Why Phenol Is Acidic but Ethanol Is Not

Phenol is used to make plastics, disinfectants and medicines, yet it behaves very differently from the alcohols in Part 1: it is acidic, it reacts instantly with bromine water, and its ring can even pick up carbon dioxide. Ethers, meanwhile, are useful precisely because they are unreactive.

This part covers preparing phenol, phenol acidity, the reactions of phenol, and preparing and reacting ethers.

Phenol can be made by heating chlorobenzene with NaOH under pressure, by fusing benzenesulphonic acid with NaOH, by warming a diazonium salt with water, or on a large scale by oxidising cumene, which also yields propanone.

The four routes:

- From chlorobenzene — NaOH at $623$ K and $300$ atm gives sodium phenoxide, which acid converts to phenol
- From benzenesulphonic acid — fusion with NaOH gives sodium phenoxide; acidify to get phenol
- From diazonium salts — benzenediazonium chloride warmed with water gives phenol and N${}_2$
- From cumene — isopropylbenzene is oxidised by air to cumene hydroperoxide, which dilute acid splits into phenol and propanone

$${\text{C}}_6{\text{H}}_5{\text{CH(CH}}_3{)}_2\stackrel{{\text{O}}_2}{\to }{\text{C}}_6{\text{H}}_5{\text{C(CH}}_3{)}_2\text{OOH}\stackrel{{\text{H}}^{+}/{\text{H}}_2\text{O}}{\to }{\text{C}}_6{\text{H}}_5\text{OH}+{\text{CH}}_3{\text{COCH}}_3$$

Worked example. One mole of cumene ($M=120$ g mol${}^{-1}$) gives one mole each of phenol ($94$ g mol${}^{-1}$) and propanone ($58$ g mol${}^{-1}$). From $240$ kg of cumene, the theoretical yields are $188$ kg of phenol and $116$ kg of propanone.

An everyday example. Many household disinfectants contain phenol derivatives made from phenol produced this way.

The substance. The cumene process wastes almost nothing — its by-product, propanone, is itself a valuable solvent.

**Phenol is more acidic than alcohols because the phenoxide ion is stabilised by resonance that spreads its negative charge over the ring; electron-withdrawing groups such as –NO${}_2$ strengthen this stabilisation and the acidity, while electron-releasing groups such as –CH${}_3$ weaken it.

Order of acidity: carboxylic acids > phenol > water > alcohols

Why phenol is acidic:**

- The $s{p}^2$ ring carbon attached to –OH draws electron density from oxygen
- The phenoxide ion is resonance-stabilised, with charge delocalised to the ortho and para carbons
- Alkoxide ions have no such stabilisation

Substituent effects:

- Electron-withdrawing groups at ortho and para positions increase acidity — p-nitrophenol is more acidic than phenol
- Electron-releasing groups decrease acidity — cresols are weaker acids than phenol
- More –NO${}_2$ groups mean more acidity: 2,4,6-trinitrophenol (picric acid) is a strong acid

Worked example. p$K_a$ values: phenol $10.0$, ethanol $15.9$, p-nitrophenol $7.1$ and p-cresol $10.2$. Since $K_a=10^{-\text{p}{K}_a}$,

$$\frac{K_a(\text{phenol})}{K_a(\text{ethanol})}=10^{15.9-10.0}=10^{5.9}\approx 8\times 10^5$$

so phenol is close to a million times more acidic than ethanol.

An everyday example. Phenol dissolves in sodium hydroxide solution but not in sodium hydrogencarbonate, which is how a laboratory test tells it apart from a carboxylic acid.

The substance. A nitro group at the meta position helps much less, because resonance never carries the phenoxide charge to the meta carbon.

**The –OH group strongly activates the ring, so phenol undergoes electrophilic substitution easily at ortho and para positions; its phenoxide takes up CO${}_2$ to give salicylic acid in Kolbe's reaction, chloroform and alkali add an aldehyde group in the Reimer-Tiemann reaction, and oxidation gives benzoquinone.

Electrophilic substitution:

- Dilute HNO${}_3$** at low temperature — o- and p-nitrophenol, separated by steam distillation
- **Concentrated HNO${}_3$ — 2,4,6-trinitrophenol
- Bromine in CS${}_2$ at low temperature — mainly monobromophenols
- Bromine water — an immediate white precipitate of 2,4,6-tribromophenol

Kolbe's reaction.** Sodium phenoxide with CO${}_2$ under pressure, then acid, gives salicylic acid, 2-hydroxybenzoic acid.

Reimer-Tiemann reaction. Phenol with CHCl${}_3$ and aqueous NaOH, then acid, gives salicylaldehyde, 2-hydroxybenzaldehyde.

Oxidation. Chromic acid oxidises phenol to benzoquinone.

Uses: methanol is a solvent for paints and varnishes and a raw material for methanal, but it is highly poisonous; ethanol is a solvent and fuel additive, denatured with copper sulphate and pyridine to prevent misuse.

Worked example. $0.94$ g of phenol ($0.010$ mol) with excess bromine water gives $0.010$ mol of 2,4,6-tribromophenol, C${}_6$H${}_2$Br${}_3$OH:

$$M=6(12)+3(1)+3(80)+16=331{\text{ g mol}}^{-1},m=0.010\times 331=3.31\text{ g}$$

An everyday example. Salicylic acid from Kolbe's reaction is found in anti-acne face washes and is used to make common painkillers.

The substance. o-Nitrophenol steam-distils away from p-nitrophenol because its internal hydrogen bond leaves it less attached to water and more volatile.

**Ethers are made by dehydrating primary alcohols with conc. H${}_2$SO${}_4$ at $413$ K or, more generally, by Williamson synthesis from an alkyl halide and a sodium alkoxide; they boil like alkanes, dissolve slightly in water, and have their C–O bond split by HX.

Preparation:

- Dehydration** — two ethanol molecules lose water at $413$ K to give ethoxyethane; this works only for primary alcohols, since others form alkenes
- Williamson synthesis — $\text{RX}+\text{R’ONa}\to \text{ROR’}+\text{NaX}$, an SN2 reaction in which the alkyl halide should be primary

Physical properties. Ethers boil close to alkanes of similar mass because they cannot hydrogen-bond with each other, yet they dissolve in water about as well as alcohols of similar size, since water hydrogen-bonds to the ether oxygen.

Reactions:

- Cleavage by HX — $\text{ROR}+\text{HI}\to \text{RI}+\text{ROH}$
- Aryl alkyl ethers — anisole with HI gives **phenol and CH${}_3$I, because the aryl–oxygen bond is too strong to break
- Electrophilic substitution** — –OCH${}_3$ activates the ring and directs ortho and para: anisole brominates, nitrates and undergoes Friedel-Crafts reactions at those positions

Worked example. To make tert-butyl methyl ether, react sodium tert-butoxide with CH${}_3$Br. The other pairing — sodium methoxide with tert-butyl bromide — gives 2-methylpropene by elimination instead.

An everyday example. Ethoxyethane has served as a solvent and an anaesthetic because it is volatile and fairly unreactive.

The substance. Which halide and which alkoxide you pair decides whether Williamson synthesis works at all.