Lecture on Alcohols: Physical Properties and Alkoxide Preparation

Comparison of Alcohols and Alkanes

• Ethane vs. Ethanol:
  • Ethane (C2H6):
    • Boiling point: -89°C
    • State at room temperature (20-25°C): Gas
  • Ethanol (C2H5OH):
    • Boiling point: 78°C
    • State at room temperature: Liquid

Boiling Points and Intermolecular Forces

• Ethane:
  • Held by weak London dispersion forces
  • Low boiling point due to ease of separation
• Ethanol:
  • Involves hydrogen bonding, a strong intermolecular force
  • Higher boiling point as more energy is required to separate molecules

Hydrogen Bonding

• Ethanol Molecules:
  • Polarized O-H bond due to electronegativity difference
  • Partial negative charge on Oxygen, partial positive on Hydrogen
  • Hydrogen bonds form between the partially charged atoms of different ethanol molecules

Solubility in Water

• Ethanol:
  • Soluble due to hydrogen bonding with water
  • Water and ethanol both polar, allowing dissolution

Structure and Solubility

• Ethanol Structure:
  • OH group provides polarity (hydrophilic)
  • Alkyl group (ethane part) is non-polar (hydrophobic)
  • Solubility decreases with increasing carbon atoms in the alkyl group

Preparation of Alkoxides

• Reaction with Strong Base:
  • Alcohol + Strong Base → Alkoxide ion
  • Example: Ethanol + sodium hydride (NaH) → Sodium ethoxide (alkoxide) + Hydrogen gas
• Properties:
  • Alcohols can be acidic with a strong enough base

Preparation from Group 1 Metals

• General Reaction:
  • Alcohol + Group 1 Metal (e.g., Na, K) → Alkoxide + Hydrogen gas
• Example:
  • Cyclohexanol + Sodium → Alkoxide ion + Hydrogen gas
  • Mechanism involves metal donating an electron, forming hydrogen gas

Summary

• Physical properties of alcohols are influenced by intermolecular forces, primarily hydrogen bonding.
• Alcohols react with strong bases and group 1 metals to form alkoxide ions, which are used in organic reactions.
• Solubility of alcohols in water is determined by the balance of hydrophilic and hydrophobic parts within the molecule.