The Chemistry of Phenol (OCR A-Level Chemistry A): Revision Notes

The Chemistry of Phenol

What are phenols?

Phenols are organic compounds where a hydroxyl group (-OH) is bonded directly to a benzene ring. This direct attachment to the aromatic ring makes phenols fundamentally different from alcohols. The simplest member of this family is phenol itself, with the molecular formula $\ce{C6H5OH}$.

For example, 2-phenylethanol ($\ce{C6H5CH2CH2OH}$) is an aromatic alcohol because the -OH group is on a carbon side chain rather than directly on the ring.

This structural distinction is crucial because phenols and alcohols show different chemical properties. The proximity of the -OH group to the delocalised π-system of the benzene ring significantly influences the behaviour of phenols, particularly their acidity and reactivity in substitution reactions.

Phenols have important practical applications. They are used in manufacturing disinfectants, detergents, plastics, paints, and even medicines like aspirin. Many commercial antiseptics contain chlorinated phenol derivatives as their active ingredients.

The manufacture of phenol

Phenol is an important industrial chemical, and several methods have been developed to produce it from benzene.

Historical method

Historically, phenol was manufactured from benzene through a multi-stage process involving sulfonation. The benzene was first treated with sulfuric acid, then with sodium hydroxide. The overall equation for this process is:

$\ce{C6H6 + H2SO4 + 2NaOH -> C6H5OH + Na2SO3 + 2H2O}$

This method is now considered obsolete due to the development of more efficient routes.

Current industrial method

Today, the majority of phenol is manufactured using the cumene process (also called the cumene-to-phenol process). This involves reacting benzene with propene in the presence of oxygen:

$\ce{C6H6 + C3H6 + O2 -> C6H5OH + CH3COCH3}$

Possible future method

Research chemists are investigating an alternative route using benzene and nitrogen(I) oxide ($\ce{N2O}$, also called nitrous oxide):

$\ce{C6H6 + N2O -> C6H5OH + N2}$

Phenol as a weak acid

Phenol is less soluble in water than typical alcohols due to the presence of the non-polar benzene ring. However, when phenol does dissolve in water, something interesting happens - it partially dissociates to release protons ($\ce{H+}$ ions).

This partial dissociation can be represented by an equilibrium:

$\ce{C6H5OH <=> C6H5O- + H+}$

In this equilibrium, phenol splits into a phenoxide ion ($\ce{C6H5O-}$) and a proton. Because phenol can release protons in solution (even though only partially), it is classified as a weak acid. The term "weak" indicates that only a small proportion of phenol molecules dissociate at any given time - most remain as neutral $\ce{C6H5OH}$ molecules.

Comparing acid strengths

We can compare the acidity of different compounds by looking at their acid dissociation constants ($K_\text{a}$). The larger the $K_\text{a}$ value, the stronger the acid.

Compound	$K_\text{a}$ at 298 K / mol dm⁻³	Acid strength
Ethanol	$1.00 \times 10^{-16}$	Weakest
Phenol	$1.26 \times 10^{-11}$	Intermediate
Ethanoic acid	$1.00 \times 10^{-5}$	Strongest

Reactions with bases

The intermediate acidity of phenol determines which bases it will react with:

• Ethanol does not react with sodium hydroxide (a strong base) or sodium carbonate (a weak base). It is too weak an acid to donate protons even to strong bases.

• Phenol reacts with strong bases like sodium hydroxide and aqueous sodium hydroxide, but does not react with the weak base sodium carbonate.

• Carboxylic acids are strong enough acids to react with both strong bases (like sodium hydroxide) and weak bases (like sodium carbonate).

Reaction with sodium hydroxide

When phenol reacts with sodium hydroxide solution, a neutralisation reaction occurs. This is an acid-base reaction where phenol acts as the acid and sodium hydroxide as the base:

$\ce{C6H5OH + NaOH -> C6H5O-Na+ + H2O}$

The products formed are sodium phenoxide (which exists as sodium ions $\ce{Na+}$ and phenoxide ions $\ce{C6H5O-}$ in solution) and water. This reaction demonstrates that phenol behaves as an acid by donating a proton to the hydroxide ion from sodium hydroxide.

The formation of the phenoxide salt is complete when excess sodium hydroxide is present, as the strong base drives the equilibrium towards products.

Electrophilic substitution reactions of phenol

Phenols, like benzene, contain an aromatic ring and can undergo electrophilic substitution reactions. However, phenol is much more reactive than benzene towards electrophiles. The reactions occur under milder conditions and proceed more readily.

Why is phenol more reactive than benzene?

The enhanced electron density in phenol is sufficient to allow reactions to occur that would require much harsher conditions with benzene, or would not occur at all.

Bromination of phenol

When bromine water (an aqueous solution of bromine) is added to phenol, an immediate reaction occurs at room temperature. No halogen carrier catalyst is required, unlike the bromination of benzene which needs a catalyst like iron(III) bromide.

The reaction produces 2,4,6-tribromophenol as a white precipitate:

$\ce{C6H5OH + 3Br2 -> C6H2Br3OH + 3HBr}$

The product contains three bromine atoms substituted at positions 2, 4, and 6 on the benzene ring (these are the ortho and para positions relative to the -OH group). The formation of the tri-substituted product demonstrates just how reactive phenol is compared to benzene, where only mono-substitution would typically occur even with a catalyst.

Nitration of phenol

Phenol reacts readily with dilute nitric acid at room temperature. This is in stark contrast to benzene, which requires concentrated nitric acid and concentrated sulfuric acid at elevated temperatures.

$\ce{C6H5OH + HNO3 -> C6H4(OH)(NO2) + H2O}$

The reaction produces a mixture of two isomers:

• 2-nitrophenol (ortho-nitrophenol) - where the $\ce{NO2}$ group is at position 2
• 4-nitrophenol (para-nitrophenol) - where the $\ce{NO2}$ group is at position 4

Comparing reactivity with benzene

Both bromination and nitration proceed much more readily with phenol than with benzene:

Feature	Benzene	Phenol
Bromination catalyst	Iron(III) bromide required	No catalyst needed
Bromination product	Bromobenzene (mono-substituted)	2,4,6-tribromophenol (tri-substituted)
Nitration conditions	Concentrated $\ce{HNO3}$ + concentrated $\ce{H2SO4}$, heat	Dilute $\ce{HNO3}$, room temperature
Electron density in ring	Lower	Higher (due to electron donation from oxygen)

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