Overview
This lecture covers the key reactions and properties of hydroxy compounds, focusing mainly on phenols as outlined in the CIE Chemistry syllabus.
Alcohols and Acyl Chlorides
- Alcohols contain the hydroxyl group (–OH).
- Alcohols react with acyl (acid) chlorides to form esters and hydrogen chloride (HCl) gas.
- The reaction is vigorous and must be performed in a fume cupboard due to toxic HCl gas.
Introduction to Phenols
- Phenols have a hydroxyl group attached directly to a benzene ring.
- Examples include 2-methylphenol and salicylic acid.
- The carbon with the –OH is numbered as carbon 1 in naming.
Acidity of Phenols
- Phenols are weak acids; they partially dissociate in water to form phenoxide ions and H+.
- Phenols are more acidic than water and ethanol due to stabilization of the phenoxide ion by the benzene ring.
- The negative charge on O– is delocalized into the aromatic ring, increasing stability.
Reactions of Phenols
- Phenols react with sodium hydroxide to form sodium phenoxide and water (neutralization).
- Phenols react with sodium metal to form sodium phenoxide and hydrogen gas.
- Phenols can react with carbonate and metal as other acids do, following typical acid-base reaction patterns.
Reactivity of Phenols with Electrophiles
- The –OH group donates electrons to the benzene ring, increasing reactivity at positions 2, 4, and 6.
- Phenols undergo electrophilic substitution more easily than benzene.
- Electrophiles tend to attack the 2, 4, and 6 positions on the phenol ring.
Specific Reactions with Phenols
- Phenol reacts with bromine water to produce 2,4,6-tribromophenol (white precipitate, antiseptic smell), decolorizing bromine water.
- Phenol reacts with dilute nitric acid to give 2-nitrophenol and 4-nitrophenol isomers.
- No halogen carrier is required for bromination or nitration of phenol, unlike benzene.
Influence of Other Groups on Substitution
- Electron-donating groups (like –OH in phenol) favor substitution at 2, 4, 6.
- Electron-withdrawing groups (like nitro in nitrobenzene) favor substitution at 3, 5 positions.
Coupling Reactions and Dyes
- Phenol reacts with benzene diazonium chloride (in alkaline conditions and cold) to form azo dyes (bright colored compounds).
- The coupling reaction links two aromatic rings, producing an orange precipitate.
Key Terms & Definitions
- Hydroxyl group (–OH) — A functional group containing oxygen and hydrogen, characteristic of alcohols and phenols.
- Phenol — Aromatic compound with a hydroxyl group attached to a benzene ring.
- Phenoxide ion — The anion formed when phenol loses a proton (O– attached to benzene).
- Acyl/Acid chloride — A reactive derivative of carboxylic acid, RCOCl.
- Electrophilic substitution — A reaction where an electrophile replaces a hydrogen atom in an aromatic ring.
- Azo dye — Colored compound formed by a coupling reaction between a diazonium ion and an aromatic compound.
Action Items / Next Steps
- Review previous topics, especially aromatic compounds (Topics 30 and 31).
- Practice reaction mechanisms for phenol with acyl chlorides, bromine, and nitric acid.
- Learn the naming and identification of substitution positions on aromatic rings.
- Revise coupling reactions and their applications in dye chemistry.