Alcohols, Phenols and Ethers
Easy Overview
Alcohols aren't just for drinking. Phenols are used in antiseptics. Ethers are solvents and anaesthetics. These three families are all about the -OH group (alcohols/phenols) or the -O- bridge (ethers). They react differently based on where the oxygen is.
Alcohols — structure and preparation
Alcohols have an -OH group attached to a carbon. Primary (1°), secondary (2°), tertiary (3°) depending on how many carbons are attached to that carbon. Made by hydration of alkenes, reduction of carbonyl compounds, or fermentation of sugar. Yes, fermentation makes ethanol — that's the drinkable kind.
Reactions of alcohols
Alcohols can be oxidized — primary → aldehyde → acid; secondary → ketone; tertiary won't oxidize (too protected). Dehydration (removing water) gives alkenes. Lucas test distinguishes between 1°, 2°, 3° alcohols based on how fast they react with HCl/ZnCl2. Tertiary reacts immediately — cloudy in seconds.
Phenols — acidity and reactions
Phenols have -OH directly attached to a benzene ring. They're more acidic than alcohols because the phenoxide ion is stabilized by resonance. Not as acidic as carboxylic acids though. Phenol reacts with FeCl3 to give a purple color — a classic test. Used in antiseptics like Dettol.
Ethers — structure and properties
Ethers have an oxygen bonded to two alkyl/aryl groups. R-O-R'. They're relatively unreactive — that's why they're great solvents. Diethyl ether was used as an anaesthetic (smells sweet). They're prepared by Williamson synthesis — alkoxide ion attacks an alkyl halide. Like clicking two LEGO blocks together.
Important reactions for exams
Kolbe's reaction: phenol + CO2 → salicylic acid (used in aspirin). Reimer-Tiemann reaction: phenol + CHCl3 + NaOH → salicylaldehyde. Friedel-Crafts: ethers on benzene rings. These are board exam favorites. Also know that ethers can cleave with HI to give alcohol + alkyl iodide.
Key Points
- •Alcohols: R-OH; Phenols: Ar-OH; Ethers: R-O-R'
- •Lucas test: 3° alcohol reacts fastest (cloudy immediately)
- •Primary alcohols oxidize to aldehydes, then carboxylic acids
- •Phenols are more acidic than alcohols but less than carboxylic acids
- •Phenol + FeCl3 → violet color (distinctive test)
- •Williamson synthesis: R-O-Na+ + R'-X → R-O-R' + NaX
- •Ethers are cleaved by HI (hot) to give alkyl halide and alcohol
Practice Questions
- Explain the mechanism of dehydration of ethanol to ethene.
- Why is phenol more acidic than ethanol but less acidic than acetic acid?
- Write the mechanism of Kolbe's reaction.
- How is diethyl ether prepared by Williamson's synthesis?