Class 12 Chemistry Chapter 10 Notes: Haloalkanes and Haloarenes | Important Questions & PYQs

 Class 12 Chemistry – Chapter 10 Haloalkanes and Haloarenes Notes

1. What are Haloalkanes and Haloarenes?

• Haloalkanes: Alkyl halides (R–X), X = F, Cl, Br, I

• Haloarenes: Aryl halides (Ar–X), halogen attached directly to benzene ring

📌 General formula:

• Haloalkane → CₙH₂ₙ₊₁X

• Haloarene → ArX

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2. Classification

(A) On the Basis of Carbon Atom

• 1° (Primary) → R–CH₂–X

• 2° (Secondary) → R₂CH–X

• 3° (Tertiary) → R₃C–X

(B) Special Types

• Allylic halide → CH₂=CH–CH₂–X

• Benzylic halide → C₆H₅–CH₂–X

• Vinylic halide → CH₂=CH–X

• Aryl halide → C₆H₅–X

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3. Nomenclature (IUPAC)

• Select longest carbon chain

• Number the chain to give lowest number to halogen

• Halogens treated as prefixes (chloro, bromo, iodo)

📌 Example:
CH₃–CH(Cl)–CH₃ → 2-Chloropropane

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4. Nature of C–X Bond

• Polar bond: Cδ⁺–Xδ⁻

• Bond strength:

$$\text{C–F}>\text{C–Cl}>\text{C–Br}>\text{C–I}$$

• Reactivity (leaving group ability):

$$\text{I⁻}>\text{Br⁻}>\text{Cl⁻}>\text{F⁻}$$

📌 Haloarenes are less reactive due to:

• Partial double bond character (resonance)

• sp² carbon–halogen bond

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5. Methods of Preparation of Haloalkanes

(A) From Alcohols

• Using HX (HCl, HBr, HI)

• Lucas reagent (ZnCl₂ + HCl) distinguishes 1°, 2°, 3° alcohols

(B) From Alkanes

• Free radical halogenation (Cl₂ / hv)

(C) Finkelstein Reaction

• R–Cl / R–Br + NaI (acetone) → R–I

(D) Swarts Reaction

• R–Cl → R–F using metal fluorides

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6. Preparation of Haloarenes

• From benzene by electrophilic substitution

• From diazonium salts (Sandmeyer, Gattermann)

📌 Diazonium reactions are very important for haloarenes.

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7. Physical Properties

• Insoluble in water (no H-bonding)

• Soluble in organic solvents

• Boiling point:

  • Increases with molecular mass

  • Decreases with branching

• p-Dichlorobenzene has highest m.p. due to symmetry

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8. Chemical Reactions of Haloalkanes

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(A) Nucleophilic Substitution

SN2 Mechanism

• One-step reaction

• Backside attack

• Inversion of configuration

• Favoured by:

$$1^{\circ }>2^{\circ }>3^{\circ }$$

SN1 Mechanism

• Two-step reaction

• Carbocation formation

• Racemisation

• Favoured by:

$$3^{\circ }>2^{\circ }>1^{\circ }$$

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(B) Elimination Reactions

• Alcoholic KOH → alkenes

• Follows Saytzeff rule

• More substituted alkene is major product

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(C) Reaction with Metals

Wurtz Reaction

• R–X + Na (dry ether) → R–R

Grignard Reagent Formation

• R–X + Mg (dry ether) → R–MgX
  ⚠️ Always prepared in anhydrous conditions

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9. Chemical Reactions of Haloarenes

• Do not undergo SN1 / SN2 easily

• Undergo nucleophilic substitution only under drastic conditions

• Benzyne mechanism possible

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10. Ambident Nucleophiles

• Nucleophiles attacking through two sites

• Example:

  • CN⁻ → gives nitrile (–C≡N) or isocyanide (–N≡C)

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11. Uses of Some Halo Compounds

• Freon-12 → refrigerant (now restricted)

• DDT → insecticide (restricted)

• CCl₄ → solvent, fire extinguisher

• Iodoform → antiseptic (earlier)

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12. Important Reason-Based Points (Exam Favourite)

✔ SN2 favoured by primary halides
✔ SN1 favoured by tertiary halides
✔ Haloarenes less reactive than haloalkanes
✔ p-Isomer → higher melting point
✔ Grignard reagents destroyed by water
✔ Alcoholic KOH → elimination
✔ Aqueous KOH → substitution

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13. Order & Trends to Remember

Leaving Group Ability

$$I^{-}>B{r}^{-}>C{l}^{-}>F^{-}$$

SN2 Reactivity

$$C{H}_{3}X>1^{\circ }>2^{\circ }>3^{\circ }$$

SN1 Reactivity

$$3^{\circ }>2^{\circ }>1^{\circ }$$

✍️ Top 10 Short Question–Answers (Board Exam Oriented)

1. What are haloalkanes?
   Organic compounds in which a halogen atom is attached to an sp³ hybridised carbon.

2. What are haloarenes?
   Compounds in which a halogen is directly attached to a benzene ring.

3. Write general formula of haloalkanes.
   CₙH₂ₙ₊₁X

4. Why is C–X bond polar?
   Because halogen is more electronegative than carbon.

5. Which halide is the best leaving group?
   I⁻ (iodide ion).

6. Why are haloarenes less reactive than haloalkanes?
   Due to resonance and partial double bond character of C–X bond.

7. What is Finkelstein reaction?
   Conversion of alkyl chloride/bromide into alkyl iodide using NaI in acetone.

8. What is Lucas reagent used for?
   To distinguish between 1°, 2° and 3° alcohols.

9. Which base favours elimination reaction?
   Alcoholic KOH.

10. Why are Grignard reagents prepared in dry ether?
    Because they are destroyed by moisture.

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📝 Long Answer Questions

1. Explain classification of haloalkanes and haloarenes.

Haloalkanes are classified on the basis of carbon attached to halogen:

• Primary (1°): R–CH₂–X

• Secondary (2°): R₂CH–X

• Tertiary (3°): R₃C–X

Special types:

• Allylic halides

• Benzylic halides

• Vinylic halides

Haloarenes have halogen directly attached to benzene ring:
Example: C₆H₅–Cl

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2. Explain SN1 and SN2 mechanisms of haloalkanes.

SN2 Mechanism

• One-step reaction

• Backside attack by nucleophile

• Inversion of configuration

• Favoured by: CH₃X > 1° > 2° > 3°

SN1 Mechanism

• Two-step reaction

• Carbocation intermediate

• Racemisation occurs

• Favoured by: 3° > 2° > 1°

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3. Explain important chemical reactions of haloalkanes.

(a) Nucleophilic Substitution

• With aqueous KOH → alcohol formation

(b) Elimination Reaction

• With alcoholic KOH → alkene formation

• Follows Saytzeff rule

(c) Reaction with Metals

• Wurtz reaction: forms higher alkanes

• Grignard reagent formation: R–MgX in dry ether

📝 PYQs (Previous Years’ Questions – CBSE Board Oriented)

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🔹 1 Mark PYQs

1. What are haloalkanes?

2. Which halogen has the best leaving group ability?

3. Why is C–I bond more reactive than C–Cl bond?

4. Name the reagent used in Finkelstein reaction.

5. What is the nature of C–X bond in haloalkanes?

6. Why are haloalkanes insoluble in water?

7. Write the product formed when CH₃Cl reacts with Mg in dry ether.

8. Which is more reactive towards SN1 reaction: 1° or 3° haloalkane?

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🔹 2 Mark PYQs

1. Differentiate between SN1 and SN2 reactions (any two points).

2. Why are haloarenes less reactive than haloalkanes towards nucleophilic substitution?

3. What is Lucas reagent? Write its use.

4. Write two differences between aqueous KOH and alcoholic KOH reactions.

5. Why are Grignard reagents prepared in dry ether?

6. What is Saytzeff rule?

7. Write two uses of haloalkanes or haloarenes.

8. What are ambident nucleophiles? Give one example.

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🔹 3 Mark PYQs

1. Explain SN1 mechanism with suitable example.

2. Explain SN2 mechanism with suitable example.

3. Describe Finkelstein reaction with equation.

4. Explain Wurtz reaction and write its limitation.

5. Why do haloarenes not undergo nucleophilic substitution easily?

6. Explain the order of reactivity of haloalkanes in SN1 and SN2 reactions.

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🔹 5 Mark PYQs

1. (a) What are haloalkanes and haloarenes?
   (b) Explain SN1 and SN2 mechanisms with suitable examples.

2. (a) Describe methods of preparation of haloalkanes from alcohols.
   (b) Explain Lucas test.

3. (a) Explain elimination reaction of haloalkanes.
   (b) What is Saytzeff rule?

4. (a) Explain preparation and properties of Grignard reagents.
   (b) Why are they highly reactive?

5. (a) Explain nucleophilic substitution reactions of haloarenes.
   (b) Why are drastic conditions required?

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🔑 Most Repeated PYQ Areas (Exam Favourite)

✔ SN1 vs SN2 (mechanism + order)
✔ Haloarenes vs haloalkanes reactivity
✔ Finkelstein & Wurtz reactions
✔ Grignard reagent (very important)
✔ Saytzeff rule
✔ Lucas test

✔ Leaving group ability order 

❓ FAQs (Concept Clarity)

1. Why does SN2 reaction show inversion of configuration?
   Due to backside attack of nucleophile.

2. Why is SN1 favoured by tertiary halides?
   Because tertiary carbocations are more stable.

3. Why are haloalkanes insoluble in water?
   Because they cannot form hydrogen bonds with water.

4. Why does p-dichlorobenzene have higher melting point?
   Due to symmetrical structure and better packing.

5. Why do haloarenes not undergo SN1/SN2 easily?
   Because C–X bond has partial double bond character due to resonance.

Chapter No.	Chapter Name	Visit
1	The Solid State	Visit
2	Solutions	Visit
3	Electrochemistry	Visit
4	Chemical Kinetics	Visit
5	Surface Chemistry	Visit
6	General Principles and Processes of Isolation of Elements	Visit
7	The p-Block Elements	Visit
8	The d and f Block Elements	Visit
9	Coordination Compounds	Visit
10	Haloalkanes and Haloarenes	Visit
11	Alcohols, Phenols and Ethers	Visit
12	Aldehydes, Ketones and Carboxylic Acids	Visit
13	Amines	Visit
14	Biomolecules	Visit
15	Polymers	Visit
16	Chemistry in Everyday Life	Visit

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