Chapter 7 — Alcohols, Phenols and Ethers
One oxygen atom, three families: alcohols, phenols and ethers. How they are named and made, why hydrogen bonding sets their boiling points, why phenol is an acid, and how the –OH and –OR groups react.
Every year India crushes hundreds of millions of tonnes of sugarcane. The sweet juice becomes sugar; the dark syrup left over, molasses, is fermented by yeast, and the ethanol distilled from it is blended into petrol at the fuel pump. That one molecule, $\mathrm{C_{2}H_{5}OH}$, is also the solvent of most medicines and perfumes, the disinfectant on a hospital trolley and, through its oxidation, the reason a breath test can catch a drunk driver. Its close relatives are just as familiar: the glycerol of soaps and cosmetics, the phenol that Joseph Lister sprayed in his operating theatre to stop wounds from going septic, the salicylic acid behind aspirin, and the ethoxyethane that made painless surgery possible.
All of these compounds contain an oxygen atom joined to carbon by single bonds. When the oxygen carries a hydrogen and sits on a saturated carbon, the compound is an alcohol; when the $\mathrm{{-}OH}$ is on a benzene ring, it is a phenol; when the oxygen links two carbon groups, it is an ether. In this chapter you will learn to tell the three families apart, to name them, and to make them; you will see how hydrogen bonding explains their boiling points and solubility, why phenol is a million times more acidic than ethanol, how alcohols lose their $\mathrm{OH}$ group or are oxidised, how the phenol ring reacts with electrophiles, and how ethers are made and split.
| Question | Key idea | Section |
|---|---|---|
| Which family, which class? | carbon bearing O: $sp^{3}$ (1°/2°/3°) or ring | 7.1 |
| How is the $\mathrm{OH}$ put in place? | hydration, hydroboration, reduction, Grignard | 7.1 |
| Why do alcohols boil so high? | $\mathrm{O{-}H\cdots O}$ hydrogen bonds | 7.1 |
| How acidic is it? | $\mathrm{p}K_{a}$: ethanol $15.9$, phenol $10.0$ | 7.2 |
| What happens to the C–O bond? | Lucas test, dehydration, oxidation | 7.2 |
| How are ethers made and split? | $\mathrm{RONa+R'X}$; $\ \mathrm{HI}$ | 7.3 |
- classify alcohols (1°, 2°, 3°, allylic, benzylic), phenols and ethers (symmetrical, unsymmetrical), and give and interpret their IUPAC names;
- prepare alcohols from alkenes (acid-catalysed hydration, hydroboration–oxidation), from carbonyl compounds by reduction and from Grignard reagents, and phenol by four routes including the cumene process;
- explain boiling points and solubilities through hydrogen bonding, and the acidity of alcohols and phenols, including the effect of substituents and $\mathrm{p}K_{a}$ values;
- predict the products of esterification, the Lucas test, dehydration (with its mechanism), oxidation, nitration and bromination of phenol, and Kolbe's and Reimer–Tiemann reactions;
- prepare ethers by dehydration and by a well-chosen Williamson synthesis, predict the products of their cleavage by $\mathrm{HI}$, and the substitution products of anisole.
Sections in this chapter
Classification, Nomenclature and Preparation
Classes and names of alcohols, phenols and ethers; shapes of the functional groups; making alcohols and phenols; hydrogen bonding, boiling points and solubility.
Properties and Reactions of Alcohols and Phenols
Acidity and $\mathrm{p}K_{a}$, esterification, the Lucas test, dehydration, oxidation, reactions of phenol, and commercial methanol and ethanol.
Ethers
Dehydration and the Williamson synthesis, physical properties, cleavage by hydrogen halides, and substitution in anisole.