How the hydroxyl group shapes alcohols, phenols and ethers
Alcohols contain an –OH group attached to a saturated carbon. They are classified as primary, secondary or tertiary depending on how many carbon atoms are bonded to the carbon carrying –OH, and named with the suffix -ol, as in propan-2-ol. They are prepared by hydration of alkenes, reduction of aldehydes and ketones, or the reaction of Grignard reagents with carbonyl compounds. Hydrogen bonding gives alcohols higher boiling points than hydrocarbons of similar mass.
Phenols have –OH directly on a benzene ring. They are more acidic than alcohols because the phenoxide ion is stabilised by resonance, and they react with NaOH while ethanol does not. The –OH group activates the ring, so phenol reacts readily with bromine water to give 2,4,6-tribromophenol. Lucas reagent distinguishes alcohol types: tertiary alcohols turn cloudy at once, secondary ones in a few minutes, and primary ones not at room temperature. Ethers, R–O–R′, are made by Williamson synthesis and cleaved by HI.
Why it matters
Ethanol is used as a fuel additive, solvent and in hand sanitisers, while methanol is poisonous and causes blindness if swallowed. Phenol derivatives are found in antiseptics, plastics such as Bakelite and medicines like aspirin. Diethyl ether was once an anaesthetic. Reactions of the –OH group are used throughout organic synthesis.
Key terms
- Hydroxyl group
- The –OH functional group present in alcohols and phenols.
- Lucas test
- A test using ZnCl₂ in concentrated HCl that distinguishes primary, secondary and tertiary alcohols by reaction speed.
- Williamson synthesis
- A method of preparing ethers by reacting an alkyl halide with a sodium alkoxide.
- Phenoxide ion
- The negative ion formed when phenol loses a proton, stabilised by resonance in the ring.