Organic Chemistry - Arenes

Edexcel International A-Level Chemistry· 18.1–18.6· 22 min read

1. Structure and Evidence for Benzene’s Delocalised Model★★☆☆☆⏱ 4 min

For many years benzene was represented by the Kekulé structure, a six-membered ring with alternating single and double C-C bonds. However, three key pieces of experimental evidence contradict this model and support the delocalised π-system model.

📘 Definition

Delocalised benzene model

Six sp²-hybridised C atoms form a planar ring, each with one unhybridised p-orbital perpendicular to the ring. These overlap side-on to form a delocalised π-electron cloud above and below the ring, with all C-C bonds equal in length (0.139 nm, between single and double bond length).

Thermochemical evidence: Hydrogenation of cyclohexene releases \ $120 k J m o l^{- 1}$ of energy. If Kekulé’s structure with three double bonds was correct, hydrogenation of benzene would release ~\ $360 k J m o l^{- 1}$ . Instead, only \ $208 k J m o l^{- 1}$ is released: the \ $152 k J m o l^{- 1}$ difference is the delocalisation energy, making benzene more stable than the hypothetical Kekulé structure.
X-ray crystallography: All C-C bond lengths in benzene are equal, not alternating between single (0.154 nm) and double (0.134 nm) bond lengths as predicted by Kekulé.
IR spectroscopy: Benzene does not show the C=C stretch peak at ~1650 cm⁻¹ typical of alkenes, confirming no localised double bonds are present.

📐 Worked Example

Calculate the delocalisation energy of benzene using the following data: ΔH hydrogenation of cyclohexene = -120 kJ mol⁻¹, ΔH hydrogenation of benzene = -208 kJ mol⁻¹

1
Step 1: Calculate the expected enthalpy of hydrogenation for the hypothetical Kekulé structure, which has three equivalent double bonds:
2
$Expected ΔH = 3 × (-120) = -360 kJ mol^{-1}$
3
Step 2: Subtract the actual experimental ΔH from the expected value to find delocalisation energy (always a positive value, representing stability gain):
4
$Delocalisation energy = |-360 - (-208)| = 152 kJ mol^{-1}$

2. Reactivity of Benzene Compared to Alkenes★★☆☆☆⏱ 3 min

Benzene’s delocalised π-system is far more stable than the localised π-bond in alkenes, so it undergoes different reactions: electrophilic substitution instead of electrophilic addition, as addition would break the delocalised system and lose the delocalisation energy stability.

📐 Worked Example

Explain why benzene does not decolourise bromine water at room temperature, while hexene does.

1
Step 1: Hexene has a localised high electron density π-bond, which readily polarises a Br₂ molecule, leading to electrophilic addition that removes the orange Br₂ colour.
2
Step 2: Benzene has a lower electron density delocalised π-system, so it cannot polarise Br₂ without a catalyst. Addition of Br₂ would require breaking the stable delocalised system, which is energetically unfavourable, so no reaction occurs at room temperature, and the orange colour remains.

3. Named Reactions of Benzene★★★☆☆⏱ 5 min

All named benzene reactions follow the electrophilic substitution mechanism, where an electrophile replaces a hydrogen atom on the ring, preserving the stable delocalised π-system.

Combustion: Benzene burns in oxygen with a smoky yellow flame due to its high carbon to hydrogen ratio, producing CO₂ and H₂O.
Halogenation: Reaction with Br₂ or Cl₂ in the presence of a Lewis acid catalyst (e.g. FeBr₃, AlCl₃) produces halobenzene.
Nitration: Reaction with concentrated HNO₃ and concentrated H₂SO₄ catalyst at 50°C produces nitrobenzene.
Sulfonation: Reaction with fuming H₂SO₄ at room temperature produces benzenesulfonic acid.
Friedel-Crafts alkylation: Reaction with a halogenoalkane and AlCl₃ catalyst produces an alkylbenzene.
Friedel-Crafts acylation: Reaction with an acyl chloride and AlCl₃ catalyst produces a phenylketone.

📐 Worked Example

Write the balanced equation for the nitration of benzene.

1
Step 1: Identify reactants: benzene, concentrated HNO₃, concentrated H₂SO₄ catalyst at 50°C
2
Step 2: Identify products: nitrobenzene and water
3
$C_6H_6 + HNO_3 \xrightarrow{conc H_2SO_4, 50°C} C_6H_5NO_2 + H_2O$

4. Electrophilic Substitution Mechanisms★★★★☆⏱ 6 min

All electrophilic substitution reactions follow three core steps: electrophile generation, electrophilic attack on the ring, and deprotonation to regenerate the aromatic ring.

📘 Definition

Wheland intermediate

The positively charged intermediate formed after electrophilic attack, with a partial delocalised π-system (represented as a horseshoe shape covering 5 carbon atoms) that temporarily loses aromaticity.

📐 Worked Example

Draw the full mechanism for the nitration of benzene, including electrophile generation.

1
Step 1: Generate the electrophile (NO₂⁺) via reaction of HNO₃ with the H₂SO₄ catalyst:
2
$HNO_3 + H_2SO_4 \rightarrow H_2NO_3^+ + HSO_4^-$
3
$H_2NO_3^+ \rightarrow NO_2^+ + H_2O$
4
Step 2: Electrophilic attack: a curly arrow from the delocalised benzene π-system attacks the positive NO₂⁺ electrophile, forming the Wheland intermediate with a positive charge.
5
Step 3: Deprotonation: the HSO₄⁻ ion removes a proton from the carbon attached to the NO₂ group, the electrons from the C-H bond reform the delocalised π-system, regenerating the H₂SO₄ catalyst and producing nitrobenzene.

Exam tip:

Always draw curly arrows originating from the delocalised ring for electrophilic attack, and use a horseshoe shape for the partial delocalised system in the Wheland intermediate to gain full marks.

5. Reactivity of Phenol★★★☆☆⏱ 4 min

Phenol consists of a benzene ring attached to an -OH group. The lone pair of electrons on the oxygen atom of the -OH group is partially delocalised into the benzene ring, increasing the electron density of the π-system, making phenol much more reactive towards electrophiles than benzene.

📐 Worked Example

Explain why phenol reacts with bromine water at room temperature without a catalyst, while benzene does not.

1
Step 1: The lone pair on the oxygen atom of the phenol -OH group is delocalised into the benzene ring, increasing the electron density of the π-system compared to benzene.
2
Step 2: This higher electron density is sufficient to polarise Br₂ molecules, so no catalyst is required for electrophilic substitution. The reaction produces 2,4,6-tribromophenol, a white precipitate, and decolourises the orange bromine water.

6. Common Pitfalls

Wrong move:

Calculating delocalisation energy as the actual enthalpy of hydrogenation of benzene, not the difference from the hypothetical Kekulé value

Why:

Delocalisation energy represents the stability gain from the delocalised system, so it is the difference between expected and experimental hydrogenation enthalpy

Correct move:

Multiply cyclohexene hydrogenation enthalpy by 3 for the expected Kekulé value, subtract the actual benzene hydrogenation enthalpy, and take the positive value as delocalisation energy

Wrong move:

Drawing electrophilic addition mechanisms for benzene instead of substitution

Why:

Addition would break the delocalised π-system and lose the large delocalisation energy, which is energetically unfavourable

Correct move:

Always draw electrophilic substitution mechanisms for benzene, where a H atom is replaced by the electrophile and the delocalised ring is regenerated

Wrong move:

Omitting the electrophile generation step in mechanism answers

Why:

Exam questions explicitly require you to show how the electrophile is formed, and this step carries separate mark weighting

Correct move:

Include the full electrophile generation reaction for every mechanism you draw, before the electrophilic attack step

Wrong move:

Stating phenol is more reactive than benzene because the -OH group is electron-withdrawing

Why:

The -OH group donates electron density to the ring via delocalisation of the oxygen lone pair, increasing ring electron density

Correct move:

Explain that the oxygen lone pair in the -OH group is delocalised into the benzene ring, increasing π-system electron density to make electrophilic attack easier

Wrong move:

Drawing alternating double bonds in the Wheland intermediate

Why:

The intermediate has a partial delocalised π-system across 5 carbon atoms, not localised double bonds

Correct move:

Represent the Wheland intermediate with a horseshoe shape covering 5 carbon atoms and a single positive charge on the ring

7. Quick Reference Cheatsheet

Reaction	Reagents & Conditions	Electrophile	Product
Halogenation	Br₂/Cl₂, FeBr₃/AlCl₃ catalyst	Br⁺/Cl⁺	Halobenzene
Nitration	Conc HNO₃ + conc H₂SO₄, 50°C	NO₂⁺	Nitrobenzene
Sulfonation	Fuming H₂SO₄, room temperature	SO₃	Benzenesulfonic acid
Friedel-Crafts alkylation	Halogenoalkane, AlCl₃ catalyst	R⁺	Alkylbenzene
Friedel-Crafts acylation	Acyl chloride, AlCl₃ catalyst	RCO⁺	Phenylketone
Phenol + bromine water	Bromine water, room temp, no catalyst	Br⁺	2,4,6-tribromophenol

8. Frequently Asked

Do I need to learn substituent directing effects for this topic?

No, directing effects are out of scope. You only need to explain the higher reactivity of phenol compared to benzene, no other activation/deactivation rules are required.

Can I use the Kekulé benzene structure in mechanism diagrams?

You may use either the delocalised ring or Kekulé representation, but ensure curly arrows originate from the π-system for electrophilic attack to get full marks.

Going deeper

syllabusEdexcel IAL Chemistry Unit 5 SpecificationOfficial topic 18 guidance

What's Next

Now you have mastered arene structure, reactions and mechanisms for Edexcel IAL Chemistry Unit 5, you can move on to related organic nitrogen chemistry topics that build on electrophilic substitution principles, including the synthesis of aromatic amines and amides. This content is frequently tested alongside organic synthesis and practical procedure questions in Unit 5 written papers, so make sure to practice drawing mechanisms from memory and applying delocalisation stability reasoning to explain reactivity trends. You should also work through past paper questions on this topic to familiarise yourself with exam phrasing and mark scheme expectations, as mechanism questions have very specific marking points for curly arrows and intermediate structure.