Why Picric Acid Is Far More Acidic Than Phenol

Phenol has an -OH group just like ethanol, but because that group sits directly on a benzene ring, phenol dissolves in sodium hydroxide and reacts with bromine water in seconds, while ethanol does neither. The ring and the -OH group change each other's behaviour — making phenol an acid and its ring unusually reactive.

This lesson covers the acidic character of phenols and the factors that affect it, and the electrophilic substitution reactions of phenol, including Kolbe's reaction.

Phenol is acidic because the phenoxide ion formed when it loses a proton is stabilised by resonance, spreading the negative charge over the benzene ring; electron-withdrawing groups make phenols more acidic, and electron-donating groups make them less acidic.

Why phenol loses a proton:

- The oxygen is attached to an $\mathrm{s}{\mathrm{p}}^2$ carbon, which draws electron density away and weakens the O-H bond
- In the phenoxide ion, the negative charge is delocalised onto the ortho and para carbons
- Phenol's own resonance structures separate charge, so the phenoxide ion gains more stability from delocalisation

Evidence of acidity:

- Phenol reacts with sodium hydroxide: ${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{OH}+\mathrm{NaOH}\to {\mathrm{C}}_6{\mathrm{H}}_5\mathrm{ONa}+{\mathrm{H}}_2\mathrm{O}$
- It does not release carbon dioxide from sodium hydrogencarbonate, because it is a weaker acid than carbonic acid
- Acidity order: carboxylic acids > phenol > water > ethanol, with approximate $\mathrm{p}{K}_a$ values of 4.8 for ethanoic acid, 10.0 for phenol and 15.9 for ethanol

Effect of substituents:

- Electron-withdrawing groups such as $-\mathrm{N}{\mathrm{O}}_2$ raise acidity, most strongly at the ortho and para positions: 4-nitrophenol has a $\mathrm{p}{K}_a$ of about 7.1
- Electron-donating groups such as $-\mathrm{C}{\mathrm{H}}_3$ lower acidity: 4-methylphenol has a $\mathrm{p}{K}_a$ of about 10.2
- Picric acid, 2,4,6-trinitrophenol, has a $\mathrm{p}{K}_a$ of about 0.7 and is strong enough to release carbon dioxide from sodium hydrogencarbonate

Worked example. A $\mathrm{p}{K}_a$ difference of $10.0-7.1=2.9$ makes 4-nitrophenol about $10^{2.9}\approx 800$ times more acidic than phenol, while picric acid, with a difference of $10.0-0.7=9.3$, is about $2\times 10^9$ times more acidic.

An everyday example. Black phenyl disinfectant used to clean floors and drains in Indian homes contains cresols, methylphenols whose weakly acidic -OH groups help them kill germs.

The substance. A meta nitro group raises acidity much less than an ortho or para one — from the meta position it withdraws electrons only inductively, so 3-nitrophenol has a $\mathrm{p}{K}_a$ of about 8.3.

The -OH group donates electrons into the benzene ring, strongly activating phenol towards electrophilic substitution at the ortho and para positions, so it brominates and nitrates under mild conditions, and its phenoxide ion reacts even with carbon dioxide in Kolbe's reaction.

Why the ring is activated. A lone pair on oxygen is delocalised into the ring, raising electron density at the ortho and para carbons.

Halogenation:

- With bromine water, phenol instantly gives a white precipitate of 2,4,6-tribromophenol: ${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{OH}+3B{\mathrm{r}}_2\to {\mathrm{C}}_6{\mathrm{H}}_2\mathrm{B}{\mathrm{r}}_3\mathrm{OH}+3HBr$
- With bromine in a non-polar solvent such as $\mathrm{C}{\mathrm{S}}_2$ at low temperature, monobromophenols form, mainly 4-bromophenol
- No Lewis acid catalyst is needed, unlike for benzene

Nitration:

- Dilute nitric acid at 298 K gives a mixture of 2-nitrophenol and 4-nitrophenol
- Steam distillation separates them: 2-nitrophenol is steam volatile because its hydrogen bond is within the molecule, while 4-nitrophenol forms hydrogen bonds between molecules and stays behind
- Concentrated nitric acid gives picric acid

Kolbe's reaction. Sodium phenoxide heated with carbon dioxide at about 400 K under pressure, then acidified, gives salicylic acid, 2-hydroxybenzoic acid:

$${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{ONa}+\mathrm{C}{\mathrm{O}}_2\to \mathrm{HO}{\mathrm{C}}_6{\mathrm{H}}_4\mathrm{COONa}\stackrel{{\mathrm{H}}^{+}}{\to }\mathrm{HO}{\mathrm{C}}_6{\mathrm{H}}_4\mathrm{COOH}$$

The phenoxide ion is even more strongly activated than phenol, so it can attack a weak electrophile like carbon dioxide.

Other reactions. In the Reimer-Tiemann reaction, chloroform and sodium hydroxide add a -CHO group at the ortho position, giving salicylaldehyde; and neutral iron(III) chloride gives phenol a violet colour.

Worked example. Acetylating salicylic acid from Kolbe's reaction with ethanoic anhydride gives aspirin, acetylsalicylic acid.

An everyday example. Aspirin tablets and many acne face washes sold in India both trace back to salicylic acid made by Kolbe's reaction.

The substance. Phenol reacts with bromine water but benzene does not — the electron-donating -OH group makes the ring so reactive that no catalyst is needed and all three activated positions are brominated.