Haloalkanes and Haloarenes Class 12 Chemistry Notes – CBSE Quick Revision

📘 Haloalkanes and Haloarenes Class 12 Chemistry Notes provide a quick revision of important definitions, chemical reactions, named reactions, reaction mechanisms, and CBSE exam questions. These short notes are useful for Class 12 students preparing for board examinations and strengthening their understanding of organic chemistry.

🎯 Chapter: Haloalkanes and Haloarenes

📚 Subject: Chemistry | Class 12 CBSE

✍️ Best for: Quick revision, reaction practice, and exam preparation

---

1. 🧪 What Are Haloalkanes and Haloarenes?

Haloalkanes

Haloalkanes are compounds formed when one or more hydrogen atoms of an alkane are replaced by halogen atoms such as fluorine (F), chlorine (Cl), bromine (Br), or iodine (I).

General formula of monohaloalkanes: C_nH_2n+1X

Example: CH₃Cl (chloromethane)

Haloarenes

Haloarenes are aromatic compounds in which a halogen atom is directly attached to an aromatic ring.

Example: C₆H₅Cl (chlorobenzene)

2. 📌 Classification of Haloalkanes

• Primary (1°): The carbon attached to the halogen is connected to one other carbon atom. Example: CH₃CH₂Cl.
• Secondary (2°): The carbon attached to the halogen is connected to two other carbon atoms. Example: CH₃CHClCH₃.
• Tertiary (3°): The carbon attached to the halogen is connected to three other carbon atoms. Example: (CH₃)₃CCl.

Other important types:

• Geminal dihalides: Two halogen atoms are attached to the same carbon atom.
• Vicinal dihalides: Two halogen atoms are attached to adjacent carbon atoms.
• Allylic halides: The halogen is attached to an sp³-hybridised carbon next to a carbon-carbon double bond.
• Benzylic halides: The halogen is attached to a carbon next to an aromatic ring.
• Vinylic halides: The halogen is directly attached to a carbon of a carbon-carbon double bond.

3. ⚡ Nature of the Carbon–Halogen Bond

Halogens are more electronegative than carbon. Therefore, the carbon-halogen bond is polar:

C^δ+ — X^δ−

• Bond length: C–F < C–Cl < C–Br < C–I
• Bond strength: C–F > C–Cl > C–Br > C–I
• Bond dissociation energy: C–F > C–Cl > C–Br > C–I

💡 The relatively weaker C–I bond makes alkyl iodides generally more reactive than corresponding alkyl bromides and chlorides in many nucleophilic substitution reactions.

4. 🔬 Preparation of Haloalkanes

A. From Alkanes: Free-Radical Halogenation

Alkanes react with chlorine or bromine in the presence of light or heat.

Example:

CH₄ + Cl₂  —(hν)→  CH₃Cl + HCl

The mechanism involves three steps:

1. Initiation: Formation of free radicals.
2. Propagation: Radicals react and generate further radicals.
3. Termination: Radicals combine to form stable molecules.

B. From Alkenes: Addition of Hydrogen Halides

Hydrogen halides (HX) add across the double bond of an alkene. For an unsymmetrical alkene, the usual orientation follows Markovnikov's rule.

Example:

CH₃CH=CH₂ + HBr → CH₃CHBrCH₃

C. Anti-Markovnikov Addition

In the presence of peroxide, HBr adds to suitable unsymmetrical alkenes according to the anti-Markovnikov rule. This peroxide effect is generally observed with HBr, not HCl or HI.

D. From Alcohols

Alcohols can be converted into haloalkanes using reagents such as:

• HCl / anhydrous ZnCl₂ — Lucas reagent
• PCl₅ or PCl₃
• SOCl₂ — thionyl chloride
• HBr or HI under suitable conditions

General reaction:

R–OH + SOCl₂ → R–Cl + SO₂ + HCl

⭐ Thionyl chloride is often preferred for preparing alkyl chlorides because the by-products SO₂ and HCl are gases that can escape from the reaction mixture.

E. Finkelstein Reaction

Alkyl chlorides or bromides react with sodium iodide in dry acetone to form alkyl iodides.

R–X + NaI → R–I + NaX ↓

Condition: Dry acetone; precipitation of NaCl or NaBr helps drive the reaction.

F. Swarts Reaction

Alkyl chlorides or bromides are converted into alkyl fluorides using suitable metallic fluorides, such as AgF or SbF₃.

R–X → R–F

5. ⚙️ Important Reactions of Haloalkanes

A. Nucleophilic Substitution Reaction

A nucleophile replaces the halogen atom in a haloalkane.

R–X + Nu⁻ → R–Nu + X⁻

Important examples:

• Aqueous KOH: R–X → R–OH (alcohol)
• KCN: R–X → R–CN (nitrile)
• AgCN: R–X → R–NC (isocyanide, generally)
• Excess alcoholic NH₃: Can produce a primary amine.
• KNO₂: Generally forms an alkyl nitrite, R–ONO.
• AgNO₂: Generally forms a nitroalkane, R–NO₂.

B. SN1 Mechanism

• SN1 means substitution nucleophilic unimolecular.
• It occurs in two main stages, with carbocation formation as the slow step.
• Rate law: Rate = k[haloalkane].
• Carbocation stability strongly influences the reaction rate.
• Typical reactivity trend for many simple substrates: 3° > 2° > 1°.
• Carbocation rearrangement may occur.
• A planar carbocation can lead to racemisation at a chiral centre, although the extent may vary.

C. SN2 Mechanism

• SN2 means substitution nucleophilic bimolecular.
• It occurs in a single concerted step.
• Rate law: Rate = k[haloalkane][nucleophile].
• Backside attack leads to inversion of configuration at a chiral centre.
• Steric hindrance affects the rate.
• Typical reactivity trend: CH₃X > 1° > 2° > 3°.

D. Elimination Reaction (Dehydrohalogenation)

When a haloalkane is heated with alcoholic KOH, HX is eliminated and an alkene is generally formed.

Haloalkane + alcoholic KOH → Alkene + KX + H₂O

Saytzeff's rule: In many elimination reactions, the more substituted alkene is the major product.

E. Reaction with Magnesium: Grignard Reagent

Haloalkanes react with magnesium in dry ether to form Grignard reagents.

R–X + Mg → R–MgX

⚠️ Grignard reagents react with water, so they must be prepared and handled under dry conditions.

F. Wurtz Reaction

Two molecules of an alkyl halide react with sodium in dry ether to form a higher alkane.

2R–X + 2Na → R–R + 2NaX

6. 🌿 Haloarenes: Preparation and Properties

Preparation by Direct Halogenation

Benzene reacts with chlorine in the presence of anhydrous FeCl₃ to form chlorobenzene.

C₆H₆ + Cl₂ → C₆H₅Cl + HCl

Preparation from Diazonium Salts

Aniline is first converted into benzene diazonium chloride using NaNO₂ and HCl at 273–278 K.

• Sandmeyer reaction: CuCl/HCl gives chlorobenzene; CuBr/HBr gives bromobenzene.
• Balz–Schiemann reaction: Used to prepare fluorobenzene from a suitable diazonium tetrafluoroborate.
• Iodination: Benzene diazonium salt reacts with KI to give iodobenzene.

Why Are Haloarenes Less Reactive Towards Nucleophilic Substitution?

• Resonance gives the carbon-halogen bond partial double-bond character.
• The carbon attached to the halogen is sp²-hybridised.
• The carbon-halogen bond is relatively shorter and stronger than in typical haloalkanes.
• A phenyl carbocation is highly unstable, making the usual SN1 pathway unfavourable.

Electrophilic Substitution in Haloarenes

Halogens deactivate the aromatic ring overall but direct incoming electrophiles mainly to the ortho and para positions.

• Halogenation: Produces ortho- and para-dihalobenzene derivatives.
• Nitration: Uses concentrated HNO₃ and H₂SO₄.
• Sulphonation: Uses concentrated H₂SO₄.
• Friedel–Crafts alkylation: Uses an alkyl halide and anhydrous AlCl₃.
• Friedel–Crafts acylation: Uses an acyl halide and anhydrous AlCl₃.

7. 🔥 Important Named Reactions at a Glance

• Finkelstein reaction: Preparation of alkyl iodides.
• Swarts reaction: Preparation of alkyl fluorides.
• Wurtz reaction: Coupling of alkyl halides to form alkanes.
• Wurtz–Fittig reaction: Coupling of an aryl halide and an alkyl halide using sodium in dry ether.
• Fittig reaction: Coupling of aryl halides using sodium in dry ether.
• Sandmeyer reaction: Replacement of the diazonium group by Cl or Br using suitable copper(I) salts.
• Balz–Schiemann reaction: Preparation of aryl fluorides.
• Hunsdiecker reaction: Decarboxylative halogenation of silver salts of carboxylic acids, commonly with bromine.

8. 🌍 Polyhalogen Compounds

• Chloroform: CHCl₃.
• Iodoform: CHI₃.
• Carbon tetrachloride: CCl₄.
• Freons: Chlorofluorocarbons formerly used in refrigeration; some damage the ozone layer.
• DDT: A persistent insecticide that can accumulate in food chains and harm wildlife.

🌱 Environmental note: Some halogen-containing compounds are persistent pollutants. Certain chlorofluorocarbons contribute to stratospheric ozone depletion, while DDT can biomagnify in food chains.

9. ⭐ Important Questions for Class 12 Chemistry

Very Short Answer Questions (1 Mark)

1. Define haloalkanes and haloarenes with one example each.
2. What is a nucleophile?
3. Write the general formula of monohaloalkanes.
4. What is a Grignard reagent? Write its general formula.
5. Name the reaction used to prepare alkyl iodides from alkyl chlorides or bromides.
6. Which positions are favoured in electrophilic substitution of haloarenes?

Short Answer Questions (2–3 Marks)

1. Differentiate between geminal and vicinal dihalides with examples.
2. Explain why haloalkanes are polar but generally insoluble in water.
3. Differentiate between SN1 and SN2 reactions on the basis of mechanism, rate law, and stereochemistry.
4. Why are haloarenes less reactive than haloalkanes towards nucleophilic substitution?
5. State Markovnikov's rule and explain the peroxide effect with HBr.
6. Write the equations for Finkelstein and Swarts reactions.
7. Why is thionyl chloride often preferred for converting alcohols into alkyl chlorides?
8. Explain Saytzeff's rule with a suitable example.

Long Answer Questions (3–5 Marks)

1. Explain the SN1 and SN2 mechanisms with suitable examples and rate laws.
2. Describe the preparation of haloalkanes from alcohols using suitable reagents and equations.
3. Explain the preparation of haloarenes from benzene diazonium chloride through Sandmeyer and Balz–Schiemann reactions.
4. Explain why halogens are deactivating but ortho- and para-directing in haloarenes.
5. Describe the important reactions of haloarenes: nitration, sulphonation, halogenation, and Friedel–Crafts reactions.
6. Write equations and explain Wurtz, Fittig, and Wurtz–Fittig reactions.

📝 Important Reaction Practice

1. CH₃CH₂Br + aqueous KOH → ?
2. CH₃CH₂Br + alcoholic KOH, heat → ?
3. C₂H₅Br + NaI in dry acetone → ?
4. C₆H₅N₂⁺Cl⁻ + CuCl/HCl → ?
5. CH₃CH=CH₂ + HBr in the presence of peroxide → ?

Revision tip: Practise each named reaction with its reagent, reaction conditions, and major product. Learn the differences between SN1 and SN2 and revise the reasons behind the lower reactivity of haloarenes.

---

🎯 Prepare for Your Chemistry Board Exam with SkillShala

Revise Class 12 Chemistry concepts, practise important reactions, and strengthen your understanding of organic chemistry with SkillShala's learning resources.

Keywords: Haloalkanes and Haloarenes Class 12 Notes, CBSE Chemistry Notes, Haloalkanes Important Questions, Haloarenes Reactions, SN1 and SN2 Mechanism, Named Reactions Class 12 Chemistry.