Alcohols, Phenols and Ethers
An alcohol has -OH attached to a carbon of an aliphatic system (CH3OH); a phenol has -OH on an aromatic ring carbon (C6H5OH); an ether has an alkoxy/aryloxy group (R-O-R or Ar-O-R).
Classification and nomenclature
- Alcohols/phenols: mono-, di-, tri- or polyhydric by number of -OH groups.
- Monohydric alcohols are primary, secondary or tertiary (Csp3-OH); also allylic, benzylic and vinylic (Csp2-OH) types.
- IUPAC: alkane 'e' replaced by 'ol'; chain numbered from the end nearest -OH. Anisole = methoxybenzene; catechol/resorcinol/hydroquinone = benzene-1,2-/1,3-/1,4-diol.
- Ethers: simple/symmetrical (same groups) or mixed/unsymmetrical; larger group is parent, smaller is alkoxy substituent.
Structure
O is sp3 in alcohols and ethers. C-O in phenol (136 pm) is short due to partial double bond character (lone-pair conjugation with the ring) and sp2 carbon. Ether C-O-C angle is slightly more than tetrahedral (bulky R-group repulsion).Preparation of alcohols
- From alkenes: acid catalysed hydration (Markovnikov) and hydroboration-oxidation (anti-Markovnikov).
- From carbonyls: reduction of aldehydes/ketones (H2/Pt, NaBH4, LiAlH4) and of acids/esters (LiAlH4).
- From Grignard reagents: methanal gives primary, other aldehydes give secondary, ketones give tertiary alcohols.
Preparation of phenols
From haloarenes (NaOH, 623 K/320 atm), benzenesulphonic acid (molten NaOH), diazonium salts (warm water/dilute acid), and industrially from cumene (gives acetone as by-product).Physical properties
Alcohols/phenols boil higher than comparable hydrocarbons and ethers due to intermolecular H-bonding; boiling point falls with branching. Solubility in water (H-bonding) decreases as the alkyl/aryl group grows.Acidity
Alcohols and phenols are weak Bronsted acids; both react with active metals. Phenols also react with aqueous NaOH (alcohols do not), so phenols are stronger acids. Phenoxide is stabilised by charge delocalisation. Electron-withdrawing groups (-NO2, especially ortho/para) increase phenol acidity; electron-releasing groups (-CH3) decrease it. pKa: phenol 10.0, ethanol 15.9.Chemical reactions
- Alcohols/phenols: esterification (acids/acid chlorides/anhydrides; acetylation gives aspirin from salicylic acid).
- Alcohols (C-O cleavage): with HX (Lucas test distinguishes 1/2/3), with PBr3, dehydration to alkenes (tertiary > secondary > primary), oxidation (1 -> aldehyde -> acid; PCC stops at aldehyde; 2 -> ketone; 3 resists).
- Phenols: electrophilic substitution (ortho/para; nitration, bromination to 2,4,6-tribromophenol with bromine water), Kolbe's reaction (salicylic acid), Reimer-Tiemann (salicylaldehyde), Zn dust (benzene), oxidation to benzoquinone.