Haloalkanes - Class 12 Gujarat Board Chemistry

Haloalkanes and Haloarenes: Concepts, Properties, and Reactions

Syllabus Structure and Exam Weightage

Haloalkanes and haloarenes are key topics with significant exam weightage. The chapter is structured around definitions, classification, nomenclature, physical and chemical properties, and core reactions. For board exams, this chapter typically carries 8 marks (5+3), with additional MCQs in competitive exams like GUJCET.

• Board Exam: 3 MCQs, 2 theory questions (2 marks and 3 marks), total 8 marks.
• GUJCET: 4 MCQs, each 1 mark.
• Unit Distribution: Chapters 6–10, total 49 marks.

Definitions and Core Concepts

Haloalkanes are organic compounds where one or…

Haloalkanes and Haloarenes: Concepts, Properties, and Reactions

Syllabus Structure and Exam Weightage

Haloalkanes and haloarenes are key topics with significant exam weightage. The chapter is structured around definitions, classification, nomenclature, physical and chemical properties, and core reactions. For board exams, this chapter typically carries 8 marks (5+3), with additional MCQs in competitive exams like GUJCET.

• Board Exam: 3 MCQs, 2 theory questions (2 marks and 3 marks), total 8 marks.
• GUJCET: 4 MCQs, each 1 mark.
• Unit Distribution: Chapters 6–10, total 49 marks.

Definitions and Core Concepts

Haloalkanes are organic compounds where one or more hydrogen atoms in an alkane are replaced by halogen atoms (F, Cl, Br, I). Haloarenes are aromatic compounds where halogen is directly attached to an aromatic ring (benzene).

• Haloalkane: Halogen attached to a non-aromatic carbon.
• Haloarene: Halogen attached directly to an aromatic (benzene) ring.
• Halogen: F, Cl, Br, I.

Classification of Haloalkanes and Haloarenes

Classification is based on the number of halogen atoms and the nature of the carbon to which the halogen is attached.

• By Number of Halogens:

  • Monohalide: One halogen atom (e.g., CH₃Cl).
  • Dihalide: Two halogen atoms (e.g., CH₂Cl₂).
    • Geminal Dihalide: Both halogens on the same carbon.
    • Vicinal Dihalide: Halogens on adjacent carbons.
  • Trihalide: Three halogen atoms.
  • Polyhalide: More than three halogen atoms.

• By Nature of Carbon:

  • Primary (1°): Halogen on a carbon attached to one other carbon.
  • Secondary (2°): Halogen on a carbon attached to two other carbons.
  • Tertiary (3°): Halogen on a carbon attached to three other carbons.
  • Allylic Halide: Halogen on sp³ carbon next to a double bond.
  • Benzylic Halide: Halogen on sp³ carbon next to a benzene ring.
  • Vinylic Halide: Halogen on sp² carbon of a double bond.
  • Aryl Halide: Halogen directly on benzene ring.

Nomenclature Rules

Nomenclature follows IUPAC rules, with halogen as a prefix and the parent hydrocarbon as the base.

• Prefixes: Fluoro (F), Chloro (Cl), Bromo (Br), Iodo (I).
• Numbering: Lowest possible number to halogen.
• Common Names: Normal (n-), iso-, sec-, tert- prefixes for alkyl groups.
• Aromatic Compounds: Ortho (1,2), meta (1,3), para (1,4) positions for disubstituted benzenes.

Physical Properties

Physical properties such as state, color, odor, melting and boiling points, density, and solubility are influenced by the type and number of halogen atoms.

• State: Lower members (methyl, ethyl halides) are gases; higher are liquids or solids.
• Color: Pure alkyl halides are colorless; bromides and iodides may be colored on exposure to light.
• Odor: Volatile haloalkanes have sweet smell.
• Boiling Point: Increases with molecular weight and number of halogens; decreases with branching.
• Melting Point: Ortho, meta, para isomers have different melting points due to lattice packing; para isomer has highest.
• Density: Increases with molecular weight and halogen atomic mass.
• Solubility: Slightly soluble in water (due to polarity), soluble in organic solvents.

Chemical Bonding and Reactivity

The C–X bond in haloalkanes is polar due to the higher electronegativity of halogens compared to carbon, resulting in partial charges.

• Bond Polarity: Halogen pulls electron density, making C partially positive and X partially negative.
• Bond Length: Increases from C–F to C–I.
• Bond Strength: Decreases from C–F (strongest) to C–I (weakest).
• Dipole Moment: Depends on the number and type of halogen atoms; for tetrahalides, net dipole can be zero.

Methods of Preparation

Haloalkanes and haloarenes can be prepared by several methods, each with specific reagents and conditions.

• From Alcohols:
  • Lucas Test: Alcohol + conc. HCl + anhydrous ZnCl₂ (Lucas reagent); 3° > 2° > 1° alcohols in reactivity.
  • PBr₃/PCl₃: Alcohol + PBr₃/PCl₃ gives alkyl bromide/chloride.
  • SOCl₂ (Thionyl chloride): Alcohol + SOCl₂ → Alkyl chloride + SO₂ (gas) + HCl (gas); gives pure product.
• Halogen Exchange:
  • Finkelstein Reaction: Alkyl chloride/bromide + NaI in acetone → Alkyl iodide.
  • Swarts Reaction: Alkyl halide + AgF → Alkyl fluoride.
• From Alkanes:
  • Free Radical Halogenation: Alkane + Cl₂/Br₂ (sunlight/UV) → Haloalkane.
    • Selectivity: Br₂ is more selective (3° > 2° > 1°).
• From Alkenes:
  • Markovnikov Rule: HX adds to unsymmetrical alkene; H goes to carbon with more hydrogens.
  • Anti-Markovnikov (Peroxide effect): With HBr and peroxide, Br adds to less substituted carbon.
  • Addition of X₂ (Br₂/Cl₂): In CCl₄, forms vicinal dihalide.
• From Benzene:
  • Electrophilic Substitution: Benzene + Cl₂/Br₂ (Lewis acid catalyst, dark) → Haloarene.
  • Sandmeyer Reaction: Benzene diazonium salt + CuX/HX → Haloarene.
  • Gattermann Reaction: Benzene diazonium salt + Cu powder + HX → Haloarene.

Mechanisms: SN1 and SN2 Reactions

Nucleophilic substitution reactions proceed via two main mechanisms, depending on substrate structure and reaction conditions.

• SN2 (Bimolecular):
  • One-step, concerted mechanism.
  • Rate depends on both substrate and nucleophile.
  • Inversion of configuration (Walden inversion).
  • Favored by primary alkyl halides and strong nucleophiles.
• SN1 (Unimolecular):
  • Two-step mechanism: formation of carbocation intermediate.
  • Rate depends only on substrate.
  • Racemization occurs (mixture of retention and inversion).
  • Favored by tertiary alkyl halides and weak nucleophiles.

Stereochemistry: Optical Activity and Isomerism

Optical activity arises from the presence of chiral (asymmetric) carbon atoms. Compounds with one chiral carbon show optical isomerism.

• Chiral Carbon: Carbon attached to four different groups.
• Optical Isomers: Non-superimposable mirror images (enantiomers).
• Dextrorotatory (+): Rotates plane-polarized light to the right.
• Levorotatory (–): Rotates plane-polarized light to the left.
• Racemic Mixture: Equimolar mixture of enantiomers; optically inactive.
• Inversion: SN2 reactions lead to inversion of configuration.
• Retention: SN1 reactions can lead to both retention and inversion (racemization).

Important Reactions and Transformations

Several named reactions and transformations are crucial for synthesis and conversions.

• Williamson Synthesis: Alkyl halide + sodium alkoxide → Ether.
• Wurtz Reaction: Alkyl halide + Na (dry ether) → Alkane (even number of carbons).
• Wurtz-Fittig Reaction: Alkyl halide + aryl halide + Na (dry ether) → Alkyl arene.
• Fittig Reaction: Aryl halide + Na (dry ether) → Biaryl.
• Grignard Reagent Formation: Alkyl halide + Mg (dry ether) → R–Mg–X.
• Elimination (Dehydrohalogenation): Alkyl halide + alcoholic KOH → Alkene.
  • Saytzeff Rule: Major product is the more substituted alkene.

Applications and Uses

Haloalkanes and haloarenes have diverse applications in medicine, industry, and daily life.

• Phosgene (COCl₂): Poisonous gas used in chemical warfare.
• Chloramphenicol: Antibiotic containing chlorine.
• Chloroquine: Antimalarial drug containing chlorine.
• Thyroxine: Iodine-containing hormone; deficiency causes goiter.
• Chloroform: Former anesthetic; replaced by halothane.
• Fluoride in Toothpaste: Prevents dental caries; used up to 1 ppm.
• Chlorine in Water Treatment: Disinfectant in swimming pools.
• Iodized Salt: Prevents iodine deficiency disorders.
• Bromine Compounds: Used as signal flares and pesticides.

Environmental and Health Hazards

Some haloalkanes and haloarenes are toxic and environmentally hazardous.

• Chloroform: Can form phosgene on exposure to air and sunlight; toxic to liver and kidneys.
• Carbon Tetrachloride: Used as fire extinguisher; carcinogenic.
• Freons (CFCs): Used as refrigerants; deplete ozone layer.
• DDT: Insecticide; persistent in environment, banned in many countries.
• Iodoform: Antiseptic; strong odor.

Key Exam Pitfalls and Shortcuts

• Geminal vs. Vicinal Dihalides: Geminal—same carbon; Vicinal—adjacent carbons.
• Markovnikov vs. Anti-Markovnikov: Know when peroxide is present.
• SN1 vs. SN2: Substrate structure determines mechanism.
• Optical Activity: Only compounds with chiral carbon show optical isomerism.
• Saytzeff Rule: Major alkene from elimination is more substituted.
• Finkelstein/Swarts: Used for halogen exchange; acetone as solvent for Finkelstein.
• Physical Properties: Boiling point increases with molecular weight and decreases with branching.
• Reactivity Order: For SN2: 1° > 2° > 3°; for SN1: 3° > 2° > 1°.
• Environmental Hazards: Chloroform, CCl₄, DDT, and Freons have specific health and environmental risks.

Table: Key Reactions and Their Features

This table summarizes the main reactions, their substrates, reagents, products, and distinguishing features as explicitly described.