9. Haloarenes
🎯 Learning Outcomes
After the lesson, learners should be able to:
- Describe the nomenclature and isomerism of haloarenes.
- Show the preparation of chlorobenzene from benzene and benzene diazonium chloride.
- State the physical properties of haloarenes.
- Explain the low reactivity of haloarenes as compared to haloalkanes in terms of nucleophilic substitution reactions.
- Explain the chemical properties of haloarenes:
- reduction of chlorobenzene,
- electrophilic substitution reactions,
- action with Na (Fittig and Wurtz-Fittig reaction),
- and action with chloral.
- State important uses of haloarenes.
1 Introduction, Nomenclature and Isomerism
Haloarenes or aryl halides are aromatic compounds in which a halogen atom is directly bonded to an aromatic ring.
General formula: Ar–X, where Ar = aryl group, and X = halogen (F, Cl, Br or I).
Name these compounds according to IUPAC rules.
2 Preparation of Haloarenes
1. From benzene (by halogenation)
(Electrophilic aromatic substitution of benzene)
2. From benzene diazonium chloride
Sandmeyer reaction
In the Sandmeyer reaction, the diazonium group (–N₂+) of benzene diazonium chloride is replaced by –Cl or –Br, using cuprous chloride or cuprous bromide dissolved in the corresponding halogen acid at room temperature or when warmed.
Gattermann reaction
Benzene diazonium chloride gives halobenzene when warmed with copper powder and hydrochloric acid or hydrobromic acid. This is called the Gattermann reaction.
The Raschig process is a chemical reaction used to produce chlorobenzene from benzene, hydrochloric acid, and oxygen in the presence of a catalyst like copper or iron chloride. This is one of the methods for commercial production of chlorobenzene. — Not in the Grade 12 NEB syllabus but appeared in 2080 NEB examination.
3 Physical Properties
| Property | Haloarenes |
|---|---|
| Physical state | Usually oily liquids or crystalline solids |
| Solubility | Insoluble in water; soluble in organic solvents |
| Chlorobenzene | Colourless liquid with characteristic odour |
4 Chemical Properties — Nucleophilic Substitution 🧪
Haloarenes are like haloalkanes, but their halogen atom is directly attached to an aromatic ring. This makes their reactions different — especially nucleophilic substitution.
Key question: Why is chlorobenzene much less reactive than chloroethane?
1. Nucleophilic substitution reaction
Unlike haloalkanes, haloarenes undergo nucleophilic substitution reactions with extreme difficulty. Such low reactivity can be explained as:
a. Carbon-to-halogen bonds in haloarenes are strong: Resonance effect
Chlorobenzene is a resonance hybrid of the following structures.
The lone pair of electrons on chlorine can participate in resonance with the benzene ring. As a result, the C–Cl bond acquires some partial double-bond character. Therefore, the bond is shorter and stronger than a normal C–Cl single bond and is difficult to break.
b. Carbon-halogen bond in haloarenes is less polar: Hybridisation effect
In haloarenes, the halogen atom is bonded to sp²-hybridised carbon, whereas in haloalkanes the halogen atom is bonded to sp³ carbon. An sp² carbon (with 33.33% s-character) holds its electrons more strongly (is more electronegative) than sp³ carbon (25% s-character). Hence, the carbon–halogen bond in haloarenes is less polar than that of haloalkanes.
sp² → 33.3% s-character → stronger hold on electrons | sp³ → 25% s-character → weaker hold on electrons
Which of the following helps explain why chlorobenzene resists nucleophilic substitution?
- The C–Cl bond has partial double-bond character.
- Chlorobenzene is ionic.
- Chlorine has no lone pair.
- Benzene contains no π electrons.
For example, chlorobenzene reacts with aqueous NaOH only at 360 °C and 300 atm, producing phenol. (Dow process)
5 Reduction of Chlorobenzene
Reduction of chlorobenzene (with Ni–Al alloy–NaOH, Δ or LiAlH₄) gives back benzene — a useful preparation of benzene.
6 Reaction with Sodium
Fittig reaction
Aryl halides, when heated with sodium in dry ether as a solvent, give biphenyl compounds, as in the Wurtz reaction; this is called the Fittig reaction.
Wurtz–Fittig reaction
Aryl halides, when heated with alkyl halides in the presence of sodium metal and dry ether as a solvent, give alkylbenzene; this is called the Wurtz–Fittig reaction.
7 Reaction with Chloral
2 C₆H₅Cl + CCl₃CHO —conc. H₂SO₄→ DDT + H₂O
- DDT is an insecticide.
8 Electrophilic Substitution Reactions
The halogen atom is deactivating but ortho/para-directing. Therefore, electrophilic substitution occurs mainly at the ortho and para positions, although more slowly than in benzene.
a. Nitration
Similarly, sulphonation and halogenation also occur at either the ortho or para positions. [practice writing these reactions]
| Reaction | Reagent | Main products |
|---|---|---|
| Nitration | conc. HNO₃/H₂SO₄ | o- and p-nitrochlorobenzene |
| Sulphonation | conc. H₂SO₄ | o- and p-chlorobenzenesulphonic acid |
| Halogenation | Cl₂/FeCl₃ | o- and p-dichlorobenzene |
Halogens are deactivating but ortho/para-directing groups.
9 Uses of Chlorobenzene
- As a starting material to prepare chlorotoluene, aniline, phenol, etc.
- To manufacture DDT.
⭐ Haloarenes — 5 Things to Remember
- Ar–X = haloarene
- Chlorobenzene can be prepared from benzene and from benzene diazonium chloride.
- Haloarenes are less reactive than haloalkanes towards nucleophilic substitution.
- Halogens are deactivating but ortho/para-directing.
- Chlorobenzene + chloral → DDT.