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Comprehensive Study Notes on Haloalkanes and Haloarenes

Haloalkanes & Haloarenes

  • Definition: Hydrocarbons where one or more hydrogen atoms are replaced by halogen atoms.
    • Category:
    • Haloalkanes: Derived from open-chain hydrocarbons (alkanes).
    • Haloarenes: Derived from aromatic hydrocarbons.

Examples of Halon and DDT

  • Halon: Used previously in firefighting, contains bromine and fluorine with one or two carbons.
    • Functionality: Interrupts chain reactions during combustion.
  • DDT (Dichloro-Diphenyl Trichloroethane):
    • Synthetic insecticide, very toxic to various insects.

Biological Importance of Haloalkanes & Haloarenes

  • Thyroxine: Contains iodine, deficiency causes goiter.
  • Chloroquine: Antimalarial drug containing chlorine.
  • Halothane: Contains fluorine, chlorine, and bromine; used as an anesthetic.
  • Chloramphenicol: Antibiotic containing chlorine; used for typhoid fever.

Classification of Haloalkanes and Haloarenes

  • Based on Number of Halogens:

    • Monohaloalkane/Monohaloarene: 1 halogen.
    • Dihaloalkane/Dihaloarene: 2 halogens.
    • Polyhaloalkane/Polyhaloarene: More than 2 halogens.

  • Types of Dihalo Compounds:

    • Vicinal: Halogens are on adjacent carbon atoms.
    • Geminal: Both halogens are on the same carbon atom.

Hybridisation in Halo Compounds

  • sp³ Carbon in Haloalkanes:
    • Classification by attached carbon numbers:
    • Primary (1°): 1 carbon attached.
    • Secondary (2°): 2 carbons attached.
    • Tertiary (3°): 3 carbons attached.
  • sp² Carbon in Haloarenes:
    • Includes Aryl and Vinylic halides based on attachment to benzene or alkenes respectively.

Characteristics of C-X Bonds

  • Nature of C-X Bond:
    • Order of reactivity based on halogen: C-F < C-Cl < C-Br < C-I.
  • Electronegativity and Bond Length:
    • Polarisation due to electronegativity differences.
    • Effect on dipole moment and bond length.

Preparation of Halo Compounds

  • From Alcohols:
    • Reagents: Hydrohalic acids, phosphorus halides, thionyl chloride.
    • Process: Alcohols react with HX to form haloalkanes.
  • Halogen exchange: To convert one halide to another.

Reactions of Halo Compounds

  • Nucleophilic Substitution Reactions: Replacing leaving groups with nucleophile.
  • Elimination Reactions: Involve removal of atoms to form double bonds.
    • E1 and E2 Mechanisms:
    • E1: Unimolecular; involves a carbocation intermediate.
    • E2: Bimolecular; proceeds in a single step.

Electrophilic Substitution in Haloarenes

  • Reactivity: Strong electron-withdrawing groups enhance nucleophilic substitution via stabilization.
  • Dow's Process: Converts chlorobenzene to phenol under high pressure with sodium hydroxide.

Polyhalogen Compounds

  • Types: Dichloromethane, trichloromethane, DDT, freons.
  • Dichloromethane:
    • Uses: Solvent, propellant, paint remover.
    • Harmful Effects: CNS impairment, skin irritation.
  • Trichloromethane: Anesthetic; can cause dizziness, liver damage.

Chiral Compounds

  • A compound is chiral if it has four different groups attached to a carbon atom leading to non-superimposable mirror images.

Summary of Elimination vs Substitution

  • E2 reactions favor strong bases under sterically unhindered conditions.
  • SN1 reactions favor stable carbocation formation, often mediated by weak nucleophiles and polar protic solvents.