A-Level Chemistry: Halogenoalkanes

Introduction

• Focus on properties, mechanisms, and environmental impact of halogenoalkanes.
• Topics covered:
  • Properties of halogenoalkanes
  • Nucleophilic substitution mechanism
  • Elimination mechanism
  • Role in ozone depletion

Definition of Halogenoalkanes

• Derived from alkanes by replacing one or more hydrogen atoms with halogen (e.g., chlorine, fluorine).
• Example: Methane (CH₄) becomes chloromethane (CH₃Cl).
• Synthetic compounds, used in refrigerants, solvents, pharmaceuticals, PVC, Teflon, anesthetics.
• More reactive than alkanes due to the carbon-halogen bond.
• General formula: CₙH₂ₙ₊₁X (X = halogen).
• Naming: Use longest unbranched carbon chain + halogen prefix (e.g., fluoroethane).
  • Prefix order: bromo, chloro, fluoro, iodo.
  • Use numbers to indicate position of halogens.

Properties of Halogenoalkanes

Bond Polarity

• Dependent on electronegativity.
• Electronegativity: increases right and up in the periodic table.
• Polar bonds due to difference in electronegativity between carbon and halogen.
• Order of polarity: C-F > C-Cl > C-Br > C-I.

Solubility

• Insoluble in water due to weak interaction between solvent (water) and solute (halogenoalkane).
• More soluble in hydrocarbons; used in dry cleaning.

Boiling Point

• Increases with chain length due to stronger Van der Waals forces.
• Decreases with branching (less surface area for intermolecular forces).
• Increases down the halogen group (I > Br > Cl > F) due to larger size/mass of halogens.

Reactivity

• Reactivity depends on the bond breaking (C-X bond breaking).
• Influenced by bond polarity and bond enthalpy.
• Iodoalkanes (C-I bond) are most reactive due to weakest bond enthalpy.

Nucleophilic Substitution

Definition

• Substitution reaction where one atom/group is replaced by another.
• Nucleophiles: electron-rich species seeking positive charge (e.g., hydroxide, cyanide, ammonia).

Mechanism

• Curly Arrows show electron movement.
• Examples:
  • Hydroxide ion (OH⁻) forms alcohol.
  • Cyanide ion (CN⁻) forms nitrile.
  • Ammonia (NH₃) forms amine.

Conditions

• Hydroxide ions from aqueous sodium/potassium hydroxide.
• Cyanide from aqueous ethanolic potassium cyanide, warm conditions.
• Ammonia requires excess concentrated ammonia solution, often with ethanol, under pressure.

Elimination Mechanism

• Produces alkenes from halogenoalkanes in the presence of potassium/sodium hydroxide and ethanol (no water).
• Larger molecule loses atoms/groups of atoms.
• Conditions differ from substitution: heated and ethanol solvent.

Environmental Impact

Ozone Depletion

• CFCs (chlorofluorocarbons) rise to ozone layer.
• UV light breaks C-Cl bond, forming chlorine free radicals.
• Chain reaction catalyzed by chlorine radicals, decomposing ozone faster.
• Led to regulation and development of CFC-free alternatives.

Summary

• Halogenoalkanes are versatile in application but have an environmental impact.
• Understanding mechanisms and conditions helps predict their behavior and reactivity.