Introduction
Haloalkanes and Haloarenes are one of the most important chapters of Class 12 Chemistry ( Haloalkanes and Haloarens Class 12 Notes) (Organic Chemistry). This chapter builds the foundation for understanding reaction mechanisms like SN1, SN2, E1 and E2, which are very important for Board exams, NEET, and JEE. Students often find this chapter confusing because of multiple reactions and mechanisms, but with proper conceptual clarity, it becomes easy and scoring.
Haloalkanes and Haloarenes Class 12 Notes are organic compounds in which one or more hydrogen atoms of alkanes or aromatic hydrocarbons are replaced by halogen atoms (F, Cl, Br, I). These compounds are widely used in medicines, pesticides, solvents, refrigerants, and polymers.
Haloalkanes and Haloarenes Class 12 Notes PDF | NCERT + Important Questions
Classification of Haloalkanes and Haloarenes
1. Haloalkanes (Alkyl Halides)
Haloalkanes are compounds in which a halogen atom is attached to an sp³ hybridised carbon atom.
General Formula: R–X (where R = alkyl group, X = halogen)
Types of Haloalkanes:
- Primary (1°): Halogen attached to a carbon bonded with one carbon atom
- Secondary (2°): Halogen attached to a carbon bonded with two carbon atoms
- Tertiary (3°): Halogen attached to a carbon bonded with three carbon atoms
Example: CH₃CH₂Cl (Ethyl chloride)
2. Haloarenes (Aryl Halides)
Haloarenes are compounds in which a halogen atom is directly attached to an sp² hybridised carbon atom of an aromatic ring.
Example: Chlorobenzene (C₆H₅Cl)
Haloarenes are less reactive than haloalkanes due to resonance and partial double bond character of C–X bond.
Nature of C–X Bond
The C–X bond in haloalkanes is polar due to difference in electronegativity between carbon and halogen.
Order of Bond Strength: C–F > C–Cl > C–Br > C–I
Order of Reactivity: RI > RBr > RCl > RF
This is because C–I bond is weakest and breaks easily.
Methods of Preparation of Haloalkanes
1. From Alcohols
Alcohols react with halogen acids (HX) to form haloalkanes.
Reaction: ROH + HX → RX + H₂O
Reactivity Order: 3° alcohol > 2° alcohol > 1° alcohol
Lucas reagent (conc. HCl + anhydrous ZnCl₂) is used to distinguish alcohols.
2. From Alkanes (Free Radical Substitution)
Alkanes react with halogens in presence of sunlight.
CH₄ + Cl₂ → CH₃Cl + HCl
This reaction is not very selective.
3. From Alkenes
Alkenes react with HX to form haloalkanes.
Markovnikov Rule: Hydrogen adds to the carbon with more hydrogen atoms.
In presence of peroxide, anti-Markovnikov addition occurs (only with HBr).
Physical Properties
- Haloalkanes are generally colorless liquids or gases
- Insoluble in water but soluble in organic solvents
- Density increases with increase in halogen size
- Boiling point increases with molecular mass
Chemical Reactions of Haloalkanes
1. Nucleophilic Substitution Reactions
(a) SN1 Reaction
- Two-step mechanism
- Carbocation formation
- Favoured by 3° haloalkanes
- Rate depends only on substrate
(b) SN2 Reaction
- One-step mechanism
- Backside attack
- Favoured by 1° haloalkanes
- Inversion of configuration occurs
2. Elimination Reactions
Haloalkanes on heating with alcoholic KOH undergo elimination to form alkenes.
Reaction: R–X + KOH → Alkene + KX + H₂O
3. Reaction with Metals (Wurtz Reaction)
2R–X + 2Na → R–R + 2NaX
Used for preparation of higher alkanes.
Haloarenes: Preparation
1. From Benzene (Electrophilic Substitution)
Benzene reacts with halogens in presence of Lewis acid.
C₆H₆ + Cl₂ → C₆H₅Cl + HCl
2. From Diazonium Salts
Sandmeyer reaction is used.
C₆H₅N₂⁺Cl⁻ + CuCl → C₆H₅Cl + N₂
Physical Properties of Haloarenes
- Generally colorless liquids
- Insoluble in water
- Higher boiling points than benzene
Chemical Reactions of Haloarenes
1. Nucleophilic Substitution
Haloarenes are less reactive due to:
- Resonance
- Partial double bond character
2. Electrophilic Substitution Reactions
Halogen is deactivating but ortho-para directing.
Uses of Haloalkanes and Haloarenes
- Solvents (chloroform, carbon tetrachloride)
- Medicines (anesthetics)
- Pesticides (DDT)
- Refrigerants (Freons)
Environmental Effects
Excessive use of halogen compounds causes:
- Ozone depletion
- Global warming
Important Exam Points
- SN1 vs SN2 differences
- Order of reactivity
- Markovnikov rule
- Uses and harmful effects
Conclusion–Haloalkanes and Haloarenes Class 12 Notes
Haloalkanes and Haloarenes is a high-weightage chapter. Proper understanding of reaction mechanisms and concepts can help students score well in exams. Regular practice of reactions and previous year questions is highly recommended.
✅ Haloalkanes and Haloarenes Class 12 Important Questions (Repeated)
⭐ 1 Mark Very Short Questions (Most Repeated)
- Haloalkanes kya hote hain?
- Haloarenes kya hote hain?
- Haloalkanes ka general formula likho.
- C–X bond polar kyun hota hai?
- C–F bond sabse strong kyun hota hai?
- Reactivity order likho: R–I, R–Br, R–Cl, R–F
- Primary, secondary, tertiary haloalkane ki definition do.
- Nucleophile kya hota hai? Example do.
- Leaving group ka matlab kya hota hai?
- SN1 reaction ka full form likho.
- SN2 reaction ka full form likho.
- Wurtz reaction me kaunsa metal use hota hai?
- Alcoholic KOH ka use kis reaction me hota hai?
- Chlorobenzene nucleophilic substitution me slow kyun hota hai?
- Markovnikov rule statement likho.
⭐ 2 Marks Questions (Short Answer)
- Haloalkanes aur Haloarenes me difference likho.
- Bond strength order aur reactivity order explain karo.
- Haloalkanes water me soluble kyun nahi hote?
- Boiling point haloalkanes ka increase kyun hota hai?
- SN1 aur SN2 me 2 differences likho.
- Primary haloalkane SN2 aur tertiary haloalkane SN1 kyun follow karta hai?
- Alcohols se haloalkanes banane ka reaction likho (HX method).
- Alkyl halide + aq. KOH se kya banta hai? Reaction likho.
- Alkyl halide + alcoholic KOH se kya banta hai? Reaction likho.
- Anti-Markovnikov addition kya hai? (HBr + peroxide) explain karo.
⭐ 3 Marks Questions (Frequently Asked)
- SN1 mechanism explain karo (carbocation formation + steps).
- SN2 mechanism explain karo (backside attack + inversion).
- SN1 vs SN2 comparison (rate law + substrate + mechanism).
- Wurtz reaction likho aur ek limitation बताओ.
- Haloarenes nucleophilic substitution resist kyun karte hain? 3 reasons लिखो.
- Sandmeyer reaction kya hai? Reaction with example.
- Finkelstein reaction aur Swarts reaction likho.
- Elimination reaction explain karo (E1/E2 basics).
⭐ 5 Marks Long Answer Questions (Board Level)
- SN1 reaction mechanism with example + rate law explain karo.
- SN2 reaction mechanism with stereochemistry (inversion of configuration).
- Haloalkanes ki preparation methods explain karo:
- From alcohols
- From alkenes
- From alkanes
- Haloarenes ki preparation explain karo:
- From benzene (halogenation)
- From diazonium salts (Sandmeyer/Gattermann)
- Haloarenes ki low reactivity ka detailed reason (resonance + partial double bond).
- Environmental effects of halogen compounds:
- CFCs & ozone depletion
- DDT pollution
⭐ Assertion–Reason Questions (Repeated Pattern)
- Assertion: Tertiary haloalkanes SN1 faster देते हैं।
Reason: Tertiary carbocation most stable होता है। - Assertion: Haloarenes nucleophilic substitution नहीं करते आसानी से।
Reason: C–X bond resonance se strong हो जाता है। - Assertion: RI सबसे reactive haloalkane होता है।
Reason: C–I bond weakest होता है। - Assertion: SN2 reaction inversion दिखाता है।
Reason: Nucleophile backside attack करता है।
⭐ Case Study / Competency Based Questions (New CBSE)
- Ek alkyl halide ko aq. KOH ke saath heat kiya गया. Product alcohol बना।
(a) Reaction type बताओ
(b) General equation लिखो
(c) Nucleophile कौन है? - Ek alkyl halide ko alcoholic KOH ke saath heat kiya गया. Alkene बना।
(a) Reaction type बताओ
(b) Why alcoholic KOH preferred?
(c) Product ka name लिखो
✅ Top “Most Repeated” Topics (Must Prepare)
🔥 SN1 vs SN2 (difference + mechanism)
🔥 Reactivity order: RI > RBr > RCl > RF
🔥 Alcoholic KOH vs Aq. KOH
🔥 Haloarenes less reactive reason (resonance)
🔥 Wurtz reaction + limitation
🔥 Markovnikov & Anti-Markovnikov (HBr + peroxide)