Understanding Halogen Derivatives in Chemistry

Aug 5, 2024

Review flashcards

Lecture Notes: Introduction to Organic Chemistry and Halogen Derivatives

Introduction

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Start of Chemistry Discussion

  • Topic: Organic Chemistry
  • Specific Topic: Halogen Derivatives
  • Also known as Haloalkanes and Haloarenes

Key Areas to Cover in Halogen Derivatives

  • Classification
  • Nomenclature
  • Methods of Preparation
  • Physical Properties
  • Chemical Properties
  • Optical Isomerism

Classification of Halogen Derivatives

Types Based on Number of Halogens

  • Monohalogen Compounds: Single halogen atom
  • Dihalogen Compounds: Two halogen atoms
  • Trihalogen Compounds: Three halogen atoms
  • Polyhalogen Compounds: More than three halogen atoms

Types Based on Structure

  • Alkyl Halides (Haloalkanes): Derived from alkanes (single bonds)
  • Aryl Halides (Haloarenes): Derived from benzene rings
  • Allyl Halides: Halogen bonded to a carbon adjacent to a double bond
  • Vinyl Halides: Halogen bonded to a carbon that is double-bonded to another carbon
  • Benzyl Halides: Halogen bonded to a carbon adjacent to a benzene ring
  • Vinylic Halides: Halogen bonded directly to a double-bonded carbon

Nomenclature of Halogen Derivatives

IUPAC Naming Rules

  • Name of the halogen as a prefix (e.g., chloro, bromo, iodo)
  • Parent hydrocarbon chain name follows
  • Numbering to give the smallest number to the halogen substituent
  • Example: 1-chloropropane, 2-bromobutane

Methods of Preparation

From Alcohols

  1. Using Hydrogen Halides (HX):
    • ROH + HX тЖТ R-X + HтВВO
    • Example: ROH with HCl in presence of ZnClтВВ (Lucas' reagent)
  2. Using Phosphorus Halides (PClтВГ, PClтВЕ):
    • ROH + PClтВГ тЖТ RCl + HтВГPOтВГ
    • ROH + PClтВЕ тЖТ RCl + POClтВГ + HCl
  3. Using Thionyl Chloride (SOClтВВ):
    • Best method as by-products (SOтВВ and HCl) are gases
    • ROH + SOClтВВ тЖТ RCl + SOтВВ + HCl

From Hydrocarbons

  1. Free Radical Halogenation:
    • Not suitable due to mixture of products
  2. Addition of HX to Alkenes:
    • Follows Markovnikov's rule: HX adds to the most substituted carbon
    • Anti-Markovnikov addition with HBr in presence of peroxide

Using Halogen Exchange

  1. Finkelstein Reaction:
    • RCl or RBr with NaI in acetone
    • RCl + NaI тЖТ RI + NaCl
  2. Swarts Reaction:
    • RCl with AgF
    • RCl + AgF тЖТ RF + AgCl

Physical Properties of Halogen Derivatives

Intermolecular Forces and Bond Types

  • Polar covalent bonds
  • Bond strength and bond length vary with halogen size
  • C-F strongest, C-I weakest

Boiling and Melting Points

  • Increase with molecular mass and decrease with branching
  • Boiling point order: RI > RBr > RCl > RF
  • Melting point order in Dihalobenzenes: para > ortho > meta

Solubility

  • Insoluble in water
  • Soluble in organic solvents

Chemical Properties of Halogen Derivatives

Nucleophilic Substitution Reactions (SN1 and SN2)

  1. SN1 Mechanism:
    • Two-step process (formation of carbocation)
    • Favored by tertiary carbons
    • Forms racemic mixtures if starting material is chiral
  2. SN2 Mechanism:
    • One-step process (backside attack)
    • Favored by primary carbons
    • Inversion of configuration (Walden inversion)

Elimination Reactions

  1. Dehydrohalogenation:
    • Requires strong base (e.g., alcoholic KOH)
    • Zaitsev's rule: more substituted alkene is major product

Reactions with Metals

  1. Grignard Reagents (RMgX):
    • Formation with Mg in dry ether
    • Reacts with water to form hydrocarbons
  2. Wurtz Reaction:
    • Alkyl halides react with sodium in dry ether to form alkanes

Environmental and Health Effects of Halogen Derivatives

  • Chloroform (Trichloromethane): Anesthetic, potential carcinogen
  • Carbon Tetrachloride: Cleaning agent, toxic to the liver
  • CFCs (Chlorofluorocarbons): Used in refrigeration, deplete ozone layer
  • DDT (Dichlorodiphenyltrichloroethane): Insecticide, persistent organic pollutant

Conclusion

  • Thorough coverage of halogen derivatives in preparation for exams
  • Emphasis on understanding both theoretical concepts and practical applications