Acidity of alcohols and phenols

A drop of sodium in ethanol

Drop a small piece of sodium into dry ethanol. It fizzes slowly and a gas bubbles off. The gas is hydrogen. Water does the same, only faster.

The OH group gave up its hydrogen to the metal, so ethanol acts as an acid. The salt is sodium ethoxide, and the anion $\text{C}_{2}\text{H}_{5}\text{O}^{-}$ is an alkoxide. Aluminium does a similar thing with propan-2-ol and tert-butyl alcohol: $6 \text{ROH} + 2 \text{Al} \to 2 \text{Al}(\text{OR})_{3} + 3 \text{H}_{2}$.

How strong an acid is ethanol? Chemists use the pKa: the smaller the pKa, the stronger the acid. Ethanol has a pKa of 15.9. Water has 15.7. So ethanol is slightly weaker than water, and sodium ethoxide with water gives back ethanol and sodium hydroxide. Among alcohols the order of acidity is methanol, then primary, then secondary, then tertiary. The pKa of methanol is 15.5, of propan-2-ol about 17 and of tert-butyl alcohol about 18. The alkyl groups push electrons towards oxygen and crowd the anion, so they make it harder for the anion to be solvated by water.

Phenol is a much stronger acid

Phenol has a pKa of 10.0. Its OH group is on a benzene ring. This one change makes it about $10^6$ times more acidic than ethanol, so phenol dissolves in aqueous sodium hydroxide.

Ethanol does not dissolve in sodium hydroxide in this way. Why is phenol so different? Two reasons help the phenoxide ion $\text{C}_{6}\text{H}_{5}\text{O}^{-}$.

• Resonance. The negative charge on the oxygen can move into the ring, to the two ortho carbons and the para carbon. The charge is shared over four atoms. A shared charge is more stable.
• The sp$^2$ carbon. The ring carbon holding the oxygen is $sp^2$. It attracts electrons a little more than an $sp^3$ carbon does. Nothing like this helps the ethoxide ion.

Phenol has resonance in its own structure too, but those forms put a positive charge on oxygen, which is not stable. So the anion gains more than the neutral molecule does.

Phenol, carboxylic acids and carbonic acid

Phenol is a weak acid. Ethanoic acid has a pKa of 4.76, so it is far stronger. A good test uses sodium hydrogencarbonate. The conjugate acid of hydrogencarbonate is carbonic acid, with a pKa of 6.35. A carboxylic acid is stronger than carbonic acid and fizzes carbon dioxide out of sodium hydrogencarbonate. Phenol is weaker and does not.

• Sodium metal: an alcohol, a phenol and a carboxylic acid all give hydrogen.
• Sodium hydroxide: an alcohol does not dissolve. A phenol and a carboxylic acid dissolve.
• Sodium hydrogencarbonate: only a carboxylic acid gives carbon dioxide. An alcohol and a phenol give nothing.

What substituents do to phenol

A group on the ring changes the pKa. An electron-withdrawing group stabilises the phenoxide ion and makes the phenol a stronger acid. An electron-donating group does the opposite.

Read the table in three steps.

• The nitro group at the ortho or para position pulls charge out of the ring by resonance as well as by induction. The negative charge can move onto the oxygen atoms of the nitro group.
• At the meta position it cannot take part in the resonance, so it acts by induction only. The effect is smaller, which is why m-nitrophenol is weaker than the other two.
• Three nitro groups make picric acid about as strong as many mineral acids. The methyl group pushes electrons into the ring, so the cresols are a little weaker than phenol.

Different books round these pKa values a little differently, in the first decimal place. The order is the same in every book.

Set pKa to 10.0 and move the pH. At pH 14, nearly all of the phenol is the anion. At pH 8.3, the pH of sodium hydrogencarbonate, only about 2 percent is, so no clear reaction is seen.

Practise with new numbers

Try it in Python

The program gives the equilibrium constant for each acid with hydroxide and with hydrogencarbonate. A large constant means the acid reacts with the base. It uses $K = 10^{\,pK_a(\text{conjugate acid of the base}) - pK_a(\text{acid})}$.

Common mistakes

• Thinking a larger pKa is a stronger acid. The smaller pKa is the stronger acid.
• Saying phenol is acidic because its OH is more polar. The main reason is the stability of the phenoxide ion.
• Putting m-nitrophenol with the ortho and para isomers. The meta nitro group cannot join the resonance.
• Expecting ethanol to dissolve in sodium hydroxide like phenol. It is a weaker acid than water.
• Using sodium hydrogencarbonate to detect phenol. Phenol gives no gas with it. A carboxylic acid does.

Remember these

• Acidity order for OH compounds: carboxylic acid, then phenol, then water, then alcohols (primary, secondary, tertiary).
• Phenoxide is stabilised by resonance. An electron-withdrawing group at ortho or para raises the acidity most.
• Electron-donating groups (methyl) lower it.
• Phenol dissolves in sodium hydroxide, not in sodium hydrogencarbonate.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Arrange ethanol, phenol and water in order of increasing acid strength.

2. Why is p-nitrophenol more acidic than phenol?

3. Why is m-nitrophenol less acidic than p-nitrophenol?

4. A student adds sodium hydrogencarbonate to two colourless liquids. One fizzes and the other does not. One is phenol and the other is ethanoic acid. Which fizzes?

References

• NCERT Chemistry Part II (Class 12), Chapter 7, the sections on acidity of alcohols and phenols.
• Lide (ed.), CRC Handbook of Chemistry and Physics, the table of dissociation constants of organic acids (the pKa values).
• Morrison and Boyd, Organic Chemistry, the chapter on phenols.