Comprehensive Notes on Alkyl Halides and Alcohols

Introduction

  • KNUST ranks no. 1 globally for the provision of quality education (SDG 4).

Alkal Halides

Introduction

  • Alkyl halides are compounds with halogen substituents (e.g., fluorine, chlorine, bromine, iodine) bonded to alkyl groups.
  • Generally represented as R-X, where R is an alkyl group and X is a halogen.
  • Examples include:
    • Ethyl bromide (CH₃-CH₂-Br)
    • Butyl bromide (CH₃-CH₂-CH₂-Br)
    • Isopropyl chloride (CH₃-CH-Cl-CH₃)

Classification

  • Alkyl halides can be categorized as:
    • Primary (1°): Halogen bonded to a carbon attached to one other carbon.
    • Secondary (2°): Halogen bonded to a carbon attached to two other carbons.
    • Tertiary (3°): Halogen bonded to a carbon attached to three other carbons.

Key Properties

Carbon-Halogen Bond
  • The size of halogen atoms increases down the periodic table, affecting bond lengths and strengths:
    • C-Halogen ext{ bond length increases and strength decreases as we go down the periodic table.}
Boiling Points
  • Alkyl halides have higher boiling points than alkanes of similar molar mass due to greater molecular weight and molecular polarity.
Solubility
  • Alkyl halides are insoluble in water but soluble in organic solvents.
    • Higher chlorinated compounds like CH₂Cl₂, CHCl₃ can be carcinogenic.

Nomenclature

  • Alkyl halides are named considering the longest carbon chain as the parent and treating halogens as substituents. Numbering should give substituents the lowest possible number.
  • Complex examples such as:
    • 1- ext{Bromo-3-chloro-4-methylpentane}
    • 2- ext{Butyl-5-bromohexane}

Synthesis of Alkyl Halides

  1. Hydrogen Halides (HX): Alcohols react with hydrogen halides to yield alkyl halides.
    • R-OH + HX
      ightarrow R-X + H₂O
  2. Thionyl Chlorides (SOCl₂) and Phosphorus Halides (PX₃):
    • R-OH + SOCl₂
      ightarrow R-Cl + SO_2 + HCl

Reactions of Alkyl Halides

Substitution Reactions

  • Two pathways: SN1 and SN2
    • SN1: Unimolecular substitution involving carbocation formation.
    • SN2: Bimolecular substitution with a concerted mechanism.

Elimination Reactions

  • Alkyl halides undergo elimination reactions yielding alkenes through dehydrohalogenation, categorized as:
    • E1: Two-step mechanism leading to carbocation formation.
    • E2: One-step mechanism with simultaneous bond breaking.

Nucleophilic Substitution Mechanisms

  • SN1:
    • Rate depends only on the substrate.
    • Product often leads to racemization due to chirality.
  • SN2:
    • Relies on steric hindrance, where primary substrates react more quickly than tertiary.

Physical Properties

Alcohols

  • Alcohols contain hydroxyl groups, categorized similarly to alkyl halides (primary, secondary, tertiary).
    • Boiling Points: Alcohols have significantly higher boiling points due to hydrogen bonding.
    • Solubility: Fewer than four carbons, the alcohol is soluble in water; more than four, insoluble.

Examples of Structures and Nomenclature

  • IUPAC naming typically replaces the 'e' with 'ol'.
  • Examples include:
    • 1-Pentanol
    • 2-Ethyl-1-pentanol

Commercial Significance

  • Alkyl halides and alcohols play significant roles in pharmaceuticals and industrial applications. Examples:
    • Chloramphenicol (antibiotic)
    • DDT (insecticide)
    • Refrigerants (like Freons)

Summary

  • The understanding of alkyl halides and alcohols is crucial in organic chemistry, especially their synthesis, reactions, and applications in the pharmaceutical industry and other sectors.