32.2 Phenol
- Syllabus
- 9701–2028–2029
- Topic
- 32.2
- Level
- A2
Learning objectives
32.2.1Preparation of phenol• Recall preparation of phenol: (a) reaction of phenylamine with HNO₂ or NaNO₂ and dil. acid below 10 °C to produce the diazonium salt; further warming of the diazonium salt with H₂O to give phenol32.2.2Reactions of phenol• Recall the chemistry of phenol, as exemplified by the following reactions:- (a) with bases, e.g. NaOH(aq) to produce sodium phenoxide- (b) with Na(s) to produce sodium phenoxide and H2(g)- (c) in NaOH(aq) with diazonium salts, to give azo compounds- (d) nitration of the aromatic ring with dil. HNO₃(aq) at room temp. to give a mixture of- 2-nitrophenol and 4-nitrophenol- (e) bromination of the aromatic ring with Br₂(aq) to form 2,4,6-tribromophenol32.2.3Acidity of phenol• Explain the acidity of phenol32.2.4Acidities of water, phenol and ethanol• Describe/explain: the relative acidities of water, phenol and ethanol32.2.5Phenol nitration and bromination conditions• Explain why the reagents/conditions for the nitration and bromination of phenol are different from those for benzene32.2.6Phenol directing effects• Recall that the hydroxyl group of a phenol directs to the 2-, 4- and 6-positions32.2.7Reactions of phenolic compounds• Apply knowledge of the reactions of phenol to those of other phenolic compounds, e.g. naphthol
Phenol can be made through a diazonium salt intermediate
Phenylamine is converted to a diazonium salt with nitrous acid (or sodium nitrite plus dilute acid) below 10 °C. Warming the diazonium salt with water replaces the diazonium group by -OH to give phenol.
The low temperature stabilises the diazonium salt during preparation; warming is used for the subsequent hydrolysis step. Treat the two stages as separate conditions.
C6H5NH2 -> diazonium salt below 10 °C, then aqueous warming -> C6H5OH. Nitrogen gas is released when the diazonium group leaves.
Do not warm the phenylamine/nitrite mixture during diazotisation or skip the diazonium intermediate in the syllabus route.
Phenol reacts at its acidic O-H group and its activated ring
Use the reagent to decide which feature of phenol reacts. A base or sodium removes the phenolic H; a diazonium ion, nitric acid or bromine substitutes on the electron-rich aromatic ring.
| Reagent and conditions | Product | Reaction evidence or key change |
|---|---|---|
| NaOH(aq) | sodium phenoxide + H2O | acid-base reaction: C6H5OH + NaOH -> C6H5ONa + H2O |
| Na(s), with phenol | sodium phenoxide + H2 | 2C6H5OH + 2Na -> 2C6H5ONa + H2; gas is evolved |
| diazonium salt added to phenol in NaOH(aq), kept cold | an azo compound | coupling forms an -N=N- link; commonly a yellow-orange product |
| dilute HNO3(aq), room temperature | mixture of 2-nitrophenol and 4-nitrophenol | one ring H is replaced by NO2 |
| Br2(aq), room temperature | 2,4,6-tribromophenol | bromine water is decolourised and a white precipitate forms |
The two O-H reactions form the same phenoxide ion, but only sodium metal releases H2. The three ring reactions reflect activation by -OH: azo coupling and nitration favour the available 2/4 positions, while aqueous bromine substitutes all three 2,4,6 positions under the stated conditions.
Do not substitute benzene conditions automatically: phenol uses dilute aqueous nitric acid and bromine water. Do not omit NaOH from azo coupling, because the alkaline phenoxide is the reacting form required here.
Phenol is acidic because phenoxide is resonance-stabilised
Phenol can donate H+ to form a phenoxide ion. The negative charge in phenoxide is delocalised into the aromatic ring, making the conjugate base more stable than an alkoxide.
Greater conjugate-base stability makes proton loss more favourable, but phenol remains a weak acid: its aqueous equilibrium is far from complete.
Phenol reacts with NaOH to form sodium phenoxide, but it does not release CO2 from aqueous sodium carbonate as a carboxylic acid does.
The oxygen-hydrogen bond is not acidic simply because oxygen is electronegative; the key comparison is the stability and delocalisation of the conjugate base.
Conjugate-base stability gives phenol > water > ethanol in acidity
A stronger acid forms a more stable, less proton-attracting conjugate base. Compare what happens to the negative charge after each O-H species loses H+.
| Acid | Conjugate base | What happens to negative charge | Consequence for acidity |
|---|---|---|---|
| phenol, C6H5OH | phenoxide, C6H5O- | delocalised from O into the aromatic pi system | conjugate base is most stabilised; phenol is strongest of the three |
| water, H2O | hydroxide, OH- | localised on O, with no alkyl group donating electron density | intermediate conjugate-base stability and acidity |
| ethanol, C2H5OH | ethoxide, C2H5O- | localised on O; the ethyl group donates electron density toward O | negative charge is least stabilised; ethanol is weakest of the three |
acid strength: phenol>water>ethanol
Phenol is acidic enough to react with NaOH(aq). Ethanol does not react appreciably with aqueous hydroxide because its conjugate base, ethoxide, is a stronger base and readily reforms ethanol.
Do not compare acidity by counting hydrogen atoms or by O-H bond polarity alone. The decisive comparison is the relative stability of phenoxide, hydroxide and ethoxide after proton loss.
The -OH group lets phenol use milder nitration and bromination conditions
One oxygen lone pair overlaps with the aromatic pi system and donates electron density into the phenol ring. The ring therefore attracts electrophiles more strongly than benzene and does not need the same strong electrophile-generating conditions.
| Reaction | Benzene | Phenol |
|---|---|---|
| nitration | concentrated HNO3 + concentrated H2SO4, 25-60 °C | dilute HNO3(aq), room temperature; gives 2- and 4-nitrophenol |
| bromination | Br2 with anhydrous AlBr3 catalyst | Br2(aq), room temperature, no halogen-carrier catalyst; gives 2,4,6-tribromophenol |
-OH lone-pair donation -> higher ring electron density -> easier electrophilic attack -> milder reagent/temperature or no Lewis-acid catalyst. The same donation also helps explain why multiple bromination occurs readily.
Phenol remains aromatic and the reaction remains electrophilic substitution. Activation changes how readily the ring reacts; it does not turn the reaction into electrophilic addition.
The hydroxyl group directs phenol substitution to the 2, 4 and 6 positions
In phenol, the -OH group is an activating, 2/4-directing substituent. Electrophiles therefore enter mainly at the two ortho positions (2 and 6) and the para position (4).
The two ortho positions are equivalent in an unsubstituted ring, so products are often described as 2-, 4- or 2,4,6-substituted phenols.
Bromination of phenol gives 2,4,6-tribromophenol because all three favoured positions are available and the ring is strongly activated.
The rule predicts preferred positions, not that every phenol reaction must give all three products; reagent amount and available ring hydrogens still matter.
Transfer phenol chemistry by checking the phenolic ring and its available sites
A phenolic compound contains -OH directly bonded to an aromatic ring. That structural feature usually carries two transferable ideas from phenol: loss of H+ can form a resonance-stabilised phenoxide-type ion, and -OH donates electron density into its ring to favour electrophilic substitution.
Apply the pattern in order: 1) locate the ring carbon bearing -OH; 2) identify which positions activated by -OH still bear a replaceable H; 3) account for fused rings or other substituents that occupy or influence those sites; 4) predict the acid-base or ring-substitution product without assuming phenol's exact product count.
In 1-naphthol, -OH is attached directly to one ring of the fused aromatic system. It can form a naphthoxide ion with base and its -OH group activates electrophilic substitution, with attack favoured at the available 2 and/or 4 positions in the syllabus example.
Transfer the reaction principle, not the drawing of phenol unchanged. A fused-ring carbon or an existing substituent may remove a ring H or alter which directed positions are available, so inspect the actual structure first.