Haloalkanes and Haloarenes Overview

Jun 29, 2025

Overview

This lecture covers the NCERT chapter on Haloalkanes and Haloarenes, focusing on structure, nomenclature, physical/chemical properties, preparation methods, important reactions, and mechanisms, with exam-oriented tips and key definitions highlighted.

Introduction & Importance

  • The chapter is based entirely on NCERT and is essential for board and competitive exams (NEET, JEE).
  • Organic chemistry causes fear; mastering NCERT line by line is vital.
  • Questions in board and entrance exams often come directly from NCERT content.

Classification & Nomenclature

  • Haloalkanes: Compounds where a halogen replaces hydrogen in an alkane (e.g., CH3Cl).
  • Haloarenes: Benzene derivatives with halogen substituents.
  • Nomenclature: Common names and IUPAC names (e.g., 2-chloropropane, isopropyl bromide).
  • Classification by number of halogens: mono-, di-, tri-halo compounds.
  • Special types: gem-dihalide (same carbon), vic-dihalide (adjacent carbons), allylic, benzylic, vinylic, and aryl halides.

Methods of Preparation

  • From alcohols: Using HX, PCl3, PCl5, SOCl2 (Lucas reagent for classification).
  • From hydrocarbons: Free radical halogenation using Cl2/Br2 and sunlight.
  • Halogen exchange: Finkelstein and Swarts reactions for preparing fluoroalkanes/iodoalkanes.
  • Preparation of Haloarenes: Electrophilic substitution with halogens and iron(III) halide catalysts.

Physical & Chemical Properties

  • Haloalkanes: Increase in boiling point with mass; branching decreases boiling point.
  • Haloarenes: Low water solubility due to poor hydrogen bonding; higher solubility in organic solvents.
  • Dipole moment and polarity: C–Cl bond is more polar than C–F, but C–F is shorter.
  • Density increases with heavier halogens.
  • Isomer boiling/melting points depend on symmetry and intermolecular forces.

Reactivity and Mechanisms

  • Nucleophilic substitution reactions: SN1 (unimolecular, through carbocation, fastest with tertiary), SN2 (bimolecular, backside attack, fastest with primary).
  • Elimination reactions: E1 and E2, depend on base/nucleophile strength and temperature.
  • Haloarenes are less reactive to nucleophilic substitution due to resonance, bond strength, and hybridization.

Stereochemistry

  • Chiral carbon: Carbon with four different groups; leads to optical activity.
  • Racemic mixture: Equimolar mixture of enantiomers with net zero optical activity.
  • Retention and inversion of configuration occur in substitution reactions.

Important Named Reactions

  • Wurtz Reaction: Coupling of haloalkanes with sodium in dry ether.
  • Finkelstein and Swarts: Halogen exchange for preparation of alkyl/aryl halides.
  • Sandmeyer Reaction: Preparation of aryl halides from diazonium salts.

Uses, Hazards & Applications

  • Polyhalogen compounds: CH2Cl2 (solvent, paint remover), CHCl3 (anesthetic, precursor for freon), CCl4 (cleaning agent, toxic), DDT (pesticide, environmental hazard).
  • Storage precautions for chemicals like chloroform (dark bottles to prevent phosgene formation).

Key Terms & Definitions

  • Haloalkane — Alkane where one or more hydrogens are replaced by halogen(s).
  • Haloarene — Aromatic compound with one or more halogen substituents.
  • Gem-dihalide — Two halogens on the same carbon atom.
  • Vic-dihalide — Two halogens on adjacent carbon atoms.
  • Chiral carbon — Carbon atom attached to four different groups.
  • Racemic mixture — 1:1 mixture of two enantiomers, optically inactive.
  • SN1 reaction — Unimolecular nucleophilic substitution via carbocation.
  • SN2 reaction — Bimolecular nucleophilic substitution via simultaneous bond making and breaking.
  • Finkelstein/Swarts Reaction — Halogen exchange reactions for haloalkanes.
  • Grignard reagent (RMgX) — Organomagnesium compound used for carbon–carbon bond formation.

Action Items / Next Steps

  • Complete all marked NCERT exercise questions (IUPAC names, structures, dipole moments, mechanisms, conversions).
  • Revise key named reactions and mechanisms for both boards and entrance exams.
  • Practice writing and identifying isomers.
  • Attend the next session on Alcohols, Phenols, and Ethers.
  • Download notes from the designated folder on the Vora Classes app and print for review.

Full transcript