Hydroxy compounds

32.1

Alcohols with acyl chlorides

Syllabus

describe the reaction with acyl chlorides to form esters using ethyl ethanoate

Source: Cambridge International syllabus

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 $\text{HCl}$.

Ethanol reacting with ethanoyl chloride to give the ester ethyl ethanoate and hydrogen chloride An acyl chloride reacts with an alcohol faster and more completely than a carboxylic acid does, giving the ester and HCl fumes

Explore

Acyl chloride with alcohol route

Follow nucleophilic acyl substitution to an ester.

Vocabulary Train

English	Chinese	Pinyin
alcohol	醇	chún
acyl chloride	酰氯	xiān lǜ
ester	酯	zhǐ

Exercise sheet

32.2

Phenol

Syllabus

recall the reactions (reagents and conditions) by which phenol can be produced: (a) reaction of phenylamine with $\text{HNO}_2$ or $\text{NaNO}_2$ and dilute acid below $10\text{ }^\circ\text{C}$ to produce the diazonium salt; further warming of the diazonium salt with $\text{H}_2\text{O}$ to give phenol
recall the chemistry of phenol, as exemplified by the following reactions: (a) with bases, for example $\text{NaOH(aq)}$ to produce sodium phenoxide (b) with $\text{Na(s)}$ to produce sodium phenoxide and $\text{H}_2\text{(g)}$ (c) in $\text{NaOH(aq)}$ with diazonium salts, to give azo compounds (d) nitration of the aromatic ring with dilute $\text{HNO}_3\text{(aq)}$ at room temperature to give a mixture of 2-nitrophenol and 4-nitrophenol (e) bromination of the aromatic ring with $\text{Br}_2\text{(aq)}$ to form 2,4,6-tribromophenol
explain the acidity of phenol
describe and explain the relative acidities of water, phenol and ethanol
explain why the reagents and conditions for the nitration and bromination of phenol are different from those for benzene
recall that the hydroxyl group of a phenol directs to the 2-, 4- and 6-positions
apply knowledge of the reactions of phenol to those of other phenolic compounds, e.g. naphthol

Source: Cambridge International syllabus

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.

A sample of phenol that has turned pink Phenol is a low-melting solid; it slowly turns pink as it oxidises in air

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:

$$\text{ethanol} < \text{water} < \text{phenol}$$

Phenol losing a proton to a phenoxide ion, with a curly arrow showing the negative charge spreading into the ring Phenol is more acidic than water or an alcohol: losing H$^+$ gives a phenoxide ion whose negative charge spreads into the ring, stabilising it

(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.

Phenol reacting with bromine water at room temperature to give 2,4,6-tribromophenol, a white precipitate Phenol's ring is activated, so bromine water brominates positions 2, 4 and 6 at room temperature with no catalyst, giving a white precipitate

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.

A pink bar of carbolic soap Carbolic soap contains phenol, which was one of the first antiseptics

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.