Formula sheet: Alcohols, Phenols and Ethers
Alcohols, Phenols and Ethers14 min readLesson 1 of 6
1.1 Structural numbers to remember
In an alcohol the oxygen sits on an sp3 carbon and the carbon oxygen bond is a single sigma bond. The carbon oxygen bond in phenol is shorter, 136 pm against 142 pm in methanol, because the oxygen lone pair is conjugated with the aromatic ring and the carbon is sp2. The tetrahedral bond angle at oxygen opens slightly in ethers because the two bulky groups push each other away.
1.2 Named reactions and conditions
| Reaction | Reagents and conditions | Product |
|---|---|---|
| Acid catalysed hydration of an alkene | Alkene with water and dilute acid | Alcohol, Markovnikov orientation |
| Hydroboration oxidation | Diborane then hydrogen peroxide in aqueous NaOH | Anti-Markovnikov alcohol |
| Reduction of a carbonyl | H2 with Pt, Pd or Ni, or NaBH4, or LiAlH4 | Alcohol |
| Reduction of an acid or ester | LiAlH4, or ester then catalytic hydrogenation | Primary alcohol |
| Grignard addition | RMgX with an aldehyde or ketone, then hydrolysis | Alcohol with a new carbon chain |
| Phenol from chlorobenzene | NaOH at 623 K and high pressure, then acid work up | Phenol |
| Phenol from cumene | Air oxidation of cumene, then dilute acid | Phenol plus acetone |
| Esterification | Carboxylic acid or acid chloride or anhydride, with a little H2SO4 or pyridine | Ester |
| Acetylation | Acetic anhydride, small amount of H2SO4 | Acetyl derivative, salicylic acid gives aspirin |
| Dehydration of an alcohol | Concentrated H2SO4 or H3PO4, heat; 443 K gives ethene, 413 K gives ether | Alkene or ether |
| Lucas test | Concentrated HCl with anhydrous ZnCl2 | Alkyl chloride appears as turbidity |
| Oxidation of an alcohol | Acidified KMnO4 or CrO3; PCC for aldehyde; Cu at 573 K for dehydrogenation | Aldehyde, acid or ketone |
| Kolbe reaction | Sodium phenoxide plus CO2, then acid work up | Salicylic acid |
| Reimer Tiemann reaction | Phenol with chloroform and aqueous NaOH | Salicylaldehyde |
| Williamson synthesis | Sodium alkoxide with a primary alkyl halide | Ether |
| Cleavage of an ether | Concentrated HI or HBr, heat | Alkyl iodide plus alcohol or phenol |
1.3 Acidity numbers
The pKa scale puts the acid strength in one line. Ethanol is a very weak acid with pKa 15.9. Phenol has pKa 10.0, so it beats ethanol by a million times. Cresols sit near 10.2 and 10.1. A nitro group pulls the value down: ortho nitrophenol is 7.2, meta is 8.3, para is 7.1. Three nitro groups make picric acid a strong acid.
The rule behind these numbers is straightforward. Phenoxide carries a delocalised negative charge, while an alkoxide keeps its negative charge on one oxygen. Anything that stabilises the phenoxide ion raises acidity, so electron withdrawing groups help and electron releasing alkyl groups weaken the acid.