Class 12 Chemistry Haloarenes Notes

Class 12 Chemistry Haloarenes Notes
Unit 9
Organic Chemistry
Class 12 Chemistry

Haloarenes

On mobile, swipe inside the PDF to read all pages and pinch to zoom.

NEB/CDC syllabus scope: Haloarenes is a 3-teaching-hour unit covering introduction, nomenclature and isomerism; preparation of chlorobenzene from benzene and benzene diazonium chloride; physical properties; the low reactivity of haloarenes toward nucleophilic substitution compared with haloalkanes; reduction of chlorobenzene; electrophilic substitution; Fittig and Wurtz–Fittig reactions; action with chloral; and important uses of haloarenes.

1. Introduction to Haloarenes

Definition Haloarenes, also called aryl halides, are aromatic compounds in which one or more hydrogen atoms directly attached to an aromatic ring are replaced by halogen atoms such as F, Cl, Br or I.
General representation Ar–X

Ar = aryl group and X = halogen.

The simplest and most important example in this chapter is chlorobenzene, C₆H₅Cl. In chlorobenzene, chlorine is bonded directly to an sp² carbon of the benzene ring.

Chlorobenzene — Simplest Haloarene Cl C₆H₅–Cl halogen bonded directly to aromatic sp² carbon

Diagram 1: Structure of chlorobenzene

2. Haloalkane vs Haloarene

FeatureHaloalkaneHaloarene
General formR–XAr–X
Carbon bonded to XUsually sp³sp² aromatic carbon
ExampleCH₃CH₂ClC₆H₅Cl
C–X bondOrdinary polar sigma bondHas partial double-bond character due to resonance
Nucleophilic substitutionUsually easierUsually more difficult
Remember If the halogen is directly bonded to the aromatic ring, the compound is a haloarene. If the halogen is on a side-chain sp³ carbon, it behaves as a haloalkane even if a benzene ring is also present.

3. Nomenclature of Haloarenes

In simple haloarenes, benzene is used as the parent name and the halogen is named as a prefix.

Formula / substitutionIUPAC/common name
C₆H₅ClChlorobenzene
C₆H₅BrBromobenzene
C₆H₅IIodobenzene
1,2-C₆H₄Cl₂1,2-Dichlorobenzene / o-dichlorobenzene
1,3-C₆H₄Cl₂1,3-Dichlorobenzene / m-dichlorobenzene
1,4-C₆H₄Cl₂1,4-Dichlorobenzene / p-dichlorobenzene

3.1 Position Isomerism

Disubstituted haloarenes may show ortho (1,2-), meta (1,3-) and para (1,4-) position isomerism.

Position Isomerism in Dichlorobenzene ortho (1,2-) Cl Cl meta (1,3-) Cl Cl para (1,4-) Cl Cl

Diagram 2: Ortho, meta and para dichlorobenzene

4. Preparation of Chlorobenzene

4.1 From Benzene

Benzene reacts with chlorine in the presence of a Lewis-acid catalyst such as anhydrous FeCl₃ or AlCl₃.

C₆H₆ + Cl₂ → C₆H₅Cl + HCl   (FeCl₃)

This is an electrophilic aromatic substitution reaction.

4.2 From Benzene Diazonium Chloride

Sandmeyer Reaction

C₆H₅N₂⁺Cl⁻ → C₆H₅Cl + N₂↑   (CuCl/HCl)

Gattermann-Type Conversion

Benzene diazonium chloride can also be converted to chlorobenzene using copper powder in the presence of hydrochloric acid.

Preparation of Chlorobenzene C₆H₅Cl chlorobenzene Benzene Cl₂ / FeCl₃ Benzenediazonium chloride CuCl / HCl Direct ring chlorination or diazonium-group replacement Both routes are important NEB exam reactions.

Diagram 3: Two syllabus routes for preparing chlorobenzene

Exam Important Do not write UV light for the benzene → chlorobenzene reaction. For ring chlorination, use a Lewis-acid catalyst such as FeCl₃.

5. Physical Properties of Haloarenes

PropertyGeneral behaviour
StateMany lower haloarenes are liquids; some higher or more symmetrical members are solids.
Solubility in waterVery low because haloarenes cannot form sufficiently strong interactions with water.
Solubility in organic solventsGenerally soluble in common organic solvents.
Boiling pointGenerally rises with molecular mass and polarizability.
DensityOften increases as heavier halogens are introduced.
OdourMany have characteristic aromatic/halogenated-organic odours.
Structural effect Para isomers can sometimes pack more efficiently in a crystal lattice than corresponding ortho or meta isomers, which may affect melting points.

6. Why Haloarenes Are Less Reactive toward Nucleophilic Substitution

Chlorobenzene is much less reactive toward ordinary nucleophilic substitution than a typical chloroalkane. Several structural factors contribute.

6.1 Resonance Gives Partial Double-Bond Character

The lone pair on chlorine can interact with the aromatic π system. Resonance structures can be written in which the C–Cl bond has partial double-bond character.

Resonance in Chlorobenzene Cl: ↔ Cl⁺ − Resonance strengthens and shortens the C–Cl bond relative to a simple alkyl C–Cl bond.

Diagram 4: Resonance contribution in chlorobenzene

6.2 sp² Carbon Forms a Stronger C–X Bond

The aromatic carbon bonded to chlorine is sp²-hybridized. Compared with an sp³ carbon, it has greater s-character, so the C–Cl bond is shorter and stronger.

6.3 Ordinary SN1 Is Unfavourable

Breaking the C–Cl bond to form a phenyl cation would produce an extremely unstable cation and disrupt aromatic stabilization. Therefore the normal haloalkane SN1 pathway is not favourable.

6.4 Ordinary SN2 Backside Attack Is Difficult

The aromatic ring geometry and electron-rich π system make normal backside displacement at the sp² carbon difficult.

ReasonEffect
ResonanceC–X bond gains partial double-bond character
sp² carbonC–X bond is shorter and stronger than in a comparable sp³ haloalkane
Phenyl cation instabilityOrdinary SN1 pathway is strongly disfavoured
Backside-attack difficultyOrdinary SN2 pathway is strongly disfavoured
Best short-answer format If asked why chlorobenzene is less reactive than chloroethane toward nucleophilic substitution, mention at least: resonance/partial double-bond character + stronger sp² C–Cl bond. For a fuller answer, also discuss why ordinary SN1 and SN2 paths are unfavourable.

7. Reduction of Chlorobenzene

Chlorobenzene can be reduced to benzene by replacing chlorine with hydrogen under suitable reducing conditions.

C₆H₅Cl + 2[H] → C₆H₆ + HCl

The important exam idea is the conversion:

chlorobenzene → benzene
Reduction of Chlorobenzene C₆H₅Cl chlorobenzene C₆H₆ benzene reduction, +2[H] −HCl Halogen is replaced by hydrogen.

Diagram 5: Reduction of chlorobenzene

8. Electrophilic Substitution Reactions of Chlorobenzene

Although chlorine deactivates the benzene ring overall, it directs new electrophilic substitution mainly to the ortho and para positions because its lone-pair resonance donation stabilizes the corresponding intermediates.

Key idea Halogens are an unusual group: deactivating but ortho/para directing.

8.1 Nitration

C₆H₅Cl + HNO₃ → o-ClC₆H₄NO₂ + p-ClC₆H₄NO₂ + H₂O   (conc. H₂SO₄)

8.2 Sulphonation

C₆H₅Cl + H₂SO₄(fuming) → o-/p-ClC₆H₄SO₃H + H₂O

8.3 Halogenation

C₆H₅Cl + Cl₂ → o-/p-C₆H₄Cl₂ + HCl   (FeCl₃)
Electrophilic Substitution in Chlorobenzene Cl o m o p Major orientation: ortho and para Chlorine deactivates the ring but directs electrophilic attack to o/p positions.

Diagram 6: Ortho/para-directing effect of chlorine

Common Mistake Do not conclude that “deactivating” means “meta directing.” Halogens are deactivating because of their −I effect, but their lone-pair resonance donation makes them ortho/para directors.

9. Fittig Reaction

Fittig reaction Two molecules of an aryl halide react with sodium metal in dry ether to form a biaryl compound.
2Ar–X + 2Na → Ar–Ar + 2NaX   (dry ether)

Example

2C₆H₅Cl + 2Na → C₆H₅–C₆H₅ + 2NaCl

The product is biphenyl.

Fittig Reaction 2 Ar–X aryl halide 2Na dry ether Ar–Ar biaryl product Chlorobenzene gives biphenyl.

Diagram 7: Fittig coupling

10. Wurtz–Fittig Reaction

Wurtz–Fittig reaction An aryl halide and an alkyl halide react with sodium in dry ether to form an alkyl-substituted aromatic hydrocarbon.
Ar–X + R–X + 2Na → Ar–R + 2NaX   (dry ether)

Example

C₆H₅Cl + CH₃Cl + 2Na → C₆H₅CH₃ + 2NaCl

The product is methylbenzene (toluene).

Fittig vs Wurtz–Fittig Fittig: aryl + aryl → biaryl.
Wurtz–Fittig: aryl + alkyl → alkyl arene.

11. Action of Chlorobenzene with Chloral

Chlorobenzene reacts with chloral, CCl₃CHO, in the presence of concentrated sulphuric acid to form DDT (dichlorodiphenyltrichloroethane).

2C₆H₅Cl + CCl₃CHO → (p-ClC₆H₄)₂CHCCl₃ + H₂O   (conc. H₂SO₄)
Chlorobenzene + Chloral → DDT 2 C₆H₅Cl chlorobenzene CCl₃CHO chloral DDT (p-ClC₆H₄)₂CHCCl₃ + H₂O conc. H₂SO₄ DDT is a persistent organochlorine insecticide. Its environmental persistence and bioaccumulation led to severe restrictions or bans in many uses.

Diagram 8: Formation of DDT from chlorobenzene and chloral

Environmental Note DDT is taught here because its preparation is part of the chemistry syllabus. It is environmentally persistent and can bioaccumulate, so its historical use should not be interpreted as a recommendation for uncontrolled application.

12. Uses of Haloarenes

  • Intermediates in the manufacture of dyes, pharmaceuticals, agrochemicals and other organic compounds.
  • Chlorobenzene is used as an industrial solvent and chemical intermediate.
  • Some haloarenes are used in the synthesis of pesticides and specialty chemicals.
  • Halogenated aromatic structures occur in some polymers, medicinal compounds and industrial materials.
Important perspective Uses depend strongly on the specific compound. Some haloarenes are useful industrial intermediates, while others are restricted because of persistence, toxicity or environmental impact.

13. High-Yield Reaction Summary

ReactionReagent / conditionMajor product
Benzene chlorinationCl₂ / FeCl₃Chlorobenzene
SandmeyerBenzenediazonium chloride + CuCl/HClChlorobenzene + N₂
ReductionReducing conditionsBenzene
NitrationConc. HNO₃ / H₂SO₄o- and p-nitrochlorobenzene
SulphonationFuming H₂SO₄o-/p-chlorobenzenesulphonic acid
Further chlorinationCl₂ / FeCl₃o-/p-dichlorobenzene
Fittig2Na / dry etherBiphenyl from chlorobenzene
Wurtz–FittigAlkyl halide + 2Na / dry etherAlkylbenzene
With chloralCCl₃CHO / conc. H₂SO₄DDT

14. Common Exam Mistakes

  • Calling benzyl chloride, C₆H₅CH₂Cl, a haloarene. It is a side-chain haloalkane because Cl is bonded to an sp³ carbon.
  • Using UV light instead of FeCl₃ for direct chlorination of benzene to chlorobenzene.
  • Forgetting that chlorobenzene is less reactive toward normal nucleophilic substitution because of resonance and the stronger sp² C–Cl bond.
  • Writing chlorobenzene as meta directing. Chlorine is deactivating but ortho/para directing.
  • Confusing Fittig with Wurtz–Fittig.
  • Forgetting dry ether and sodium in coupling reactions.
  • Writing chloral incorrectly. Chloral is CCl₃CHO.
  • Writing DDT formation as a simple substitution reaction without the chloral reagent and acid condition.
  • Claiming haloarenes never undergo nucleophilic substitution. The correct statement is that they are much less reactive under ordinary haloalkane SN1/SN2 conditions.

15. Worked Examples

Worked Example 1: Haloarene or haloalkane?

C₆H₅Cl: Cl is directly bonded to aromatic sp² carbon → haloarene.

C₆H₅CH₂Cl: Cl is bonded to CH₂ sp³ carbon → haloalkane (benzyl chloride).

Worked Example 2: Product of Fittig reaction 2C₆H₅Cl + 2Na → C₆H₅–C₆H₅ + 2NaCl

Product: biphenyl.

Worked Example 3: Product of Wurtz–Fittig reaction C₆H₅Cl + C₂H₅Cl + 2Na → C₆H₅C₂H₅ + 2NaCl

Product: ethylbenzene.

Worked Example 4: Orientation in nitration

Chlorobenzene undergoes nitration mainly at the ortho and para positions. Chlorine reduces overall ring reactivity by its inductive effect but donates electron density by resonance to ortho/para positions.

16. Important Exam Questions

Short-Answer Questions

  1. Define haloarene and give two examples.
  2. Differentiate between haloalkane and haloarene.
  3. Give the IUPAC names of the three dichlorobenzene isomers.
  4. How is chlorobenzene prepared from benzene?
  5. How is chlorobenzene prepared from benzenediazonium chloride?
  6. Why is chlorobenzene less reactive than chloroethane toward nucleophilic substitution?
  7. Why does the C–Cl bond in chlorobenzene have partial double-bond character?
  8. What is the product obtained on reduction of chlorobenzene?
  9. Why is chlorine called deactivating but ortho/para directing?
  10. What are the major products of nitration of chlorobenzene?
  11. Define Fittig reaction with an example.
  12. Define Wurtz–Fittig reaction with an example.
  13. What is chloral? Write its formula.
  14. What product is formed when chlorobenzene reacts with chloral in concentrated H₂SO₄?
  15. State two uses of haloarenes.

Long-Answer Questions

  1. Explain preparation of chlorobenzene from benzene and benzenediazonium chloride with equations.
  2. Explain why haloarenes are less reactive than haloalkanes toward nucleophilic substitution.
  3. Describe electrophilic substitution reactions of chlorobenzene: nitration, sulphonation and halogenation.
  4. Explain why chlorine is deactivating yet ortho/para directing.
  5. Explain Fittig and Wurtz–Fittig reactions with equations and products.
  6. Describe the reaction of chlorobenzene with chloral and identify the product.

Conversion / Reaction Questions

  1. Benzene → chlorobenzene.
  2. Benzenediazonium chloride → chlorobenzene.
  3. Chlorobenzene → benzene.
  4. Chlorobenzene → nitrochlorobenzene.
  5. Chlorobenzene → biphenyl.
  6. Chlorobenzene + methyl chloride → toluene.
  7. Chlorobenzene + chloral → DDT.

Diagram Questions

  1. Draw chlorobenzene and show the halogen directly attached to the aromatic ring.
  2. Draw ortho, meta and para dichlorobenzene.
  3. Draw a preparation map for chlorobenzene.
  4. Draw resonance structures showing partial double-bond character of C–Cl.
  5. Show ortho/para directing positions in chlorobenzene.
  6. Draw the Fittig reaction scheme.
  7. Draw the chlorobenzene + chloral → DDT reaction map.
Exam Strategy Haloarenes is a short unit. Focus on reaction conditions, the reason for low nucleophilic-substitution reactivity, ortho/para orientation, Fittig/Wurtz–Fittig distinction, and DDT formation.

17. One-Minute Revision

  • Haloarenes are aromatic compounds with X directly attached to an aromatic sp² carbon.
  • General form: Ar–X.
  • Chlorobenzene is C₆H₅Cl.
  • Disubstituted haloarenes show ortho, meta and para position isomerism.
  • Benzene + Cl₂/FeCl₃ gives chlorobenzene.
  • Benzenediazonium chloride + CuCl/HCl gives chlorobenzene + N₂.
  • Chlorobenzene is less reactive toward ordinary nucleophilic substitution than haloalkanes.
  • Resonance gives the aromatic C–Cl bond partial double-bond character.
  • The sp² C–Cl bond is shorter and stronger than a comparable sp³ C–Cl bond.
  • Reduction of chlorobenzene gives benzene.
  • Chlorine is deactivating but ortho/para directing.
  • Nitration, sulphonation and halogenation mainly give ortho/para products.
  • Fittig: Ar–X + Ar–X → Ar–Ar.
  • Wurtz–Fittig: Ar–X + R–X → Ar–R.
  • Both coupling reactions use Na in dry ether.
  • Chloral = CCl₃CHO.
  • Chlorobenzene + chloral / conc. H₂SO₄ gives DDT.
  • Some haloarenes are important industrial intermediates, while some persistent compounds are environmentally restricted.

18. Diagram Practice

Students should practice these diagrams for the NEB examination:

  1. Structure of chlorobenzene.
  2. Ortho, meta and para dichlorobenzene.
  3. Preparation routes to chlorobenzene.
  4. Resonance of chlorobenzene showing partial C–Cl double-bond character.
  5. Reduction of chlorobenzene to benzene.
  6. Ortho/para directing positions in chlorobenzene.
  7. Fittig reaction scheme.
  8. Formation of DDT from chlorobenzene and chloral.
Source handling: The original Nepal eNotes PDF remains embedded above. The typed section follows the verified NEB/CDC syllabus and is designed as a searchable, responsive study companion. Where the PDF viewer does not expose handwritten page text, the typed section is a syllabus-aligned reconstruction and is not claimed to be a word-for-word transcription.
KEEP LEARNING View All Posts

Discussion

Share a helpful question, idea, or explanation with other students.

Leave a Comment

Write a clear question, answer, or helpful explanation.
Your email will not be published.

Download Our Offline App

Study class-wise notes even when internet is not available. Get the app from Play Store.

Nepal eNotes offline app preview
Get it on Google Play