Edexcel IAL Chemistry Topic 18 Organic Chemistry: Arenes

Topic 18 questions connect benzene stability evidence with delocalised bonding, then test bromination, nitration, sulfonation, Friedel-Crafts reactions and phenol behaviour.

Exam points

Use thermochemical, X-ray or isomer evidence to justify benzene delocalisation.
Describe p-orbital overlap and delocalised π bonding in aromatic rings.
Select reagents and conditions for nitration, sulfonation and Friedel-Crafts reactions.

Covered syllabus points

18.1—Thermochemical, X-ray diffraction and infrared data as evidence for the structure and stability of the benzene ring Students 18.2—The delocalised model for the structure of benzene involves overlap of p-orbitals to form π-bonds 18.3—Why benzene is resistant to bromination, compared to alkenes, in terms of delocalisation of π-bonds in benzene compared 18.4—The following reactions of benzene 18.5—The mechanism of the electrophilic substitution reactions of benzene in halogenation, nitration and Friedel-Crafts reactions 18.6—The reaction of phenol with bromine water and the reasons for the relative ease of this reaction compared to benzene

[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 Topic 18: Organic Chemistry – Arenes 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 Topic 18: Organic Chemistry – Arenes 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 Topic 18: Organic Chemistry – Arenes 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 ]

All question bank results loaded

Covered syllabus points

Topic 18: Organic Chemistry – Arenes question 1

Question (a)

Question (b)

Question (c)

Question (i)

Question (d)

Question (e)

Question (i)

Topic 18: Organic Chemistry – Arenes question 2

Question (a)

Question (b)

Question (c)

Question (d)