Chemistry (9701) | Phenol

Phenol: The Aromatic Alcohol – Study Notes (9701 A Level Chemistry)

Welcome to the fascinating world of Phenol! Phenol, sometimes called hydroxybenzene, is an organic compound that looks like an alcohol (it has an -OH group) but behaves in a fundamentally different way because that -OH group is directly attached to a benzene ring. This aromatic influence changes everything!

Why is this important? Phenols are widely used in medicine (antiseptics), plastics, and dyes. Understanding their unique acidity and reactivity is crucial for A Level success and for further organic synthesis.

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1. Structure and Bonding of Phenol

1.1 What is Phenol?

Phenol has the formula \( \text{C}_6\text{H}_5\text{OH} \). It belongs to the homologous series of phenolic compounds.

• Structure: A hydroxyl group (\( \text{-OH} \)) is bonded directly to one of the carbon atoms in a benzene ring.
• Key Feature: The presence of the benzene ring means the bonding is highly influenced by the delocalised \(\pi\) electron system.

Skeletal Formula:

1.2 The Unexpected Acidity: Phenol vs. Alcohol

Phenol is weakly acidic, meaning it can donate a proton (\( \text{H}^+ \)). Crucially, phenol is a stronger acid than water or simple aliphatic alcohols (like ethanol).

The Chemistry Behind the Acidity:

When phenol loses a proton, it forms the phenoxide ion (or phenolate ion).

Phenol (\( \text{C}_6\text{H}_5\text{OH} \)) \(\rightleftharpoons\) Phenoxide ion (\( \text{C}_6\text{H}_5\text{O}^- \)) + \( \text{H}^+ \)

Why is the phenoxide ion stable? (The Resonance Effect)

1. The negative charge on the oxygen atom of the phenoxide ion can be shared (delocalised) into the delocalised \(\pi\) electron system of the benzene ring.
2. This delocalisation spreads out the charge, reducing the charge density on the oxygen atom.
3. A less concentrated charge means the ion is more stable.

Analogy: Imagine a hot potato. If you hold it in one spot (high charge density), it burns you. If you spread the heat across your whole hand (delocalisation), it feels much cooler (more stable).

Ethanol (\( \text{CH}_3\text{CH}_2\text{OH} \)) forms the ethoxide ion (\( \text{CH}_3\text{CH}_2\text{O}^- \)), which has no benzene ring to delocalise the charge. The charge remains concentrated on the oxygen, making the ethoxide ion highly unstable and reactive. This is why ethanol is a much weaker acid than phenol.

Key Takeaway: Phenol’s acidity comes from the resonance stabilisation of the phenoxide ion, a feature absent in simple alcohols.

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2. Preparation of Phenol (Syllabus 32.2.1)

The syllabus requires recalling the preparation route starting from phenylamine (\( \text{C}_6\text{H}_5\text{NH}_2 \)). This is a two-step sequence involving the formation of a highly reactive intermediate: the diazonium salt.

Step 1: Diazotisation – Forming the Diazonium Salt

Phenylamine reacts with nitrous acid (\( \text{HNO}_2 \)), which is generated in situ (in the reaction mixture) from sodium nitrite (\( \text{NaNO}_2 \)) and dilute acid (e.g., \( \text{HCl} \)).

• Reagents: \( \text{NaNO}_2 \) and dilute acid (e.g., \( \text{HNO}_3 \))
• Conditions: Temperature below \( 10^\circ\text{C} \). (This reaction is very temperature-sensitive; above \( 10^\circ\text{C} \), the diazonium salt decomposes rapidly.)
• Product: Benzenediazonium salt (\( \text{C}_6\text{H}_5\text{N}_2^+ \text{Cl}^- \) or \( \text{C}_6\text{H}_5\text{N}_2^+ \text{NO}_3^- \)).

Step 2: Hydrolysis – Forming Phenol

The diazonium salt is unstable. When it is heated (warmed) with water (\( \text{H}_2\text{O} \)), it hydrolyses.

• Conditions: Further warming of the diazonium salt with \( \text{H}_2\text{O} \).
• Product: Phenol is formed, along with nitrogen gas (\( \text{N}_2 \)) and acid.

Key Takeaway: Phenol synthesis via phenylamine involves controlling the temperature carefully during the diazonium salt formation.

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3. Chemical Reactions of Phenol

The reactions of phenol show both its acidic nature (reacting with bases) and its highly activated aromatic ring (reacting easily with electrophiles).

3.1 Reactions Showing Acidity (Syllabus 32.2.2 a & b)

Since phenol is acidic, it can react with strong bases and reactive metals, similar to carboxylic acids (but weaker than them).

1. Reaction with Sodium Hydroxide (Strong Base)

Phenol dissolves in aqueous sodium hydroxide to form a salt, sodium phenoxide (or sodium phenolate).

Reaction:
\( \text{C}_6\text{H}_5\text{OH}(\text{aq}) + \text{NaOH}(\text{aq}) \to \text{C}_6\text{H}_5\text{O}^-\text{Na}^+(\text{aq}) + \text{H}_2\text{O}(\text{l}) \)

Note: Phenol does NOT react with weak bases like sodium carbonate or sodium hydrogencarbonate, proving it is a weaker acid than carbonic acid, but stronger than water.

2. Reaction with Sodium Metal

Phenol reacts with solid sodium metal, displacing hydrogen gas, confirming the presence of an acidic hydrogen atom.

Reaction:
\( 2\text{C}_6\text{H}_5\text{OH}(\text{l}) + 2\text{Na}(\text{s}) \to 2\text{C}_6\text{H}_5\text{O}^-\text{Na}^+(\text{s}) + \text{H}_2(\text{g}) \)

3.2 Electrophilic Substitution (Enhanced Reactivity) (Syllabus 32.2.2 d & e)

The hydroxyl group (\( \text{-OH} \)) is a powerful activating group. It donates its lone pair of electrons into the benzene ring by resonance, significantly increasing the electron density. This makes the ring much more susceptible to attack by electrophiles.

Key Fact: Compared to benzene, phenol requires much milder conditions for electrophilic substitution.

Directing Effect: The -OH group directs incoming electrophiles to the 2- (ortho), 4- (para), and 6- (ortho) positions (Syllabus 32.2.6).

1. Nitration

Benzene requires concentrated \( \text{HNO}_3 \) and concentrated \( \text{H}_2\text{SO}_4 \) catalyst at \( 50^\circ\text{C} \).

Phenol requires only dilute \( \text{HNO}_3(\text{aq}) \) at room temperature.

• Reagents: Dilute \( \text{HNO}_3(\text{aq}) \)
• Conditions: Room temperature.
• Products: A mixture of 2-nitrophenol and 4-nitrophenol.

2. Bromination

Benzene requires liquid \( \text{Br}_2 \) and a catalyst (\( \text{FeBr}_3 \)) and heat.

Phenol reacts immediately with aqueous bromine (\( \text{Br}_2(\text{aq}) \)) even without a catalyst, leading to multiple substitutions.

• Reagents: Aqueous Bromine (\( \text{Br}_2(\text{aq}) \))
• Conditions: Room temperature.
• Product: A white precipitate of 2,4,6-tribromophenol is formed. This reaction is also used as a characteristic test for phenol.

Why the Milder Conditions? (Syllabus 32.2.5)
The lone pair on the oxygen of the -OH group is delocalised into the ring, greatly increasing the electron density. This makes the benzene ring a much better nucleophile and significantly lowers the activation energy for electrophilic attack.

3.3 Azo Coupling (Diazonium Salts) (Syllabus 32.2.2 c)

Phenol reacts with the benzenediazonium ion (\( \text{C}_6\text{H}_5\text{N}_2^+ \)) to form an azo compound. This reaction only works in alkaline conditions (usually in \( \text{NaOH}(\text{aq}) \)) because phenol must first be converted into the more reactive phenoxide ion.

• Reagents: Diazonium salt (e.g., benzenediazonium chloride) and \( \text{NaOH}(\text{aq}) \).
• Conditions: Below \( 10^\circ\text{C} \) (to maintain the stability of the diazonium salt).
• Product: An azo dye (often a vibrant yellow, orange, or red solid).
• The structure of the dye contains the azo group: \( \mathbf{-N=N-} \).

Did you know? Azo coupling is the standard industrial method for producing vibrant synthetic dyes. The different colours result from the extensive delocalised electron systems present in these molecules.

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4. Relative Acidity Explained (Syllabus 32.2.4)

We must compare the stability of the conjugate bases formed when ethanol, water, and phenol lose a proton.

The stability of the conjugate base determines the strength of the acid: More stable conjugate base \(\to\) Stronger acid.

4.1 Ethanol (Weakest Acid)

• Acid: Ethanol (\( \text{CH}_3\text{CH}_2\text{OH} \))
• Conjugate Base: Ethoxide ion (\( \text{CH}_3\text{CH}_2\text{O}^- \))
• Problem: The ethyl group (\( \text{CH}_3\text{CH}_2- \)) is an electron-donating group. It pushes electron density towards the already negative oxygen atom, intensifying the negative charge. This makes the ethoxide ion highly unstable.
• Conclusion: Ethanol is a very weak acid.

4.2 Water (Intermediate Acid)

• Acid: Water (\( \text{H}_2\text{O} \))
• Conjugate Base: Hydroxide ion (\( \text{OH}^- \))
• Stability: The negative charge is localised on the oxygen atom, but there are no groups pushing extra electron density onto it.

4.3 Phenol (Strongest Acid of the three)

• Acid: Phenol (\( \text{C}_6\text{H}_5\text{OH} \))
• Conjugate Base: Phenoxide ion (\( \text{C}_6\text{H}_5\text{O}^- \))
• Stabilisation: The negative charge on oxygen is delocalised into the benzene ring (resonance stabilisation). This spreading of charge makes the phenoxide ion significantly more stable than the hydroxide or ethoxide ion.
• Conclusion: Phenol is the strongest acid among the three, reacting with NaOH (whereas alcohols do not).

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5. Application of Phenol Chemistry

The knowledge of phenol's reactivity applies to other compounds containing the hydroxyl group attached directly to an aromatic ring, such as naphthol (Syllabus 32.2.7).

5.1 Common Mistakes to Avoid

1. Confusing Phenol and Alcohols: Do not treat phenol like a simple alcohol. Phenol reacts with \( \text{NaOH}(\text{aq}) \); ethanol does not.
2. Nitration Conditions: Do not use concentrated \( \text{H}_2\text{SO}_4 \) catalyst for phenol nitration. Dilute \( \text{HNO}_3 \) is sufficient due to the high reactivity of the ring.
3. Bromination Products: Bromination of phenol is so vigorous that it results in trisubstitution (2,4,6-tribromophenol), whereas benzene usually results in monosubstitution.

Key Takeaway: Phenol’s chemistry is dominated by the interaction between the hydroxyl group and the delocalised ring system, leading to unique acidic and substitution properties.