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• Alkyl halides are hydrocarbons where one or more hydrogen atoms have been replaced by a halogen atom.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:
  

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:
     $ext{C}<em>3 ext{H}</em>8 + ext{Br}<em>2 ightarrow ext{C}</em>3 ext{H}_7 ext{Br} + ext{HBr}$
   • 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:
     $ext{RCH}= ext{CR} + ext{X}<em>2 ightarrow ext{RCH}</em>2 ext{C} ext{R}X$
   • Hydrohalogenation:
     $ext{RCH}= ext{CR} + ext{HX} <br /> ightarrow ext{RCH}_2 ext{C} ext{R}X$

B. Key Reactions of Alkyl Halides

1. Nucleophilic Substitution Reactions:

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

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:
     $ext{CH}<em>2 ext{H}- ext{CH}</em>2 ext{X} + ext{OH}^- <br /> ightarrow ext{CH}<em>2= ext{CH}</em>2 + X^- + ext{H}_2 ext{O}$
   • Example:
     $ext{1-chloropropane} + ext{OH}^- <br /> ightarrow ext{propene} + ext{Cl}^- + ext{H}_2 ext{O}$

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.