Haloalkanes: Nomenclature and Classification

• Halogen derivatives of alkanes are compounds formed when one or more hydrogen atoms in an alkane molecule are replaced by halogen atoms (fluorine, chlorine, bromine, or iodine).

Nomenclature

• Common names: Alkyl halides are often named by first naming the alkyl group and then the halide (e.g., methyl chloride, ethyl bromide).
• IUPAC nomenclature:
  • Identify the longest continuous carbon chain.
  • Number the carbon atoms in the main chain to give the carbon bearing the halogen the lowest possible number.
  • Name the halogen substituent with the prefixes fluoro-, chloro-, bromo-, or iodo-.
  • List the substituents in alphabetical order with appropriate numbering.

Classification

• Based on the number of halogen atoms:
  • Monohaloalkanes: Contain one halogen atom.
  • Dihaloalkanes: Contain two halogen atoms.
  • Polyhaloalkanes: Contain more than two halogen atoms.
• Based on the type of carbon atom to which the halogen is attached:
  • Primary (1°) haloalkanes: Halogen is attached to a primary carbon atom (carbon attached to one other carbon).
  • Secondary (2°) haloalkanes: Halogen is attached to a secondary carbon atom (carbon attached to two other carbons).
  • Tertiary (3°) haloalkanes: Halogen is attached to a tertiary carbon atom (carbon attached to three other carbons).

Methods of Preparation

• Halogenation of Alkanes:
  • Alkanes react with halogens in the presence of ultraviolet light or heat via a free radical mechanism, leading to mono- and polyhalogenated products.
  • The reaction is generally non-selective, resulting in a mixture of products.
• Addition of Hydrogen Halides to Alkenes:
  • Alkenes react with hydrogen halides (HCl, HBr, HI) to form haloalkanes.
  • Follows Markovnikov's rule: the hydrogen atom adds to the carbon with more hydrogen atoms, and the halogen adds to the carbon with fewer hydrogen atoms.
• Reaction of Alcohols with Hydrogen Halides:
  • Alcohols react with hydrogen halides in the presence of a catalyst (e.g., ZnCl2) to form haloalkanes.
  • The reactivity order of hydrogen halides is HI > HBr > HCl.
  • Tertiary alcohols react readily, while primary and secondary alcohols require a catalyst and heat.
• Reaction of Alcohols with Phosphorus Halides or Thionyl Chloride:
  • Alcohols react with phosphorus halides (PCl5, PCl3, PBr3) or thionyl chloride (SOCl2) to form haloalkanes.
  • These reactions provide better yields and cleaner products than direct reaction with hydrogen halides.

Physical Properties

• Boiling Points:
  • Haloalkanes have higher boiling points than corresponding alkanes due to increased molecular weight and stronger intermolecular forces (dipole-dipole interactions).
  • Boiling points increase with increasing molecular weight (i.e., larger halogen atom or longer carbon chain).
  • For isomeric haloalkanes, the boiling point order is: primary > secondary > tertiary.
• Density:
  • Haloalkanes are generally denser than water.
  • Density increases with increasing number and atomic weight of halogen atoms.
• Solubility:
  • Haloalkanes are generally insoluble in water due to their nonpolar or weakly polar nature.
  • They are soluble in organic solvents.

Chemical Properties

• Nucleophilic Substitution Reactions:
  • Haloalkanes undergo nucleophilic substitution reactions, where the halogen atom is replaced by a nucleophile.
  • SN1 Reactions:
    • Unimolecular nucleophilic substitution.
    • Occurs in two steps: ionization to form a carbocation intermediate, followed by nucleophilic attack.
    • Favored by tertiary haloalkanes, polar protic solvents, and weak nucleophiles.
  • SN2 Reactions:
    • Bimolecular nucleophilic substitution.
    • Occurs in one step: simultaneous bond breaking and bond formation.
    • Favored by primary haloalkanes, polar aprotic solvents, and strong nucleophiles.
• Elimination Reactions:
  • Haloalkanes undergo elimination reactions (dehydrohalogenation) to form alkenes.
  • E1 Reactions:
    • Unimolecular elimination.
    • Occurs in two steps: ionization to form a carbocation intermediate, followed by proton removal.
    • Favored by tertiary haloalkanes, polar protic solvents, and weak bases.
  • E2 Reactions:
    • Bimolecular elimination.
    • Occurs in one step: simultaneous proton removal and leaving group departure.
    • Favored by strong bases, heat, and a transition state where the proton being removed and the leaving group are anti-periplanar.
    • Zaitsev's Rule: The major product is the more substituted alkene.
• Reaction with Metals:
  • Grignard Reagents:
    • Haloalkanes react with magnesium metal in dry ether to form Grignard reagents (RMgX).
    • Grignard reagents are highly reactive and can react with a variety of electrophiles (e.g., carbonyl compounds, water).
  • Wurtz Reaction:
    • Haloalkanes react with sodium metal in dry ether to form higher alkanes.
    • The reaction involves the coupling of two alkyl groups.
• Reduction:
  • Haloalkanes can be reduced to alkanes using reducing agents such as zinc and hydrochloric acid (Zn/HCl) or lithium aluminum hydride (LiAlH4).

Uses

• Solvents: Many haloalkanes (e.g., chloroform, dichloromethane) are used as solvents in laboratories and industries.
• Refrigerants: Chlorofluorocarbons (CFCs) were widely used as refrigerants but have been phased out due to their ozone-depleting properties.
• Anesthetics: Some haloalkanes (e.g., halothane) have been used as anesthetics.
• Pesticides: Some haloalkanes are used as pesticides and insecticides.
• Intermediates in Organic Synthesis: Haloalkanes are versatile intermediates in the synthesis of various organic compounds through nucleophilic substitution and elimination reactions.