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Overview

🌳 Concept tree

Aromatic Hydrocarbons and Benzene Chemistry
├── What are aromatic compounds?

├── Structure of benzene

├── Industrial preparation of benzene

├── Nomenclature of benzene derivatives

├── Orientation→where does the second group go?

├── Properties of benzene

└── Revision key points for Part A

Lesson 4: Aromatic Hydrocarbons and Benzene Chemistry · 1 / 9

What are aromatic compounds?

🧊 3D Delocalized π Electron Cloud & Aromatic Resonance of Benzene

D Scientific Model→Spatial 3D molecular render showcasing the planar hexagonal carbon frame with equal C-C bond lengths (1.39 Å), accompanied by continuous toroidal delocalized π-electron clouds residing symmetrically above and below the molecular plane.

Aromatic compounds contain one or more benzene rings.
Benzene C₆H₆ is the parent.
The group also includes naphthalene (C₁₀H₈, two fused rings) and anthracene (C₁₄H₁₀, three fused rings).
Note the petrol (gasoline) used as car fuel is a mixture of alkanes and is not the same as benzene.

Lesson 4: Aromatic Hydrocarbons and Benzene Chemistry · 2 / 9

Structure of benzene

The structure of benzene puzzled chemists for years because it behaves as unsaturated (it can add hydrogen or chlorine) but usually reacts by substitution not addition; and all six C–C bond lengths are equal and intermediate between a single and a double bond.
Kekulé proposed a hexagonal ring with alternating single and double bonds; modern theory says the six π electrons are delocalised over the whole ring, so there are no real separate single/double bonds.
This is represented by a hexagon with a circle inside.
All six carbons (and hydrogens) are identical, so a substituent can attack any carbon.

Lesson 4: Aromatic Hydrocarbons and Benzene Chemistry · 3 / 9

Industrial preparation of benzene

Comparison aspect	Main idea	Keywords
From coal tar	coal heated in the absence of air (destructive distillation) gives coke coal gas ammoniacal liquor and a black liquid coal tar. Fractional distillation of coal tar gives benzene (b.p. 80 °C) toluene phenol naphthalene etc. — very important raw materials	heatedabsencedestructivedistillation
From petroleum (catalytic reforming)	n-hexane passed at high temperature over platinum: C₆H₁₄ —(Pt heat)→ C₆H₆ + 4H₂. In the same way n-heptane gives toluene (methylbenzene): C₇H₁₆ → C₆H₅CH₃ + 4H₂	hexanepassedhightemperature
Polymerisation of ethyne	3C₂H₂ → C₆H₆ (red-hot iron or copper tube ≈ 500 °C)	3C₂H₂C₆H₆ironcopper
Reduction of phenol	phenol vapour passed over heated zinc dust: C₆H₅OH + Zn → C₆H₆ + ZnO	vapourpassedoverheated
Laboratory	heat sodium benzoate with soda lime (like methane): C₆H₅COONa + NaOH —(CaO heat)→ C₆H₆ + Na₂CO₃	heatsodiumbenzoatesoda

Lesson 4: Aromatic Hydrocarbons and Benzene Chemistry · 4 / 9

Nomenclature of benzene derivatives

Comparison aspect	Main idea	Example	Keywords
Mono-substituted	the name of the group followed by "benzene": chlorobenzene C₆H₅Cl nitrobenzene C₆H₅NO₂ ethylbenzene methylbenzene (toluene) hydroxybenzene (phenol) aminobenzene (aniline). The radical derived by removing one H from benzene is the phenyl radical C₆H₅– (an aryl radical Ar–)	—	namegroupfollowedbenzene
Di-substituted	three isomers named by ortho (o- positions 1 meta (m- 3) and para (p- 4) or by numbers: o-dichlorobenzene = 1 2-dichlorobenzene m-xylene = 1 3-dimethylbenzene p-nitrotoluene = 1-methyl-4-nitrobenzene	—	threeisomersnamedortho
Tri- or poly-substituted	use numbers only number so that the sum of numbers is the lowest and list substituents alphabetically	4-bromo-1-chloro-2-nitrobenzene 6-trinitrotoluene (TNT) 6-trinitrophenol (picric acid)	numbersnumberlowestlist

Lesson 4: Aromatic Hydrocarbons and Benzene Chemistry · 5 / 9

Orientation: where does the second group go?

The group already on the ring decides where a new substituent enters:

ortho/para-directing groups–R (alkyl), –OH, –NH₂, halogens (–Cl, –Br …). Example: toluene + Cl₂ (FeCl₃) → o-chlorotoluene + p-chlorotoluene.

meta-directing groups–CHO, –COR (C=O), –COOH, –NO₂. Example: nitrobenzene + Cl₂ (FeCl₃) → m-chloronitrobenzene (3-chloronitrobenzene).

Lesson 4: Aromatic Hydrocarbons and Benzene Chemistry · 6 / 9

Properties of benzene

Comparison aspect	Main idea	Keywords
Physical	colourless mobile pleasant-smelling liquid immiscible with water (a good organic solvent) b.p. 80 °C flammable and toxic (carcinogenic) — handle with care	colourlessmobilepleasantsmelling
Chemical	burns with a sooty (black smoky) flame because of its high carbon percentage: 2C₆H₆ + 15O₂ → 12CO₂ + 6H₂O. It undergoes two types of reaction	burnssootyblacksmoky
•	A. Addition (difficult needs special conditions)	Additiondifficultneedsspecial

Comparison aspect	Main idea	Keywords
Hydrogenation	C₆H₆ + 3H₂ —(Ni high T and pressure)→ C₆H₁₂ (cyclohexane)	C₆H₆highpressureC₆H₁₂
Halogenation in sunlight (UV)	C₆H₆ + 3Cl₂ —(UV)→ C₆H₆Cl₆ (1 6-hexachlorocyclohexane "gammexane" an insecticide)	C₆H₆3Cl₂C₆H₆Cl₆hexachlorocyclohexane
•	B. Substitution (the characteristic reaction)	Substitutioncharacteristicreaction

HalogenationC₆H₆ + Cl₂ —(FeCl₃, no light)→ C₆H₅Cl + HCl (chlorobenzene).

Alkylation (Friedel–Crafts)C₆H₆ + CH₃Cl —(anhydrous AlCl₃)→ C₆H₅CH₃ (toluene) + HCl. [Or with C₂H₅Cl → ethylbenzene.]

NitrationC₆H₆ + HNO₃ —(conc. H₂SO₄, ≈ 55 °C)→ C₆H₅NO₂ (nitrobenzene, yellow oily liquid with almond-like smell) + H₂O.

SulphonationC₆H₆ + H₂SO₄ (conc.) → C₆H₅SO₃H (benzenesulphonic acid) + H₂O.

Explosives (TNT). Polynitro organic compounds are explosive: each molecule contains its own "fuel" (carbon) and the oxygen of the nitro groups; the weak N–O bond (≈ 201 kJ/mol) breaks and very strong bonds form (the C=O bonds of CO₂ and especially the N≡N bond of N₂, 941 kJ/mol), releasing a great deal of energy and large volumes of gas suddenly. TNT (2,4,6-trinitrotoluene) is prepared by nitrating toluene with a 1:1 mixture of concentrated nitric and sulphuric acids:
C₆H₅CH₃ + 3HNO₃ —(conc. H₂SO₄)→ C₆H₂(NO₂)₃CH₃ + 3H₂O

Detergents.→Sulphonation of alkylbenzene gives alkylbenzenesulphonic acid; neutralisation with NaOH gives its sodium salt, a synthetic detergent:

R–C₆H₄–SO₃H + NaOH → R–C₆H₄–SO₃⁻Na⁺ + H₂O

The detergent molecule has a long hydrocarbon tail (hydrophobic, dissolves in oil/grease) and an ionic head (hydrophilic, dissolves in water) — this lets it lift grease into water.

Lesson 4: Aromatic Hydrocarbons and Benzene Chemistry · 7 / 9

Revision key points for Part A

Organic = carbon compounds (not oxides, carbonates, cyanides); Wöhler (1828), urea.
Alkane CₙH₂ₙ₊₂ (substitution, cracking); alkene CₙH₂ₙ (addition, Baeyer, Markovnikov, polymerisation); alkyne CₙH₂ₙ₋₂ (addition twice; CaC₂ + H₂O).
Benzene substitution (halogenation, nitration, sulphonation, alkylation); addition only under severe conditions.
Know all equations, conditions and tests (Br₂/CCl₄, alkaline KMnO₄ for unsaturation; CuO + anhydrous CuSO₄ + lime water for C and H).
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Lesson 4: Aromatic Hydrocarbons and Benzene Chemistry · 8 / 9

Part 9

📖

🔍 In-Depth Explanations, Deductions & Multi-Book Insights:

Chemistry Thanaweya Amma (2026/2027)

Lesson 4 – Aromatic Hydrocarbons and Benzene Chemistry
Detailed Academic Lesson Overview:
Benzene (C6H6) is the parent aromatic hydrocarbon.
Discovered by Faraday and structurally elucidated by Kekulé as a resonant hexagonal ring with alternating single and double bonds where 6 π electrons are delocalized across the ring.

🌳 Hierarchy Classification & Conceptual Roadmap

Aromatic Hydrocarbons (Arenes)

├── 1. Kekulé Benzene Ring Structure & Resonance

Comparison aspect	Main idea	Keywords
•	│ ├── Delocalization of 6 π electrons all C-C bonds are intermediate length (1.39 Å) between single and double	Delocalizationelectronsbondsintermediate
•	│ └── Undergoes substitution easily addition requires harsh conditions	Undergoessubstitutioneasilyaddition
•	├── 2. Industrial & Laboratory Preparations of Benzene	IndustrialLaboratoryPreparationsBenzene
│ ├── From Coal Tar	Fractional distillation (80-82°C)	Fractionaldistillation
│ ├── From Aliphatic Hydrocarbons	Cyclic trimerization of Ethyne (3 C2H2 in red-hot nickel tube)	CyclictrimerizationEthyneC2H2
│ ├── From n-Hexane	Catalytic reforming over Platinum at high temperature	CatalyticreformingoverPlatinum
│ ├── From Phenol	Reduction with hot Zinc dust (C6H5OH + Zn → C6H6 + ZnO)	ReductionZincdustC6H5OH
│ └── From Sodium Benzoate	Dry distillation with soda lime	distillationsodalime
•	└── 3. Electrophilic Aromatic Substitution (SEAr)	ElectrophilicAromaticSubstitutionSEAr
├── Halogenation	C6H6 + Cl2 (FeCl3 catalyst) → Chlorobenzene + HCl	C6H6FeCl3catalystChlorobenzene
├── Nitration	C6H6 + HNO3 (conc. H2SO4 at 50°C) → Nitrobenzene + H2O	C6H6HNO3concH2SO4
├── Sulfonation	C6H6 + conc. H2SO4 → Benzene sulfonic acid + H2O	C6H6concH2SO4Benzene
└── Friedel-Crafts Alkylation	C6H6 + CH3Cl (anhydrous AlCl3) → Toluene + HCl	C6H6CH3ClanhydrousAlCl3
•	📋 Master Teacher's Blackboard Rules & Exam Secrets	MasterTeacherBlackboardRules
RULE 1	Directing groups on Benzene ring	DirectinggroupsBenzenering
– Ortho/Para directors (Activate ring, single bonds)	-OH -NH2 -R (alkyl) -X (halogens)	alkylhalogens
•	– Meta directors (Deactivate ring multiple bonds to heteroatom): -NO2 -COOH -CHO -COR -SO3H	MetadirectorsDeactivatering
RULE 2	Trinitrotoluene (TNT) is prepared by nitrating toluene with a nitrating mixture (conc. HNO3 + conc. H2SO4 1:1) at high temperature it is a powerful explosive	Trinitrotoluenepreparednitratingtoluene
RULE 3	DDT (Dichlorodiphenyltrichloroethane) is the most historically infamous insecticide known as the ugliest chemical compound due to environmental bio-accumulation	Dichlorodiphenyltrichloroethanemosthistoricallyinfamous
📊 Detailed Comparison Matrix	Directing Influences in Disubstituted Benzene Synthesis	DirectingInfluencesDisubstitutedBenzene

Substituent on Ring	Electronic Nature	Directing Orientation	Example Product
Methyl group (-CH3) in Toluene	Electron-donating (+I / Hyperconjugation)	Ortho and Para	o-chlorotoluene & p-chlorotoluene
Nitro group (-NO2) in Nitrobenzene	Electron-withdrawing (-M)	Meta position only	m-chloronitrobenzene
Hydroxyl group (-OH) in Phenol	Electron-donating via resonance	Ortho and Para	2,4,6-trinitrophenol (Picric acid)
Carboxylic group (-COOH)	Electron-withdrawing	Meta position only	m-nitrobenzoic acid

Lesson 4: Aromatic Hydrocarbons and Benzene Chemistry · 9 / 9

Curriculum

Chemistry

Chapter 1 - Transition Elements and Iron Metallurgy

Chapter 2 - Chemical Analysis

Chapter 3 - Chemical Equilibrium

Chapter 4 - Electrochemistry

Chapter 5 - Organic Chemistry

More lessons

Lesson 4: Aromatic Hydrocarbons and Benzene Chemistry

Lesson 4: Aromatic Hydrocarbons and Benzene Chemistry

Overview

🌳 Concept tree

├── Structure of benzene

├── Industrial preparation of benzene

├── Nomenclature of benzene derivatives

├── Properties of benzene

└── Revision key points for Part A

What are aromatic compounds?

🧊 3D Delocalized π Electron Cloud & Aromatic Resonance of Benzene

Structure of benzene

Industrial preparation of benzene

Nomenclature of benzene derivatives

Orientation: where does the second group go?

Properties of benzene

R–C₆H₄–SO₃H + NaOH → R–C₆H₄–SO₃⁻Na⁺ + H₂O

Revision key points for Part A

Part 9

🔍 In-Depth Explanations, Deductions & Multi-Book Insights:

Chemistry Thanaweya Amma (2026/2027)

🌳 Hierarchy Classification & Conceptual Roadmap

Aromatic Hydrocarbons (Arenes)

├── 1. Kekulé Benzene Ring Structure & Resonance

Well done! You finished this lesson

Curriculum

Chemistry

Chapter 1 - Transition Elements and Iron Metallurgy

Chapter 2 - Chemical Analysis

Chapter 3 - Chemical Equilibrium

Chapter 4 - Electrochemistry

Chapter 5 - Organic Chemistry

More lessons

Lesson 4: Aromatic Hydrocarbons and Benzene Chemistry

🌳 Concept tree

├── Structure of benzene

├── Industrial preparation of benzene

├── Nomenclature of benzene derivatives

├── Properties of benzene

└── Revision key points for Part A

🧊 3D Delocalized π Electron Cloud & Aromatic Resonance of Benzene

R–C₆H₄–SO₃H + NaOH → R–C₆H₄–SO₃−Na+ + H₂O

🔍 In-Depth Explanations, Deductions & Multi-Book Insights:

Chemistry Thanaweya Amma (2026/2027)

🌳 Hierarchy Classification & Conceptual Roadmap

Aromatic Hydrocarbons (Arenes)

├── 1. Kekulé Benzene Ring Structure & Resonance

Well done! You finished this lesson

R–C₆H₄–SO₃H + NaOH → R–C₆H₄–SO₃⁻Na⁺ + H₂O