Alkyl Halides Study Notes

Module 03: Alkyl Halides

Introduction

  • Alkyl halides are hydrocarbons where one or more hydrogen atoms have been replaced by a halogen atom.
    • Halogens and their prefixes:
    • Fluorine (F) - prefix is fluoro-
    • Chlorine (Cl) - prefix is chloro-
    • Bromine (Br) - prefix is bromo-
    • Iodine (I) - prefix is iodo-
  • General formula for alkyl halides:
    R-X or Ar-X
    where R = alkyl group, Ar = aromatic group, X = halogen.

Properties and Uses

1. Properties of Alkyl Halides

  • Sweet Smelling: Characteristic odor.
  • Melting Point (MP) and Boiling Point (BP):
    • Increase with molecular weight.
    • Higher MP and BP than hydrocarbons of similar molecular weight due to stronger intermolecular forces (dipole-dipole compared to dispersion forces).
  • Density: Denser than water.
  • Solubility: Good solvents for non-polar solutes like grease and oil due to long alkyl chains and small polar portions.
  • Stability: Relatively inert.

2. Nomenclature

  • Identifying the Longest Chain: This is also called the parent chain.
  • Numbering the Parent Chain: Number it so that the first substituent (halogen or alkyl group) gets the lowest number.
  • Alphabetical Order of Substituents: Halogen substituents are listed alphabetically along with other prefixes:
    • F = fluoro-
    • Cl = chloro-
    • Br = bromo-
    • I = iodo-

3. Common Compounds and IUPAC Names

  • Chloromethane (CH₃Cl) - Methyl chloride
  • Bromoethane (CH₃CH₂Br) - Ethyl bromide
  • 2-fluoropropane (CH₃CH(Br)CH₃) - Isopropyl fluoride
  • 3,4-dichloro-1-butene:
    3,4-dichloro-1-butene

Reactions of Alkyl Halides

1. Nature of the Alkyl Halide Bond

  • Polar Carbon-to-Halogen Bonds:
    • Leads to a tendency for the bond to break during chemical reactions, producing a carbocation (positive charge) and a halide ion (negative charge).

2. Types of Reactions

A. Synthesis of Alkyl Halides

  1. Substitution Reactions:

    • Alkanes can undergo substitution reactions where a hydrogen atom is replaced by a halogen atom, typically requiring UV light as a catalyst.
    • Example:
      ext{C}3 ext{H}8 + ext{Br}2 ightarrow ext{C}3 ext{H}_7 ext{Br} + ext{HBr}
    • Here, bromination of propane yields 1-bromopropane.

  2. Addition Reactions:

    • Alkenes undergo addition reactions to form alkyl halides via halogenation or hydrohalogenation of the double bond.
    • Markovnikov’s Rule: The hydrogen atom attaches to the carbon atom with the most hydrogen atoms already bonded.
    • Example:
      ext{RCH}= ext{CR} + ext{X}2 ightarrow ext{RCH}2 ext{C} ext{R}X
    • Hydrohalogenation:
      ext{RCH}= ext{CR} + ext{HX}
      ightarrow ext{RCH}_2 ext{C} ext{R}X
B. Key Reactions of Alkyl Halides

  1. Nucleophilic Substitution Reactions:

    • A nucleophile attacks the alkyl halide, substituting for the halogen atom.
    • General reaction:
      ext{R}-X + : ext{Nu}^-
      ightarrow ext{R}- ext{Nu} + X^-
      Where X is the leaving group (halogen).
    • Example:
      ext{1-bromopropane} + ext{OH}^-
      ightarrow ext{1-propanol} + ext{Br}^-

  2. Elimination Reactions:

    • A strong base (good nucleophile) attacks the alkyl halide, removing a hydrogen and expelling a halide to form an alkene.
    • General reaction:
      ext{CH}2 ext{H}- ext{CH}2 ext{X} + ext{OH}^-
      ightarrow ext{CH}2= ext{CH}2 + X^- + ext{H}_2 ext{O}
    • Example:
      ext{1-chloropropane} + ext{OH}^-
      ightarrow ext{propene} + ext{Cl}^- + ext{H}_2 ext{O}

Summary

  • The polarity of the carbon-halogen bond results in differing physical properties compared to hydrocarbons.
  • Alkyl halides can be synthesized through substitution reactions of alkanes and addition reactions with alkenes.
  • Known for their diverse applications, alkyl halides undergo significant nucleophilic substitution and elimination reactions.