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Haloalkanes and Haloarenes Overview

Jun 29, 2025

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Overview

This lecture covers the fundamentals of Haloalkanes and Haloarenes as per the NCERT curriculum, emphasizing key definitions, concepts, classification, nomenclature, physical and chemical properties, and important reaction mechanisms. It also reviews exam-focused tips and practice questions relevant for board exams and competitive tests.

Introduction to Haloalkanes and Haloarenes

  • Haloalkanes are hydrocarbons where one or more hydrogens are replaced by halogen atoms on an sp³ carbon.
  • Haloarenes are aromatic compounds where halogen is attached directly to the benzene ring.
  • Classification is based on the number of halogen atoms: mono-, di-, tri-halo compounds.

Classification and Isomerism

  • Alkyl halides: halogen attached to an aliphatic sp³ carbon.
  • Primary, secondary, and tertiary halides depend on how many carbon groups attach to the halogen-bearing carbon.
  • Special classes: allylic (next to double bond), benzylic (next to benzene ring), vinylic (attached to double bond), and aryl halides (directly attached to aromatic ring).
  • Isomers: compounds with same molecular formula but different structures.

Nomenclature

  • IUPAC names: count longest carbon chain, assign lowest locants to halogens.
  • Common naming system: group names like isopropyl, sec-butyl, tert-butyl, neo-pentyl.
  • Special names: geminal (same carbon), vicinal (adjacent carbons) dihalides.

Preparation of Haloalkanes and Haloarenes

  • Alcohols react with halogenating agents (Lucas reagent, PCl₃, PCl₅, SOCl₂) to form haloalkanes.
  • Side reactions and stability of carbocation intermediates influence the outcome.
  • Phenols do not form haloarenes by this method due to strong C–O bond.

Physical Properties

  • Lower haloalkanes are gases; higher ones are liquids or solids.
  • Melting and boiling points increase with molecular mass and decrease with branching.
  • Haloalkanes are slightly soluble in water, more soluble in organic solvents.
  • Dipole moments and density increase down the halogen group.

Chemical Properties

Nucleophilic Substitution (SN1 & SN2)

  • SN2 mechanism: one-step, backside attack, inversion of configuration, best for methyl/primary halides.
  • SN1 mechanism: two-step, carbocation intermediate, racemization, favored for tertiary halides.
  • Reactivity order for SN2: methyl > primary > secondary > tertiary.
  • Reactivity order for SN1: tertiary > secondary > primary > methyl.
  • Practice identifying which compounds undergo SN1/SN2 faster.

Elimination Reactions

  • Dehydrohalogenation with alcoholic KOH yields alkenes (beta elimination).
  • Major product: more substituted alkene (Zaitsev's rule).
  • Heat favors elimination; bulky bases favor elimination over substitution.

Special Reactions and Name Reactions

  • Grignard reagent (RMgX): strong nucleophile, reacts with water/alcohol to form alkanes.
  • Wurtz and Wurtz-Fittig reactions: coupling of alkyl/aryl halides using sodium.
  • Finkelstein and Swarts reactions: halogen exchange methods.

Electrophilic Aromatic Substitution

  • Halogens are ortho/para-directing but deactivating due to electron withdrawal.
  • Nitro groups activate nucleophilic substitution at ortho/para positions.
  • Mechanistic questions around activators/deactivators and resonance effects.

Stereochemistry

  • Chiral (asymmetric) carbon: four different groups, causes optical activity.
  • Optical activity: measured by rotation of plane-polarized light (dextro, levo rotatory).
  • Racemic mixture: equal amounts of enantiomers, optically inactive.
  • Identifying chiral centers and drawing mirror images (enantiomers).

Polyhalogen Compounds

  • Methylene chloride, chloroform, carbon tetrachloride, DDT: uses and health hazards.
  • Environmental and health impacts discussed.

Key Terms & Definitions

  • Haloalkane (Alkyl halide) — Hydrocarbon with halogen on an sp³ carbon.
  • Haloarene (Aryl halide) — Halogen attached directly to an aromatic ring.
  • SN1 reaction — Unimolecular nucleophilic substitution via carbocation.
  • SN2 reaction — Bimolecular nucleophilic substitution via backside attack.
  • Grignard reagent — Organomagnesium halide (RMgX), a strong nucleophile.
  • Zaitsev's rule — Major alkene formed is the most substituted.
  • Chiral carbon — Carbon with four different substituents, showing optical activity.
  • Racemic mixture — 1:1 mix of enantiomers, optically inactive.

Action Items / Next Steps

  • Practice NCERT exercise questions and conversions as marked in your class notes.
  • Solve IUPAC nomenclature, structure, and mechanism problems (yellow highlights).
  • Review marked example problems and practice all assigned conversions.
  • Prepare for next session on Alcohols, Phenols, and Ethers.
  • Download the Vora Classes app for additional notes and resources.

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