Class 12 Chemistry Notes Chapter 10 (Haloalkanes and Haloarenes) – Examplar Problems Book

Alright class, let's get started with the essential concepts from Chapter 10: Haloalkanes and Haloarenes. This chapter is fundamental, and understanding it well is crucial for your competitive exams. We'll focus on the key points, reactions, and concepts, keeping the NCERT Exemplar perspective in mind.

Chapter 10: Haloalkanes and Haloarenes - Detailed Notes for Exam Preparation

1. Introduction & Classification:

• Definition: Halogen derivatives of hydrocarbons obtained by replacing one or more hydrogen atoms with corresponding halogen atoms (F, Cl, Br, I).
• Haloalkanes (Alkyl Halides): Halogen attached to an sp³ hybridized carbon atom of an alkyl group (R-X).
• Haloarenes (Aryl Halides): Halogen attached to an sp² hybridized carbon atom of an aryl group (Ar-X).
• Classification (Based on Number of Halogen Atoms):
  • Mono-, Di-, Tri-, Tetra- haloalkanes/haloarenes.
  • Gem-dihalides: Halogen atoms on the same carbon (e.g., Ethylidene chloride, CH₃CHCl₂).
  • Vic-dihalides: Halogen atoms on adjacent carbon atoms (e.g., Ethylene dichloride, CH₂Cl-CH₂Cl).
• Classification (Based on Nature of Carbon Bearing Halogen - sp³ C-X):
  • Primary (1°), Secondary (2°), Tertiary (3°): Based on whether the halogen-bearing carbon is attached to one, two, or three other carbon atoms, respectively.
  • Allylic Halides: Halogen atom bonded to an sp³-hybridized carbon next to a carbon-carbon double bond (C=C—C-X). Example: CH₂=CH-CH₂Cl (Allyl chloride).
  • Benzylic Halides: Halogen atom bonded to an sp³-hybridized carbon atom next to an aromatic ring (Ar—C-X). Example: C₆H₅CH₂Cl (Benzyl chloride).
• Classification (Based on Nature of Carbon Bearing Halogen - sp² C-X):
  • Vinylic Halides: Halogen atom bonded to an sp²-hybridized carbon atom of a carbon-carbon double bond (C=C-X). Example: CH₂=CHCl (Vinyl chloride).
  • Aryl Halides: Halogen atom bonded directly to an sp²-hybridized carbon atom of an aromatic ring (Ar-X). Example: C₆H₅Cl (Chlorobenzene).

2. Nomenclature:

• Common Names: Often derived from the alkyl group followed by the halide (e.g., Ethyl bromide). For simple haloarenes, prefixes ortho- (1,2), meta- (1,3), para- (1,4) are used for disubstituted arenes.
• IUPAC Names: Follow standard rules. Halogen is treated as a substituent (prefix: Fluoro-, Chloro-, Bromo-, Iodo-). Numbering gives the lowest number to the substituent(s). Alphabetical order is followed for multiple different substituents.

3. Nature of C-X Bond:

• Polarity: Halogen atoms are more electronegative than carbon. The C-X bond is polar covalent (Cᵅ⁺—Xᵅ⁻).
• Bond Length: Increases down the group (C-F < C-Cl < C-Br < C-I) as the size of the halogen atom increases.
• Bond Enthalpy: Decreases down the group (C-F > C-Cl > C-Br > C-I) as bond length increases.
• Dipole Moment: Generally decreases down the group (CH₃Cl > CH₃F > CH₃Br > CH₃I). Note the exception: CH₃Cl has a higher dipole moment than CH₃F due to the combined effect of electronegativity and bond length (µ = q × d).

4. Methods of Preparation:

• From Alcohols (R-OH → R-X):
  • Using HX (Lucas Test): R-OH + HX → R-X + H₂O. Reactivity order of HX: HI > HBr > HCl. Reactivity order of alcohols: 3° > 2° > 1°. Anhydrous ZnCl₂ is used as a catalyst for 1° and 2° alcohols with HCl (Lucas Reagent).
  • Using Phosphorus Halides:
    • R-OH + PCl₅ → R-Cl + POCl₃ + HCl
    • 3R-OH + PCl₃ → 3R-Cl + H₃PO₃
    • (PBr₃ and PI₃ are usually generated in situ by reacting red phosphorus with Br₂ or I₂ respectively).
  • Using Thionyl Chloride (SOCl₂): R-OH + SOCl₂ → R-Cl + SO₂↑ + HCl↑ (Darzen's Process). Preferred method for preparing alkyl chlorides as gaseous byproducts (SO₂, HCl) escape, yielding pure R-Cl.
• From Hydrocarbons:
  • Free Radical Halogenation (Alkanes): R-H + X₂ (in UV light/heat) → R-X + HX. Gives a mixture of mono- and polyhalogenated products. Not suitable for pure alkyl halide preparation, except for alkanes with equivalent hydrogens. Reactivity: F₂ > Cl₂ > Br₂ >> I₂ (Iodination is reversible and slow; done in presence of oxidizing agents like HIO₃/HNO₃). Selectivity: Br₂ is more selective than Cl₂ (prefers 3° H).
  • Electrophilic Addition to Alkenes:
    • Alkene + HX → Alkyl Halide. Follows Markovnikov's Rule (Negative part of addendum goes to the carbon with fewer H atoms). Exception: Peroxide Effect/Kharasch Effect (Only with HBr in presence of peroxide) leads to Anti-Markovnikov addition via a free-radical mechanism.
    • Alkene + X₂ (in CCl₄) → Vicinal Dihalide. Used as a test for unsaturation (decolorization of Br₂ water).
  • Halogen Exchange:
    • Finkelstein Reaction: R-Cl/R-Br + NaI (in dry acetone) → R-I + NaCl/NaBr↓. Used for preparing alkyl iodides. Acetone precipitates NaCl/NaBr, driving equilibrium forward (Le Chatelier's Principle).
    • Swarts Reaction: R-Cl/R-Br + Metallic Fluoride (AgF, Hg₂F₂, CoF₂, SbF₃) → R-F. Used for preparing alkyl fluorides.
• Preparation of Haloarenes:
  • Electrophilic Substitution (Direct Halogenation): Arene + X₂ (in presence of Lewis acid like FeCl₃, FeBr₃, anhyd. AlCl₃ and in dark) → Aryl Halide + HX. Iodination requires an oxidizing agent. Fluorination is too violent.
  • From Diazonium Salts (Sandmeyer's Reaction): ArN₂⁺X⁻ + CuX' (where X' = Cl, Br) → ArX' + N₂. For Iodoarenes: ArN₂⁺X⁻ + KI (aq) → ArI + N₂ + KX.
  • Gattermann Reaction: Modification of Sandmeyer using Copper powder instead of cuprous halide. ArN₂⁺X⁻ + Cu/HX' → ArX' + N₂ + CuX. Yields are often lower than Sandmeyer.
  • Balz-Schiemann Reaction: ArN₂⁺Cl⁻ + HBF₄ → ArN₂⁺BF₄⁻ (precipitate) → (Heat) → ArF + BF₃ + N₂. Used for preparing fluorobenzene.

5. Physical Properties:

• State: Lower members (CH₃Cl, CH₃Br, C₂H₅Cl) are gases at room temp. Higher members are liquids or solids.
• Colour: Pure alkyl/aryl halides are generally colourless. Bromides and Iodides develop colour when exposed to light (decomposition).
• Melting & Boiling Points: Higher than parent hydrocarbons due to greater polarity and higher molecular mass leading to stronger intermolecular forces (dipole-dipole and van der Waals).
  • Trend: R-I > R-Br > R-Cl > R-F (due to increasing size/mass → stronger van der Waals forces).
  • For isomeric haloalkanes, boiling point decreases with branching (decreased surface area → weaker van der Waals forces). 1° > 2° > 3°.
  • For isomeric dihalobenzenes, boiling points are nearly the same. Melting point of para-isomer is significantly higher than ortho- and meta- isomers due to greater symmetry, leading to better packing in the crystal lattice.
• Density: Bromo-, iodo-, and polychloro- derivatives are denser than water. Density increases with increasing number of C atoms, halogen atoms, and atomic mass of halogen. ρ: R-I > R-Br > R-Cl > R-F.
• Solubility: Haloalkanes are only very slightly soluble in water. They cannot form H-bonds with water, and energy required to break existing H-bonds in water and C-X bonds is not sufficiently compensated by energy released when new interactions are set up. They are soluble in organic solvents.

6. Chemical Reactions:

A. Reactions of Haloalkanes:

• Nucleophilic Substitution Reactions (SN): The Cᵅ⁺—Xᵅ⁻ bond is susceptible to attack by nucleophiles (Nu⁻). Nu⁻ + R—X → R—Nu + X⁻ (Leaving Group).

  • SN2 (Substitution Nucleophilic Bimolecular):
    • Mechanism: Single step, concerted reaction. Back-side attack of nucleophile. Transition state involves both reactant and nucleophile.
    • Kinetics: Rate = k[RX][Nu⁻] (Second order).
    • Stereochemistry: Complete inversion of configuration (Walden Inversion). If reactant is chiral, product is also chiral but with inverted configuration.
    • Reactivity Order (Substrate): Methyl > 1° > 2° >> 3° (due to steric hindrance).
    • Favoured by: Strong nucleophiles, polar aprotic solvents (like DMSO, acetone, DMF - they solvate cations but not anions effectively, increasing nucleophile reactivity).
  • SN1 (Substitution Nucleophilic Unimolecular):
    • Mechanism: Two steps. Step 1 (slow, RDS): Formation of carbocation intermediate (R—X → R⁺ + X⁻). Step 2 (fast): Attack of nucleophile on carbocation (R⁺ + Nu⁻ → R—Nu).
    • Kinetics: Rate = k[RX] (First order). Independent of nucleophile concentration (usually).
    • Stereochemistry: Carbocation is planar (sp²). Nucleophile can attack from either face, leading to racemization (formation of equal amounts of enantiomers - retention + inversion).
    • Reactivity Order (Substrate): 3° > 2° > 1° > Methyl (due to stability of carbocation: 3° > 2° > 1°). Allylic and benzylic halides show high reactivity due to resonance stabilization of the carbocation.
    • Favoured by: Weak nucleophiles, polar protic solvents (like water, alcohol, acetic acid - they stabilize carbocation and leaving group via solvation).
  • Factors Affecting SN1/SN2:
    1. Substrate Structure: 1° → SN2; 3° → SN1; 2° → Both (depends on other factors).
    2. Nature of Nucleophile: Strong Nu⁻ → SN2; Weak Nu⁻ → SN1.
    3. Nature of Leaving Group: Better leaving groups (weaker bases, stable anions like I⁻ > Br⁻ > Cl⁻ >> F⁻) increase rate of both SN1 and SN2.
    4. Nature of Solvent: Polar protic → SN1; Polar aprotic → SN2.
  • Ambident Nucleophiles: Nucleophiles with two nucleophilic sites (e.g., CN⁻, NO₂⁻).
    • Reaction with KCN (ionic): Gives alkyl cyanides (R-CN) as major product (C-attack).
    • Reaction with AgCN (covalent): Gives alkyl isocyanides (R-NC) as major product (N-attack).
    • Reaction with KNO₂ (ionic): Gives alkyl nitrites (R-O-N=O) as major product (O-attack).
    • Reaction with AgNO₂ (covalent): Gives nitroalkanes (R-NO₂) as major product (N-attack).
  • Other Important SN Reactions:
    • Hydrolysis (with aq. KOH/NaOH or H₂O): R-X → R-OH
    • Williamson Synthesis (with R'ONa): R-X + R'ONa → R-O-R' (Ether) + NaX. Best for preparing mixed ethers using 1° alkyl halide and sodium alkoxide/phenoxide. If 2°/3° alkyl halide is used, elimination dominates.
    • Reaction with NH₃ (Ammonolysis): R-X + NH₃ → RNH₂ (1° amine) → R₂NH (2°) → R₃N (3°) → R₄N⁺X⁻ (Quaternary salt). A mixture is usually obtained. Excess NH₃ favours 1° amine.
    • Reaction with KSH: R-X → R-SH (Thiol)
    • Reaction with R'COOAg: R-X → R'COOR (Ester)
    • Reaction with LiAlH₄: R-X → R-H (Reduction)

• Elimination Reactions (Dehydrohalogenation / β-Elimination):

  • Haloalkanes with β-hydrogen atom(s) heated with alcoholic solution of KOH undergo elimination of HX to form alkenes.
  • Saytzeff's (Zaitsev's) Rule: In dehydrohalogenation reactions, the preferred product is the alkene which has the greater number of alkyl groups attached to the doubly bonded carbon atoms (i.e., the more substituted alkene is the major product).
  • Reactivity Order: 3° > 2° > 1° (follows stability of alkene formed).
  • Competition between Substitution and Elimination:
    • 1° RX: Favours SN2 (with strong Nu⁻/weak base like I⁻, RS⁻) or E2 (with strong, bulky base like (CH₃)₃CO⁻K⁺).
    • 2° RX: Competes. Strong base/high temp → E2; Weak base/low temp/good Nu⁻ → SN2/SN1.
    • 3° RX: Favours E2 (with strong base) or SN1 (with weak Nu⁻/base in protic solvent).

• Reaction with Metals:

  • Grignard Reagent Formation: R-X + Mg (in dry ether) → R-Mg-X (Alkylmagnesium halide). Extremely reactive, react with any source of proton (H₂O, alcohols, amines) to give hydrocarbons. Must be prepared under anhydrous conditions. C-Mg bond is covalent but highly polar (C⁻-Mg⁺). Acts as a source of R⁻ (carbanion).
  • Wurtz Reaction: 2R-X + 2Na (in dry ether) → R-R + 2NaX. Used to prepare symmetrical alkanes with double the number of carbon atoms. Not suitable for odd-number alkanes (gives mixture). Methane cannot be prepared. 3° halides prefer elimination.

B. Reactions of Haloarenes:

• Nucleophilic Substitution (Generally Difficult): Haloarenes are much less reactive than haloalkanes towards nucleophilic substitution. Reasons:
  1. Resonance Effect: C-Cl bond acquires partial double bond character due to resonance, making it shorter and stronger.
  2. Difference in Hybridization: Carbon in C-X bond is sp² hybridized (more s-character, more electronegative) in haloarenes vs sp³ in haloalkanes. C-X bond is stronger.
  3. Instability of Phenyl Cation: Self-ionization (SN1 path) is ruled out.
  4. Repulsion: Electron-rich nucleophile is repelled by the electron-rich benzene ring.
• Conditions for Nucleophilic Substitution: Reaction occurs only under drastic conditions (high temp & pressure).
  • Dow's Process: Chlorobenzene + NaOH (aq) → Phenol (at 623 K, 300 atm, followed by acidification).
  • Effect of Electron-Withdrawing Groups (EWG): Presence of EWGs (like -NO₂, -CN, -COOH) at ortho and para positions increases the reactivity of haloarenes towards nucleophilic substitution by stabilizing the intermediate carbanion (Meisenheimer complex). More EWGs at o/p positions → easier substitution. Meta position has little effect.
• Electrophilic Substitution Reactions: Halogen atom is deactivating (due to -I effect) but ortho, para-directing (due to +R effect, resonance stabilization of intermediates for o/p attack is greater). Overall rate is slower than benzene, but substitution occurs at o/p positions.
  • Halogenation: Chlorobenzene + Cl₂ (anhyd. FeCl₃) → 1,2-Dichlorobenzene (minor) + 1,4-Dichlorobenzene (major).
  • Nitration: Chlorobenzene + Conc. HNO₃ + Conc. H₂SO₄ → o-Nitrochlorobenzene (minor) + p-Nitrochlorobenzene (major).
  • Sulphonation: Chlorobenzene + Conc. H₂SO₄ (fuming) → o-Chlorobenzenesulphonic acid (minor) + p-Chlorobenzenesulphonic acid (major).
  • Friedel-Crafts Alkylation: Chlorobenzene + R-Cl (anhyd. AlCl₃) → o-Alkylchlorobenzene + p-Alkylchlorobenzene.
  • Friedel-Crafts Acylation: Chlorobenzene + RCOCl (anhyd. AlCl₃) → o-Chloroacetophenone + p-Chloroacetophenone.
• Reaction with Metals:
  • Wurtz-Fittig Reaction: Aryl halide + Alkyl halide + 2Na (dry ether) → Alkylarene + 2NaX. (Ar-X + R-X + 2Na → Ar-R + 2NaX).
  • Fittig Reaction: 2 Aryl halide + 2Na (dry ether) → Biphenyl/Diphenyl + 2NaX. (2Ar-X + 2Na → Ar-Ar + 2NaX).

7. Polyhalogen Compounds (Uses and Environmental Effects):

• Dichloromethane (CH₂Cl₂): Solvent (paint remover), propellant, metal cleaning. Harmful (CNS effects, dizziness, nausea).
• Trichloromethane (Chloroform, CHCl₃): Formerly used as anesthetic. Solvent for fats, waxes, rubber. Production of freon R-22. Suspected carcinogen, causes liver damage. Oxidation in air/light produces poisonous phosgene (COCl₂), stored in dark bottles filled to brim with ethanol added as stabilizer.
• Triiodomethane (Iodoform, CHI₃): Antiseptic properties (due to liberation of free iodine). Objectionable smell.
• Tetrachloromethane (Carbon Tetrachloride, CCl₄): Solvent, cleaning agent, formerly fire extinguisher (Pyrene), refrigerant manufacture. Toxic to liver, suspected carcinogen, depletes ozone layer. Use severely restricted.
• Freons (Chlorofluorocarbons - CFCs): e.g., CCl₂F₂ (Freon-12). Refrigerants, aerosol propellants, solvents. Stable, non-toxic, non-corrosive. Major cause of ozone layer depletion in stratosphere. Use largely phased out under Montreal Protocol.
• p,p’-Dichlorodiphenyltrichloroethane (DDT): Effective insecticide (against malaria mosquitoes, lice). Highly persistent in environment, accumulates in fatty tissues (biomagnification). Toxic to fish. Banned in many countries but still used for malaria control in some areas.

Key Concepts for Exams:

• SN1 vs SN2: Mechanisms, stereochemistry, factors affecting reactivity.
• Named Reactions: Finkelstein, Swarts, Sandmeyer, Gattermann, Balz-Schiemann, Wurtz, Wurtz-Fittig, Fittig, Darzen's Process.
• Markovnikov's Rule & Anti-Markovnikov Addition (Peroxide Effect).
• Saytzeff's Rule.
• Reactivity of Haloarenes vs Haloalkanes (reasons).
• ortho, para-directing nature of halogens in electrophilic substitution.
• Grignard Reagents: Preparation and reactivity.
• Ambident Nucleophiles (KCN/AgCN, KNO₂/AgNO₂).
• Uses and environmental impact of polyhalogen compounds (especially Chloroform, CCl₄, Freons, DDT).
• Stereochemistry: Chirality, enantiomers, racemization, inversion, retention.

Multiple Choice Questions (MCQs):

1. Which of the following undergoes nucleophilic substitution exclusively by SN1 mechanism?
   (a) Ethyl chloride
   (b) Isopropyl chloride
   (c) Chlorobenzene
   (d) Benzyl chloride

2. The reaction of t-butyl bromide with sodium methoxide produces mainly:
   (a) Isobutane
   (b) Isobutylene
   (c) t-butyl methyl ether
   (d) Sodium t-butoxide

3. Which reagent is used for the Finkelstein reaction?
   (a) NaI / Dry Acetone
   (b) AgF / DMSO
   (c) NaCl / Water
   (d) CuCl / HCl

4. Identify the compound that will react fastest in an SN2 reaction with OH⁻:
   (a) CH₃CH₂I
   (b) (CH₃)₂CHI
   (c) (CH₃)₃CI
   (d) CH₃CH₂Cl

5. Chlorobenzene reacts with Mg in dry ether to give a compound (A), which further reacts with ethanol to yield:
   (a) Phenol
   (b) Benzene
   (c) Ethylbenzene
   (d) Phenetole

6. Which of the following is an example of a vinylic halide?
   (a) Allyl chloride
   (b) Vinyl chloride
   (c) Benzyl chloride
   (d) Chlorobenzene

7. Reaction of C₆H₅CH₂Br with aqueous sodium hydroxide follows:
   (a) SN1 mechanism
   (b) SN2 mechanism
   (c) Elimination reaction
   (d) Electrophilic substitution

8. Which of the following is expected to be most reactive towards SN2 reaction?
   (a) C₆H₅Cl
   (b) C₆H₅CH₂Cl
   (c) CH₃Cl
   (d) CH₂=CHCl

9. In the reaction of chlorobenzene with Cl₂/FeCl₃, the major product is:
   (a) 1,2-Dichlorobenzene
   (b) 1,3-Dichlorobenzene
   (c) 1,4-Dichlorobenzene
   (d) 1,3,5-Trichlorobenzene

10. Treatment of ethyl bromide with silver nitrite (AgNO₂) mainly gives:
    (a) Ethyl nitrite
    (b) Nitroethane
    (c) Ethane
    (d) Ethene

Answer Key for MCQs:

1. (d) Benzyl chloride (Stabilized carbocation favours SN1)
2. (b) Isobutylene (t-butyl bromide is 3°, methoxide is a strong base; E2 elimination dominates)
3. (a) NaI / Dry Acetone
4. (a) CH₃CH₂I (1° halide, good leaving group I⁻, less steric hindrance than Cl⁻)
5. (b) Benzene (A is Phenylmagnesium bromide, C₆H₅MgBr. Reacts with proton from ethanol C₂H₅OH to give C₆H₆)
6. (b) Vinyl chloride (Halogen on sp² carbon of C=C)
7. (a) SN1 mechanism (Benzyl bromide forms a stable benzylic carbocation)
8. (c) CH₃Cl (Least steric hindrance, primary halide favour SN2. Benzyl chloride is also reactive via SN2 but methyl is generally considered fastest if comparing simple alkyls and benzyl) Correction: While Benzyl chloride is highly reactive, methyl chloride has the absolute least steric hindrance, making it fastest for SN2 among simple, unactivated halides.
9. (c) 1,4-Dichlorobenzene (Halogen is o,p-directing, para product is major due to steric reasons)
10. (b) Nitroethane (AgNO₂ is covalent, N-attack is preferred)

Study these notes thoroughly, focusing on understanding the reaction mechanisms and the factors influencing them. Practice more questions from the Exemplar book itself. Good luck with your preparation!