Edexcel IAL Chemistry A2 Topic 18 Arenes

[Maximum number: 12]

This question is about benzene, C_6 H_6, a colourless liquid first isolated in 1825, and some related compounds.

Three C_6 H_6 structures proposed in the 1860 s are shown.

Table for Question 1 — Edexcel A-Level Chemistry A2

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.

Figure for Question 1 — Edexcel A-Level Chemistry A2

(a)

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

[ 2 ]

suitable test: bromine water $/ \mathrm{Br}_{2}(\mathrm{aq})$ (1)

- result of test: decolourises (from orange) with Dewar structure (and no change with benzene) (1)

Allow bromine $/ \mathrm{Br}_{2}((1)) / \mathrm{Br}_{2}$ in organic solvent

Do not award $\mathrm{Br} / \mathrm{Br}^{-}$

Accept potassium manganate((VII))/ $\mathrm{KMnO}_{4}$ and acidified/ named $\operatorname{acid} / \mathrm{H}^{+}$

Do not award hydrogenation

Do not award combustion

Allow does not decolourise with benzene

Allow brown/orange/yellow for colour of bromine water

Allow red/brown/orange for colour of bromine

Allow pink/purple for colour of potassium manganate((VII))

Ignore colour fades for decolourises

Ignore reference to addition/substitution

Do not award any reference to decolourisation/reaction with benzene

(b)

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

[ 2 ]

(c)

The Ladenburg and Dewar structures both isomerise to benzene.

The enthalpy changes are -376 kJ mol^-1 and -297 kJ mol^-1 respectively.

[ 2 ]

(i)

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.

\mathrm{H} / \mathrm{kJ} \mathrm{~mol}^{-1}

[ 2 ]

(d)

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

Figure for Question (d) — Edexcel A-Level Chemistry A2

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

Table for Question (d) — Edexcel A-Level Chemistry A2

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.

[ 2 ]

(f)(i)

- skeletal formula of any one isomer

- skeletal formulae of second and third isomers (1)

Example of correct skeletal formulae:
(1,2-isomer)
(1,3-isomer)
(1,4-isomer)

Allow Kekulé benzene ring

Allow structural/displayed $\mathrm{CH}_{3}$ and $\mathrm{CH}_{3} \mathrm{CO}$ groups

If no other mark awarded, 1,2-, 1,3- and 1,4-isomers with incorrect side chain/cyclohexane ring scores (1)

(e)

Methylbenzene, C_6 H_5 CH_3, reacts with ethanoyl chloride, CH_3 COCl, in the presence of the catalyst aluminium chloride, AlCl_3, to form a mixture of organic products with the formula CH_3 COC_6 H_4 CH_3.

\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{3}+\mathrm{CH}_{3} \mathrm{COCl} \rightarrow \mathrm{CH}_{3} \mathrm{COC}_{6} \mathrm{H}_{4} \mathrm{CH}_{3}+\mathrm{HCl}

[ 4 ]

(i)

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.

\mathrm{CH}_{3} \mathrm{COCl}+\mathrm{AlCl}_{3} \rightarrow \mathrm{CH}_{3} \mathrm{CO}^{+}+\mathrm{AlCl}_{4}^{-}

Figure for Question (i) — Edexcel A-Level Chemistry A2

[ 4 ]

A mechanism including:

- curly arrow from on or within circle to $\mathrm{C}^{+}$ of $\mathrm{CH}_{3} \mathrm{CO}^{+}$

(1)

- structure of intermediate ion

(1)

- curly arrow from C-H bond to within ring and correct product

(1)

- balanced equation for regeneration of catalyst (1)

Example of mechanism:

Figure for Answer to Question (i) — Edexcel A-Level Chemistry A2

Allow Kekulé benzene ring

‘Horseshoe’ facing tetrahedral carbon and covering at least three carbons with some part of positive sign within 'horseshoe'

Allow methyl at 1,2- or 1,3-positions

Ignore missing methyl substituent

Do not award dotted/dashed C-H / C-C bonds unless 3D structure

Allow 1,2- or 1,3-product from corresponding intermediate Do not award missing methyl substituent

$\mathrm{AlCl}_{4}^{-}+\mathrm{H}^{+} \rightarrow \mathrm{AlCl}_{3}+\mathrm{HCl}$

[Maximum number: 8]

In 1865, Friedrich August Kekulé suggested a structure for benzene which consisted of alternating single and double carbon-carbon bonds.

Figure for Question 2 — Edexcel A-Level Chemistry A2

Kekulé structure
However, modern analytical techniques indicate a structure in which the electrons are delocalised.

delocalised structure

(a)

Explain how the results of X-ray diffraction experiments on benzene suggest a delocalised structure rather than the Kekulé structure.

[ 2 ]

An explanation that makes reference to the following points:

- (the x-ray diffraction shows) the bonds are the same length

(1)

Either

- (therefore) electrons are evenly distributed (around the benzene ring)

(1)

Or

- Kekule structure would have shorter double bonds / longer single bonds

(1)

Ignore references to bond angles

Allow it is a regular hexagon Ignore just it is a hexagon Ignore bond energy is the same

Ignore electrons are delocalised / there is a ring of electrons

Ignore shorter $\pi$ bonds / longer $\sigma$ bonds Ignore double bonds and single bonds are of different length

(b)

The compound 1,2-dichlorobenzene exists as only one structure. Explain how this supports the delocalised structure of benzene rather than the Kekulé structure.

[ 2 ]

(c)

State how the enthalpy changes of hydrogenation for cyclohexene and benzene provide evidence for the delocalised structure.

Figure for Question (c) — Edexcel A-Level Chemistry A2

[ 1 ]

An answer that makes reference to the following points:
- the Kekulé structure would be expected to have an enthalpy change of hydrogenation of $3 \times-118 \mathrm{~kJ} \mathrm{~mol}^{-1}$ $/-354 \mathrm{~kJ} \mathrm{~mol}^{-1}$
(which is significantly different from the actual value of
$-205 \mathrm{~kJ} \mathrm{~mol}^{-1}$ )

Allow 3 times enthalpy change of cyclohexene
Award the actual value is $149 \mathrm{~kJ} \mathrm{~mol}^{-1}$ less
exothermic than expected
Award the actual value is $149 \mathrm{~kJ} \mathrm{~mol}^{-1}$ more stable
than expected

(d)

Describe the structure of benzene in terms of the atomic orbitals involved, the bonds formed and the delocalised electrons.

[ 3 ]

[Maximum number: 6]

The synthetic drugs of the 'Caine' family are used as local anaesthetics. Caine drugs prevent nerve activity by binding to sodium channel receptors in the lipid cell membranes of neurons.

benzocaine

procaine

lidocaine

articaine

(a)

Lidocaine can be prepared in the synthesis shown.

Step 1

Step 3

lidocaine

[ 6 ]

(i)

Give the mechanism for Step 1.

Include an equation for the formation of the electrophile.

[ 4 ]

Ignore omission or incorrect placement of methyl groups in M2 and M3

$\mathrm{HNO}_{3}+\mathrm{H}_{2} \mathrm{SO}_{4} \rightarrow \mathrm{NO}_{2}{ }^{+}+\mathrm{HSO}_{4}{ }^{-}+\mathrm{H}_{2} \mathrm{O}$

OR

$\mathrm{HNO}_{3}+2 \mathrm{H}_{2} \mathrm{SO}_{4} \rightarrow \mathrm{NO}_{2}{ }^{+}+2 \mathrm{HSO}_{4}{ }^{-}+\mathrm{H}_{3} \mathrm{O}^{+}$

OR

$\mathrm{HNO}_{3}+\mathrm{H}_{2} \mathrm{SO}_{4} \rightarrow \mathrm{H}_{2} \mathrm{NO}_{3}{ }^{+}+\mathrm{HSO}_{4}{ }^{-}$ and $\mathrm{H}_{2} \mathrm{NO}_{3}{ }^{+} \rightarrow \mathrm{NO}_{2}{ }^{+}+\mathrm{H}_{2} \mathrm{O}$

Expert

TE on electrophile from M1 provided positively charged

Do not award lone pair on N of $\mathrm{NO}_{2}{ }^{+}$

An answer that makes reference to the following points:

- equation for formation of nitronium ion

(1)

- curly arrow from within hexagon to

(1) anywhere on $\mathrm{NO}_{2}{ }^{+}$

- structure of intermediate ion

(1)

- curly arrow from C-H bond to within ring and

correct product

and

$\mathrm{H}^{+}$

(1)

(ii)

A low yield of 2,6-dimethylnitrobenzene is obtained in Step 1 due to the formation of additional organic products.

Give possible structures for two additional organic products.

[ 2 ]

An answer that makes reference to the following points:

- structure of 2,4-dimethylnitrobenzene

- structure of 3,5-dimethylnitrobenzene

Figure for Answer to Question (ii) — Edexcel A-Level Chemistry A2

Allow structures with di-, tri- or tetranitro substitution

Ignore connectivity of $\mathrm{NO}_{2}$ group

Penalise omission of delocalised ring once only

Examples of structures:

Graduate

[Maximum number: 3]

In 1865, Friedrich August Kekulé suggested a structure for benzene which consisted of alternating single and double carbon-carbon bonds.

Figure for Question 4 — Edexcel A-Level Chemistry A2

Kekulé structure
However, modern analytical techniques indicate a structure in which the electrons are delocalised.

delocalised structure

Describe the structure of benzene in terms of the atomic orbitals involved, the bonds formed and the delocalised electrons.

[Maximum number: 2]

This question is about benzene, C_6 H_6, a colourless liquid first isolated in 1825, and some related compounds.

Three C_6 H_6 structures proposed in the 1860 s are shown.

Table for Question 5 — Edexcel A-Level Chemistry A2

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.

Figure for Question 5 — Edexcel A-Level Chemistry A2

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

suitable test: bromine water $/ \mathrm{Br}_{2}(\mathrm{aq})$ (1)

- result of test: decolourises (from orange) with Dewar structure (and no change with benzene) (1)

Allow bromine $/ \mathrm{Br}_{2}((1)) / \mathrm{Br}_{2}$ in organic solvent

Do not award $\mathrm{Br} / \mathrm{Br}^{-}$

Accept potassium manganate((VII))/ $\mathrm{KMnO}_{4}$ and acidified/ named $\operatorname{acid} / \mathrm{H}^{+}$

Do not award hydrogenation

Do not award combustion

Allow does not decolourise with benzene

Allow brown/orange/yellow for colour of bromine water

Allow red/brown/orange for colour of bromine

Allow pink/purple for colour of potassium manganate((VII))

Ignore colour fades for decolourises

Ignore reference to addition/substitution

Do not award any reference to decolourisation/reaction with benzene

[Maximum number: 2]

The synthetic drugs of the 'Caine' family are used as local anaesthetics. Caine drugs prevent nerve activity by binding to sodium channel receptors in the lipid cell membranes of neurons.

benzocaine

procaine

lidocaine

articaine

Lidocaine can be prepared in the synthesis shown.

Step 1

Step 3

lidocaine

A low yield of 2,6-dimethylnitrobenzene is obtained in Step 1 due to the formation of additional organic products.

Give possible structures for two additional organic products.

An answer that makes reference to the following points:

- structure of 2,4-dimethylnitrobenzene

- structure of 3,5-dimethylnitrobenzene

Figure for Answer to Question 6 — Edexcel A-Level Chemistry A2

Allow structures with di-, tri- or tetranitro substitution

Ignore connectivity of $\mathrm{NO}_{2}$ group

Penalise omission of delocalised ring once only

Examples of structures:

Graduate

[Maximum number: 4]

The synthetic drugs of the 'Caine' family are used as local anaesthetics. Caine drugs prevent nerve activity by binding to sodium channel receptors in the lipid cell membranes of neurons.

benzocaine

procaine

lidocaine

articaine

Lidocaine can be prepared in the synthesis shown.

Step 1

Step 3

lidocaine

Give the mechanism for Step 1.

Include an equation for the formation of the electrophile.

Ignore omission or incorrect placement of methyl groups in M2 and M3

$\mathrm{HNO}_{3}+\mathrm{H}_{2} \mathrm{SO}_{4} \rightarrow \mathrm{NO}_{2}{ }^{+}+\mathrm{HSO}_{4}{ }^{-}+\mathrm{H}_{2} \mathrm{O}$

OR

$\mathrm{HNO}_{3}+2 \mathrm{H}_{2} \mathrm{SO}_{4} \rightarrow \mathrm{NO}_{2}{ }^{+}+2 \mathrm{HSO}_{4}{ }^{-}+\mathrm{H}_{3} \mathrm{O}^{+}$

OR

$\mathrm{HNO}_{3}+\mathrm{H}_{2} \mathrm{SO}_{4} \rightarrow \mathrm{H}_{2} \mathrm{NO}_{3}{ }^{+}+\mathrm{HSO}_{4}{ }^{-}$ and $\mathrm{H}_{2} \mathrm{NO}_{3}{ }^{+} \rightarrow \mathrm{NO}_{2}{ }^{+}+\mathrm{H}_{2} \mathrm{O}$

Expert

Figure for Answer to Question 7 — Edexcel A-Level Chemistry A2

TE on electrophile from M1 provided positively charged

Do not award lone pair on N of $\mathrm{NO}_{2}{ }^{+}$

An answer that makes reference to the following points:

- equation for formation of nitronium ion

(1)

- curly arrow from within hexagon to

(1) anywhere on $\mathrm{NO}_{2}{ }^{+}$

- structure of intermediate ion

(1)

- curly arrow from C-H bond to within ring and

correct product

and

$\mathrm{H}^{+}$

(1)

[Maximum number: 6]

This question is about benzene and some of its compounds.

Benzene reacts with bromine in the presence of an iron(III) bromide catalyst to form bromobenzene.

Figure for Question 8 — Edexcel A-Level Chemistry A2

Compare and contrast the reaction of benzene with bromine with the reactions of cyclohexene with bromine and of phenol with bromine.

Include reasons for any differences.
Detailed mechanisms for these reactions are not required.
8

*

This question assesses a student's ability to show a coherent and logically structured answer with linkages and fully-sustained reasoning.

Marks are awarded for indicative content and for how the answer is structured and shows lines of reasoning.

The following table shows how the marks should be awarded for indicative content.

Number of indicative marking

points seen in answer

Number of marks awarded for

indicative marking points

6

5-4

3-2

1

0

The following table shows how the marks should be awarded for structure and lines of reasoning.

Figure for Answer to Question 8 — Edexcel A-Level Chemistry A2

Comment:

Look for the indicative marking points first, then consider the mark for structure of answer and sustained line of reasoning

Indicative content

- IP1 - Similarity

Guidance on how the mark scheme should be applied:

The mark for indicative content should be added to the mark for lines of reasoning. For example, an answer with five indicative marking points that is partially structured with some linkages and lines of reasoning scores 4 marks ( 3 marks for indicative content and 1 mark for partial structure and some linkages and lines of reasoning).

If there are no linkages between points, the same five indicative marking points would yield an overall score of 3 marks (3 marks for indicative content and no marks for linkages).

In general it would be expected that 5 or 6 indicative points would get 2 reasoning marks, and 3 or 4 indicative points would get 1 mark for reasoning, and 0,1 or 2 indicative points would score zero marks for reasoning.

General points to note

If there is any incorrect chemistry, deduct mark(s) from the reasoning. If no reasoning mark(s) awarded do not deduct mark(s).

Accept structures for names throughout If name and formula given both must be correct

Deduct a reasoning mark if there is no comparison given for IP1 to IP3

Do not penalise unbalanced / incomplete equations Deduct (mark) from reasoning if any products given are incorrect

All are attacked by / react with electrophiles

- IP2 - Types of reaction

Cyclohexene undergoes addition reactions but benzene and/or phenol undergo substitution reactions

- IP3 - Conditions

Cyclohexene and/or phenol react with (aqueous) bromine / without a catalyst and benzene needs

(a Friedel-Crafts catalyst / iron / iron(III) bromide)

- IP4 - Benzene

Benzene has delocalised electrons and is (kinetically) stable so the reaction has a high activation energy

- IP5 - Cyclohexene

Cyclohexene has localised electron density in one $\pi$ bond (which increases the electron density and makes it more susceptible to electrophilic attack)

- IP6 - Phenol

Phenol has a lone pair of electrons on the oxygen which is delocalised (within the ring)

and

makes it more susceptible to electrophilic attack

All three need to be mentioned for this IP - evidence for
phenol reacting with an electrophile may be seen in IP6
Accept benzene forms monobromo product /
bromobenzene, cyclohexene forms dibromo product / 1,2-
dibromocyclohexane and phenol forms tribromo product /
2, 4, 6-tribromophenol
Allow HBr is produced with benzene and phenol but
cyclohexene only forms one product
Allow react under normal laboratory conditions / room
temperature and pressure
Allow reference to $\mathrm{AlBr}_{3} / \mathrm{AlCl}_{3}$
This IP can be awarded if benzene equation has catalyst and
other equation(s) do not
Ignore references to specific temperatures
Allow delocalised ( $\pi$ ) electron ring in benzene is (very)
stable
Allow delocalisation of electrons in $\pi$ bonds which
decreases the electron density (of the ring) and makes it less
susceptible to electrophilic attack
If neither IP4 or IP5 awarded then allow (1) for benzene has
delocalised electrons but cyclohexene does not
Allow the lone pair (of electrons) on the oxygen/OH in
phenol
and increases the electron density of the (benzene)
ring/overlaps with the delocalised ring

Edexcel A-Level Chemistry A2 18 Organic Chemistry Arenes Questions

Exam points

Covered syllabus points

Covered syllabus points

Question 1

Question (a)

Question (b)

Question (c)

Question (i)

Question (d)

Question (e)

Question (i)

Question 2

Question (a)

Question (b)

Question (c)

Question (d)

Question 3

Question (a)

Question (i)

Question (ii)

Question 4

Question 5

Question 6

Question 7

Question 8