Overview
This lecture covers hydroxy compounds for CIE Chemistry, focusing on the structure, reactions, and properties of alcohols and phenols, with special emphasis on phenols' reactivity.
Hydroxy Compounds & Alcohols
- Hydroxy compounds contain an -OH (hydroxyl) group.
- Alcohols react with acyl chlorides to form esters and hydrogen chloride gas.
- Acid chlorides are reactive due to polar C=O and C-Cl bonds.
Phenols: Structure and Acidity
- Phenol is an aromatic compound with an -OH group attached to a benzene ring.
- Phenols are more acidic than water and ethanol because the phenoxide ion is stabilized by delocalization into the benzene ring.
- Phenols only partially dissociate, acting as weak acids and existing in equilibrium with phenoxide ions.
Reactions of Phenols
- Phenols react with sodium hydroxide (base) to form sodium phenoxide and water (neutralization).
- Phenols react with sodium metal to form sodium phenoxide and hydrogen gas.
- Phenols follow general acid reactions: acid + base → salt + water; acid + metal → salt + hydrogen.
Electrophilic Substitution & Reactivity
- The -OH group donates electrons into the benzene ring, increasing reactivity at carbons 2, 4, and 6.
- Phenols undergo electrophilic substitution more easily than benzene.
- Phenols react with bromine water to form 2,4,6-tribromophenol (white precipitate, antiseptic smell).
- Phenols react with dilute nitric acid to form 2-nitrophenol and 4-nitrophenol.
Effects of Substituents on Benzene
- Electron-donating groups (like -OH) favor substitution at positions 2, 4, and 6.
- Electron-withdrawing groups (like -NO₂) favor substitution at positions 3 and 5.
Coupling Reactions: Diazonium Salts
- Phenol reacts with benzene diazonium chloride in the presence of sodium hydroxide to form azo dyes (orange precipitate).
- The reaction is called a coupling reaction and is important for dye synthesis.
Key Terms & Definitions
- Acyl chloride — derivative of a carboxylic acid with -Cl instead of -OH.
- Phenol — benzene ring with an attached -OH group.
- Phenoxide ion — the anion formed when phenol loses a proton.
- Electrophilic substitution — reaction where an electrophile replaces a hydrogen atom on benzene.
- Azo dye — colored compounds formed from coupling reactions involving diazonium salts.
- Coupling reaction — reaction joining two aromatic rings via an azo linkage, typically for dye production.
Action Items / Next Steps
- Review previous topics (30 & 31) on aromatic compounds and benzene.
- Study the first-year topic on hydroxy compounds for foundational understanding.
- Practice drawing mechanisms for phenol reactions.
- Prepare for questions on reactivity patterns and electrophilic substitution.