Phenol: Preparation & Acidity

CHEPhenols

This subtopic covers how phenol is manufactured (cumene and Dow processes) and the reasoning behind its acidity relative to alcohols and carbonic acid. The acidity of phenol — and how substituents shift it — is one of the most heavily examined ideas in the chapter.

Cumene process

C_6H_5CH(CH_3)_2 \xrightarrow{O_2} C_6H_5C(CH_3)_2OOH \xrightarrow{\text{dil. } H_2SO_4} C_6H_5OH + (CH_3)_2CO

Dow's process

C_6H_5Cl + NaOH \xrightarrow[623\ K,\ 200\ atm]{} C_6H_5ONa \xrightarrow{H^+} C_6H_5OH

From benzenesulphonic acid

C_6H_5SO_3H + NaOH\,(\text{fused}) \rightarrow C_6H_5ONa \xrightarrow{HCl} C_6H_5OH

From diazonium salt

C_6H_5N_2^+Cl^- + H_2O \xrightarrow{\Delta} C_6H_5OH + N_2\uparrow + HCl

Acid ionisation of phenol

C_6H_5OH \rightleftharpoons C_6H_5O^- + H^+ \qquad pK_a \approx 10

Acidity order with substituents

p\text{-nitrophenol} > \text{phenol} > p\text{-cresol} > \text{ethanol}

Cumene process: cumene $\xrightarrow{O_2}$ cumene hydroperoxide $\xrightarrow{dil.\ H_2SO_4}$ phenol + acetone; preferred industrially because it is cheap and gives a useful by-product (acetone).
Phenol is more acidic than ethanol because the phenoxide ion $C_6H_5O^-$ is resonance-stabilised, delocalising the negative charge into the ring; alkoxide ions have no such resonance.
Phenol ( $pK_a \approx 10$ ) is less acidic than carbonic acid, so it dissolves in NaOH but not in $NaHCO_3$ — a standard distinguishing observation.
Electron-withdrawing groups ( $\text{-}NO_2$ ) increase acidity; electron-donating groups ( $\text{-}CH_3$ ) decrease it: p-nitrophenol > phenol > p-cresol (p-methylphenol).
The $\text{-}NO_2$ group raises acidity most strongly at the o- and p- positions, where it stabilises the phenoxide by resonance as well as induction.
o-Nitrophenol shows intramolecular H-bonding (chelation), so it is steam-volatile and boils lower; p-nitrophenol shows intermolecular H-bonding, giving a higher boiling point.
Dow process: chlorobenzene + NaOH at high temperature and pressure $\rightarrow$ sodium phenoxide $\rightarrow$ phenol on acidification; harsh conditions are needed because the aryl C–Cl bond is strong.
Other lab preparations: from benzene sulphonic acid (fuse sodium benzenesulphonate with NaOH), and from aniline via the diazonium salt ( $C_6H_5N_2^+$ warmed with water gives phenol $+ N_2$ ).
In phenol itself, the oxygen lone pair is delocalised into the ring (partial $C\text{-}O$ double-bond character), which weakens the $O\text{-}H$ bond and makes it more acidic than alcohols.
Resonance contributes most to acidity: the negative charge in phenoxide is shared by the ortho and para ring carbons, which is why o-/p-electron-withdrawing groups boost acidity far more than meta ones.
2,4,6-trinitrophenol (picric acid) is a very strong phenol-type acid (comparable to mineral acids) because three $\text{-}NO_2$ groups powerfully stabilise the anion.
Acidity comparison to remember: carboxylic acid > carbonic acid > phenol > water > alcohol — it explains the NaOH-yes / $NaHCO_3$ -no behaviour of phenol.

Where the marks go

Explaining phenol's acidity by induction only — the dominant reason is resonance stabilisation of the phenoxide ion; alkoxides lack this.
Claiming phenol reacts with $NaHCO_3$ — it is weaker than carbonic acid, so it dissolves in NaOH but gives no $CO_2$ with $NaHCO_3$ .
Putting a meta-nitro substituent on a par with ortho/para — only o-/p- $NO_2$ can stabilise phenoxide by resonance, so m-nitrophenol is less acidic than the o-/p- isomers.
Reversing the o-/p-nitrophenol volatility — the ortho isomer is steam-volatile (intramolecular H-bond), the para isomer is not (intermolecular H-bond, higher b.p.).
Using conc. $H_2SO_4$ for the final cumene step — the cleavage of cumene hydroperoxide uses DILUTE acid, and acetone is the essential by-product to mention.

How the board asks it

Give reasonsresonance stabilisation of phenoxide ion
Account for the fact that phenol is more acidic than ethanol but less acidic than carboxylic acids.
Distinguish $NaOH$ -soluble / $NaHCO_3$ -insoluble behaviour
How will you distinguish between phenol and benzoic acid by a simple chemical test?
Give reasonssubstituent effects on acidity
Arrange the following in increasing order of acidic strength, giving a reason: $p$ -cresol, phenol and $p$ -nitrophenol.
Conversioncumene process
How is phenol manufactured by the cumene process? Give the equations involved.
Give reasonsintramolecular vs intermolecular H-bonding
Give a reason: $o$ -nitrophenol is steam-volatile and has a lower boiling point than $p$ -nitrophenol.
Predict the producthydrolysis of benzene diazonium chloride
Name the product and write the equation formed when benzene diazonium chloride is warmed with water.

Practise this topic

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Written for Sublevo. Question text quoted anywhere in these notes is the Council’s and carries its year and paper; the board’s own diagrams are not reproduced.