Hydroxy compounds · ⁨ヒドロキシ化合物⁩

At A Level you meet one more reaction of an alcohol 醇: it reacts with an acyl chloride 酰氯 to make an ester 酯. This works faster and more completely than using a carboxylic acid. For example, ethanol and ethanoyl chloride give ethyl ethanoate, plus fumes of $\text{HCl}$.

Phenol 苯酚 has an $\text{–OH}$ group joined directly to a benzene 苯 ring. This changes the chemistry of both the $\text{–OH}$ group and the ring.

Making phenol

Cool phenylamine 苯胺 with $\text{NaNO}_2$ and dilute acid below $10\,°\text{C}$ to make a diazonium salt 重氮盐. Warming this salt with water then gives phenol (and nitrogen gas).

Reactions of phenol

• with a base such as $\text{NaOH(aq)}$: phenol reacts to give sodium phenoxide and water. (Ordinary alcohols do not react with $\text{NaOH}$ — this shows phenol is more acidic.)
• with sodium metal: gives sodium phenoxide and hydrogen.
• with a diazonium salt in $\text{NaOH(aq)}$: forms a coloured azo compound 偶氮化合物 (used in dyes).
• nitration 硝化 with dilute $\text{HNO}_3$ at room temperature: gives a mixture of 2-nitrophenol and 4-nitrophenol.
• bromination 溴化 with bromine water at room temperature (no catalyst): gives a white precipitate of 2,4,6-tribromophenol.

Acidity of phenol

Phenol loses its $\text{H}^+$ to form a phenoxide ion. This ion is stabilised because its negative charge is spread (delocalised) into the ring. So phenol's acidity 酸性 is higher than that of water or ethanol:

(Phenol is still a weak acid — weaker than a carboxylic acid.)

Why the conditions are milder than for benzene

A lone pair on the phenol oxygen is partly delocalised 离域 into the ring. This makes the ring more electron-rich, so it attracts electrophiles more strongly. Phenol therefore reacts faster and under much milder conditions than benzene — no catalyst is needed, and dilute reagents work at room temperature.

The $\text{–OH}$ group directs new substituents to the 2-, 4- and 6-positions. The same ideas apply to other phenolic compounds, such as naphthol.

Worked example. Phenol, ethanol and ethanoic acid are each shaken with aqueous $\text{NaOH}$ and then with aqueous $\text{Na}_2\text{CO}_3$. Predict what happens, and order the three by acidity. Acidity depends on how well the anion left behind is stabilised. Ethanol's ethoxide keeps its charge stuck on one oxygen, so ethanol is weakest and reacts with neither. Phenol's phenoxide spreads the charge into the ring, making phenol acidic enough to react with the strong base $\text{NaOH}$ but not with the weaker $\text{Na}_2\text{CO}_3$ - so no fizzing. Ethanoate spreads the charge over two oxygens, the most effective of the three, so ethanoic acid reacts with both and fizzes with the carbonate. Order: ethanol < phenol < ethanoic acid. The carbonate is the discriminating test - only a carboxylic acid fizzes, which is exactly how you tell phenol from an acid.

• Phenol is a stronger acid than alcohols (its anion is stabilised by delocalisation) but weaker than carboxylic acids — it does not react with carbonates.
• Phenol decolourises bromine water and gives a white precipitate without a catalyst (the ring is activated by oxygen).
• Acyl chlorides react with alcohols/phenols to give esters readily (better than the reversible acid route); misty HCl fumes are the observation.