoric(V) acid.

What is the systematic name of H₃PO₃?

Phosphine, PH₃, is a poisonous gas.

Phosphine reacts with oxygen gas to form phosphorus(V) oxide and water.

Write the equation for this reaction.

Aqueous silver nitrate, AgNO₃, is reduced by PH₃. The unbalanced equation is shown below.

Balance the equation and use oxidation numbers to explain why this is a redox reaction.

Explanation........

When phosphorus(V) chloride, PCl₅, and ammonium chloride are heated together, the compound P₃N₃Cl₆ is formed, together with HCl gas.

P₃N₃Cl₆ has a cyclic structure, like the Kekulé structure of benzene.

Write an equation for the reaction of PCl₅ and ammonium chloride to form P₃N₃Cl₆.

Calculate the percentage by mass of P in P₃N₃Cl₆.

Give your answer to 2 decimal places.

Suggest one example of evidence that could show that P₃N₃Cl₆ has a Kekulé structure rather than a delocalised structure.

In a molecule of P₃N₃Cl₆ all the N and Cl atoms are bonded to P atoms.

Suggest a possible structure for a molecule of P₃N₃Cl₆.

Which statement supports the delocalised model of benzene and not the Kekulé model?

A Sigma bonds overlap to form a -system.

B The carbon-carbon bond lengths are all the same.

C The enthalpy change of hydrogenation is more exothermic than expected.

D Benzene is more reactive than alkenes with bromine.

What is the number of sigma bonds in a molecule of methylbenzene?

A 7

B 10

C 12

D 15

Which of the following statement(s) provide(s) evidence for the delocalised model of benzene rather than the Kekulé model?

1 The enthalpy change of hydrogenation is more exothermic than expected.

2 Benzene only reacts with bromine in the presence of a halogen carrier.

3 The carbon-carbon bonds all have the same length.

A 1, 2 and 3

B Only 1 and 2

C Only 2 and 3

D Only 1

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.

Dettol ® is a disinfectant containing the antiseptic chloroxylenol, shown below.

Chloroxylenol is a weak Brønsted–Lowry acid.

What is the systematic name of chloroxylenol?

Predict the number of peaks in a¹³C NMR spectrum of chloroxylenol.

Name the functional group responsible for the acidity of chloroxylenol and describe a simple test which would confirm the presence of this group.

Functional group........

Test........

A student measures the pH of the contents in a bottle of Dettol ® as 5.14.

The label on the bottle shows the percentage of chloroxylenol in Dettol ® as 4.80% i.e. 100 cm³ of Dettol^® contains 4.80 g of chloroxylenol.

Assume the following:

• Chloroxylenol is the only acidic component in Dettol ®.

• Chloroxylenol is a weak monobasic acid.

• The density of Dettol ® is 1.00 g cm −3.

Write the equation, using molecular formulae, for the acid dissociation of chloroxylenol.

Calculate the acid dissociation constant, K_a, for chloroxylenol.

K_a =........ mol dm⁻³

Dettol^® contains other chemicals including -terpineol, shown below.

-Terpineol is a chiral compound.

Show with an asterisk, (*), the chiral centre(s) in the structure of -terpineol.

-Terpineol meets the requirements for E / Z isomerism. However, only one E / Z isomer of -terpineol exists.

Explain

• why -terpineol meets the requirements for E / Z isomerism

• whether -terpineol is an E- or Z- isomer

• why only one E / Z isomer of -terpineol exists.

-Terpineol contains two functional groups.

For each functional group, choose a reagent that reacts with that group only. Draw the structures for the organic products of the reactions.

Show structures for organic compounds.

Reagent(s)........

Name of functional group that reacts........

Structure of organic product

Reagent(s)........

Name of functional group that reacts........

Structure of organic product

Which statement(s) support(s) the delocalised model for the structure of benzene?

1 All carbon–carbon bonds have the same length.

2 The enthalpy change of hydrogenation of benzene is less exothermic than expected.

3 Bromine reacts with benzene less readily than with cyclohexene.

A 1, 2 and 3

B Only 1 and 2

C Only 2 and 3

D Only 1