Haloalkanes and Haloarenes

May 15, 2024

Lecture on Haloalkanes and Haloarenes

Overview

  • This lecture focused on the chemistry of haloalkanes and haloarenes.
  • Discussion included definitions, structures, nomenclature, preparation methods, properties, and applications.

Definitions and Introduction

  • Haloalkanes: Formed by replacing a hydrogen atom in an alkane with a halogen (e.g., chlorine, bromine, iodine).
  • Haloarenes: Aromatic hydrocarbons (typically benzene) with one or more hydrogen atoms replaced by halogens.

Structure and Nomenclature

Haloalkanes

  • Derived from alkanes by substituting a hydrogen with a halogen.
  • Common examples: Methyl chloride, Ethyl chloride.
  • Example structure: CH<sub>3</sub>-CH<sub>2</sub>-CH<sub>2</sub>-Br.
  • Naming rules: The position of the halogen is indicated with a number.

Haloarenes

  • Derived from aromatic hydrocarbons like benzene.
  • Example structure: Benzene ring with chlorine (Chlorobenzene).
  • Nomenclature involves indicating positions like ortho, meta, para.

Preparation Methods

From Alkanes

  1. Halogenation: Involves replacing hydrogens in alkanes with halogens using sunlight or UV light.
    • Problems: May produce isomers, less selectivity.

From Alkenes

  1. Addition of HX (hydrogen halides): Forms haloalkanes via Markovnikov's rule.
    • Example: Propene + HBr → 2-Bromopropane.
  2. Halogen Addition: Forms vicinal dihalides (Example: Ethene + Br<sub>2</sub>).
    • Results in trans-dihalides.

From Alcohols

  1. Using HX: Direct substitution (Example: ROH + HCl → RCl + H<sub>2</sub>O).
  2. Using PCl<sub>3</sub>, PCl<sub>5</sub>, SOCl<sub>2</sub>: Reagents convert alcohols to haloalkanes with SO<sub>2</sub> and HCl as byproducts.
    • Advantage with SOCl2: gaseous byproducts make it easy to purify.
  3. Haloform Reaction: Converts methyl ketones and related alcohols to haloforms.

From Arenes

  • Electrophilic Aromatic Substitution: Replacing hydrogen in benzene under catalytic conditions and no light.
    • Example: Benzene + Cl<sub>2</sub> (with FeCl<sub>3</sub> catalyst) → Chlorobenzene.

Properties

Physical Properties

  1. State: Lower members are gases; higher ones are liquids or solids.
  2. Color: Pure compounds are colorless; some might develop color due to impurities.
  3. Boiling Points: Higher than comparable alkanes due to mass and polarity; branching decreases boiling points.
  4. Density: Generally denser than water due to the presence of heavier halogens.
  5. Solubility: Slightly soluble in water, more soluble in organic solvents.

Chemical Properties

  1. Nucleophilic Substitution Reaction (SN1, SN2): Replacement of the halogen by a nucleophile like OH<sup>-</sup>, CN<sup>-</sup>.
  2. Elimination Reactions (E1, E2): Formation of alkenes under strong bases.

Applications in Daily Life and Medicine

  • Chloroform and Iodoform: Historically used as anesthetics.
  • Thyroxine: Contains iodine, critical for human health.
  • Freons: Used in refrigeration (though environmentally harmful).
  • Synthetic Uses: Precursors to other chemical compounds.

Important Concepts and Rules

  • Markovnikov's Rule: During addition of HX to alkenes, halogen attaches to the more substituted carbon.
  • Polarity of C-Halogen Bond: Increases with a more electronegative halogen.
  • Reactivity Trends: Primary, secondary, tertiary alkyl halides show different reactivity patterns.

Next lecture will cover advanced topics on haloalkanes and haloarenes, including their role in synthetic chemistry and industrial applications. Be sure to review these concepts as they form the basis for more complex reactions.