Organic Chemistry: Haloalkanes and Haloarenes

Introduction

• Topic: Haloalkanes and Haloarenes (Class 12th Organic Chemistry)
• Objective: Lay foundation for organic chemistry in class 12th
• Key Concept: Organic chemistry is logical; understanding reactions is more important than memorizing them
• Lecture Structure: 1. Preparation of Haloalkanes 2. Chemical Properties of Haloalkanes 3. Preparation of Haloarenes 4. Chemical Properties of Haloarenes

Haloalkanes

Definition

• Haloalkanes: Alkanes with one or more hydrogen atoms substituted by halogen atoms (X)
• General Formula: R-X (R=Alkyl group, X=Halogen)

Preparation Methods

• From Alkanes: Via halogenation using X2 in the presence of light (Hν)
  • Example: Methane (CH4) + Cl2 -> CH3Cl (Chloromethane)
  • Reactivity Order of Halogens: Cl2 > Br2 > I2 (Cl2 most reactive, I2 least reactive)
• From Alcohols: By treating with HX
  • Example: R-OH + HX -> R-X + H2O
  • Mechanism involves formation of carbocation (electrophile, E+) and nucleophile (Nu-)
• From Halogen Exchange: Using reagents like AgF, SbF3, Hg2F2, CoF2
  • Schwarz Reaction: For producing fluoroalkanes
  • Finkelstein Reaction: For producing iodoalkanes using NaI

Chemical Properties

• Nucleophilic Substitution: SN1 and SN2 mechanisms
  • SN1 (Unimolecular Nucleophilic Substitution): Involves formation of carbocation; two-step process
    • Rate depends on the stability of the carbocation
    • Results in partial racemization
    • Example: Tertiary haloalkanes
  • SN2 (Bimolecular Nucleophilic Substitution): Direct displacement of the leaving group by nucleophile; single-step mechanism
    • Results in inversion of configuration (Walden Inversion)
    • Example: Primary haloalkanes
• Elimination Reactions: E1 and E2 mechanisms
  • E1 (Unimolecular Elimination): Two-step mechanism involving carbocation intermediate
  • E2 (Bimolecular Elimination): Single-step mechanism involving simultaneous removal of proton and leaving group
• Reaction with Metals: Wurtz Reaction
  • Formation of alkanes by coupling of two haloalkane molecules in the presence of sodium

Haloarenes

Definition

• Haloarenes: Aromatic compounds where one or more hydrogen atoms are substituted by halogen atoms
• General Formula: Ar-X (Ar=Aryl group)

Preparation Methods

• Electrophilic Substitution: Using halogen (X2) with a Lewis acid (e.g., FeX3)
  • Example: Benzene + Cl2 + FeCl3 -> Chlorobenzene
• From Aniline: Via diazotization and subsequent substitution
  • Example: Aniline + HNO2 + HCl -> Benzene Diazonium Chloride; treated with CuCl, CuBr, KI, etc.
  • Sandmeyer Reaction: For producing haloarenes from diazonium salts
  • Balz-Schiemann Reaction: For producing fluoroarenes using HBF4

Chemical Properties

• Reactivity: Lower than haloalkanes due to resonance stabilization
• Nucleophilic Substitution: Via addition-elimination or elimination-addition mechanisms
  • Addition-Elimination Mechanism: Involves formation of a Meisenheimer complex
  • Elimination-Addition Mechanism: Involves formation of benzyne intermediate
• Electrophilic Substitution: Similar to benzene but halogens direct ortho/para positions
  • Friedel-Crafts Alkylation/Acylation
  • Nitration, Sulfonation, and Halogenation

Special Reactions

• Wurtz-Fittig Reaction: Coupling of haloarene with haloalkane using sodium
• Fittig Reaction: Coupling of two haloarenes

Important Reactions and Mechanisms

• Schwarz Reaction: For preparing fluoroalkanes
• Finkelstein Reaction: For preparing iodoalkanes
• Sandmeyer Reaction: For preparing haloarenes from diazonium salts
• Balz-Schiemann Reaction: For preparing fluoroarenes
• Wurtz Reaction: Coupling of haloalkanes
• Wurtz-Fittig Reaction: Coupling of haloarenes with haloalkanes
• Fittig Reaction: Coupling of haloarenes

Recap and Summary

• Understanding and identifying electrophiles and nucleophiles is crucial
• Different mechanisms (SN1, SN2, E1, E2) have specific conditions and outcomes
• Haloarenes are less reactive towards nucleophilic substitution due to resonance
• Various synthesis methods and reactions are essential for preparation and transformation of haloalkanes and haloarenes

Practice Questions

1. Identify the major product in a given reaction involving nucleophilic substitution or elimination.
2. Compare the reactivity of different haloalkanes and haloarenes in various reactions.
3. Predict the stereochemical outcome of SN1 and SN2 reactions.
4. Discuss the mechanism and conditions required for specific named reactions.

Note: Ensure you understand the role of solvents, stability of intermediates, and the effects of substituents on reactivity.