Give two reactions that show the acidic nature of phenol. Compare acidity of phenol with that of ethanol.

Exercises · 7.14

Q. Give two reactions that show the acidic nature of phenol. Compare acidity of phenol with that of ethanol.

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Concept understanding — Acidity Of Phenol

Inductive Effect on Acidity – From Intuition to Precision

Imagine you are holding a rope tied to a heavy box. If you pull the rope, the box moves toward you. Now imagine the rope is made of rubber bands — the pull still reaches the box, but it gets weaker the farther away you are. That is exactly how the inductive effect works inside a molecule.

The Core Intuition

An acid donates a proton ( $H^+$ ). After it does, the remaining part (the conjugate base) carries a negative charge. The stability of that negative charge determines how willing the molecule is to give up the proton. More stable conjugate base → stronger acid.

Now, some atoms or groups are electron-withdrawing — they pull electron density toward themselves through the sigma bonds. If such a group is attached near the acidic proton, it pulls some electron density away from the negative charge on the conjugate base. That spreads out (delocalises) the negative charge, making the conjugate base more stable. The acid becomes stronger.

Conversely, electron-donating groups push electron density toward the negative charge, concentrating it and making the conjugate base less stable. The acid becomes weaker.

Note

The inductive effect operates through sigma bonds only. It does not involve pi bonds or resonance. It is a permanent, through-bond polarisation.

The Precise Statement

Inductive effect on acidity: The acidity of a compound increases with the presence of electron-withdrawing groups (EWGs) near the acidic site, and decreases with electron-donating groups (EDGs). The effect is strongest when the group is closest to the acidic proton, and diminishes rapidly with distance.

Mathematically, for a series of substituted carboxylic acids:

\text{R-COOH} \quad \text{where R = substituent}

The acid dissociation constant $K_a$ changes as:

If R is electron-withdrawing (e.g., $-\text{Cl}$ , $-\text{NO}_2$ , $-\text{CF}_3$ ): $K_a$ increases → stronger acid.
If R is electron-donating (e.g., $-\text{CH}_3$ , $-\text{C}_2\text{H}_5$ ): $K_a$ decreases → weaker acid.

Why Distance Matters

The inductive effect falls off with distance because sigma bonds are localised. Each bond attenuates the effect by roughly a factor of 2–3. For example, compare:

Compound	$pK_a$	Explanation
$\text{CH}_3\text{COOH}$	4.76	Reference (no EWG)
$\text{ClCH}_2\text{COOH}$	2.86	Cl withdraws through one bond
$\text{Cl}_2\text{CHCOOH}$	1.29	Two Cl atoms, stronger withdrawal
$\text{Cl}_3\text{CCOOH}$	0.65	Three Cl atoms, strongest withdrawal
$\text{CH}_3\text{CH}_2\text{COOH}$	4.87	Ethyl group is electron-donating (slightly weaker acid)

Notice: $\text{ClCH}_2\text{COOH}$ is about 100 times stronger than acetic acid ( $\Delta pK_a \approx 1.9$ ). But if the Cl is moved further away:

Compound	$pK_a$
$\text{ClCH}_2\text{CH}_2\text{COOH}$	4.08
$\text{ClCH}_2\text{CH}_2\text{CH}_2\text{COOH}$	4.52

The effect fades as the chlorine moves farther from the carboxyl group. …

Why this formula?

Acidity of Phenol: Why It's More Acidic Than Alcohols

Let's build this from first principles — understanding why phenol is acidic is the key to mastering organic chemistry.

1. The Core Observation

Phenol ( $\ce{C6H5OH}$ ) has a pKa ≈ 10, while ethanol ( $\ce{CH3CH2OH}$ ) has a pKa ≈ 16.

This means phenol is about 1 million times more acidic than a typical alcohol.

The question: Why does the O–H bond in phenol break so much more easily?

2. The Key: Stability of the Conjugate Base

Acidity is determined by the stability of the conjugate base after losing $\ce{H+}$ .

Alcohol conjugate base: $\ce{CH3CH2O-}$ (alkoxide ion) — negative charge is localized on oxygen.
Phenol conjugate base: $\ce{C6H5O-}$ (phenoxide ion) — negative charge is delocalized into the benzene ring.

The Resonance Explanation

The phenoxide ion has multiple resonance structures:

\ce{C6H5O-} \leftrightarrow \text{(several resonance forms where negative charge moves to ortho/para carbons)}

Draw the structures mentally:

One structure has the negative charge on oxygen.
Other structures show the negative charge on carbon atoms at the ortho and para positions of the ring.

This delocalization spreads the negative charge over more atoms, making the ion more stable.

Key principle: The more stable the conjugate base, the stronger the acid.

3. Why Alcohols Can't Do This

In an alkoxide ion ( $\ce{RO-}$ ), the negative charge is stuck on oxygen.

There are no empty p-orbitals or conjugated $\pi$ systems nearby to accept the charge.

Result: The alkoxide is less stable, so the alcohol is less acidic.

4. The Inductive Effect Also Helps (But Resonance Dominates)

The benzene ring is slightly electron-withdrawing (due to its $sp^2$ carbons being more electronegative than $sp^3$ ).

This inductive effect pulls electron density away from the O–H bond, making the proton slightly more positive and easier to remove.

However, resonance stabilization of the conjugate base is the dominant factor — inductive effects alone cannot explain the million-fold difference.

5. The Quantitative Picture (pKa Values)

Compound	pKa	Conjugate base stability
Ethanol	~16	Localized charge on O
Phenol	~10	Delocalized charge via resonance
Acetic acid	~4.76	Even more resonance (two O atoms)

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Phenol shows acidic nature through reactions with active metals (like Na) and strong bases (like NaOH), forming sodium phenoxide. Phenol is far more acidic than ethanol because the phenoxide ion is stabilised by resonance delocalisation of the negative charge into the benzene ring, whereas the ethoxide ion has no such stabilisation.

Why Phenol Is Acidic — The Core Idea

Acidity in organic compounds comes down to one thing: how stable the conjugate base is after losing a proton ( $H^+$ ). The more stable the anion, the stronger the acid.

Phenol ( $C_6H_5OH$ ) has a hydroxyl group directly attached to a benzene ring. When it loses $H^+$ , it forms the phenoxide ion ( $C_6H_5O^-$ ). This negative charge is not stuck on the oxygen — it gets delocalised into the aromatic ring through resonance. That spreading out of charge makes the phenoxide ion much more stable than a simple alkoxide ion like ethoxide ( $C_2H_5O^-$ ), where the negative charge is localised on oxygen.

This resonance stabilisation is the single most important reason phenol is acidic. Ethanol, by contrast, has no such mechanism — its conjugate base is a high-energy, charge-localised anion.

C_6H_5OH \rightleftharpoons C_6H_5O^- + H^+

Two Reactions That Show Phenol Is Acidic

1. Reaction with Sodium Metal

Phenol reacts with metallic sodium to liberate hydrogen gas — exactly the same behaviour as a typical acid.

2 C_6H_5OH + 2 Na \rightarrow 2 C_6H_5ONa + H_2 \uparrow

The sodium phenoxide ( $C_6H_5ONa$ ) formed is a salt. This reaction is analogous to how mineral acids or carboxylic acids behave with active metals. Ethanol also reacts with sodium, but much less vigorously — the difference in rate itself hints at the difference in acidity.

2. Reaction with Aqueous Sodium Hydroxide

This is the more telling reaction. Phenol dissolves in aqueous NaOH to form sodium phenoxide and water.

C_6H_5OH + NaOH \rightarrow C_6H_5ONa + H_2O

This is a straightforward acid-base neutralisation. Phenol donates a proton to the strong base $OH^-$ , forming water. Ethanol does not react with aqueous NaOH at all — it is simply not acidic enough to protonate the hydroxide ion.

Watch out

A common mistake is to think that because ethanol reacts with sodium, it is as acidic as phenol. But the NaOH test is the real discriminator: ethanol fails it completely. The sodium reaction is not selective — it just shows that both have a labile O–H hydrogen, not that they are equally acidic.

Comparing Acidity: Phenol vs Ethanol

Property	Phenol ( $C_6H_5OH$ )	Ethanol ( $C_2H_5OH$ )
$pK_a$ value	~10	~16
Relative acidity	Weak acid (but stronger than water)	Very weak (almost neutral)
Reaction with NaOH	Yes, forms salt	No reaction
Reaction with Na	Yes, liberates $H_2$	Yes, but slower
Conjugate base	Phenoxide ion — resonance stabilised	Ethoxide ion — charge localised

The $pK_a$ difference of 6 units means phenol is about $10^6$ times more acidic than ethanol. That is a million-fold difference.

Tip

A quick way to remember: any compound whose conjugate base can be stabilised by resonance will be more acidic than one that cannot. For phenols, the negative charge moves into the ring. For alcohols, it stays put on oxygen — and that is energetically costly.

Why the Difference? The Resonance Argument …

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Here are the common mistakes students make on this question about the Acidity of Phenol, along with clear strategies to avoid each.

Mistake 1: Writing the wrong reactions to show acidic nature

The error: Students often write reactions that show phenol reacting (e.g., with bromine water or FeCl₃) but not reactions that prove it is an acid.

Acidity is shown by:

Reaction with a base (like NaOH) to form a salt.
Reaction with an active metal (like Na) to liberate H₂ gas.

How to avoid:

Only use these two standard reactions:

With sodium hydroxide (neutralization):

\ce{C6H5OH + NaOH -> C6H5ONa + H2O}

(Phenol acts as a weak acid, donating a proton to the strong base.)

With sodium metal (liberation of hydrogen):

\ce{2 C6H5OH + 2 Na -> 2 C6H5ONa + H2}

(This is analogous to how acids like HCl or H₂SO₄ react with metals.)

Key point: Both reactions produce sodium phenoxide ( $\ce{C6H5ONa}$ ) — a salt — proving phenol is acidic.

Mistake 2: Saying phenol is more acidic than ethanol because of the –OH group

The error: Both phenol and ethanol have an –OH group, so this reason is incomplete and often leads to a wrong comparison.

The correct reason:

Phenol is more acidic than ethanol because the phenoxide ion ( $\ce{C6H5O-}$ ) is stabilized by resonance with the benzene ring, while the ethoxide ion ( $\ce{C2H5O-}$ ) has no resonance stabilization.

How to avoid:

Always explain using resonance stabilization of the conjugate base:

Phenol: After losing H⁺, the negative charge on oxygen is delocalized into the benzene ring (resonance structures). This makes the phenoxide ion more stable, so phenol loses H⁺ more easily.
Ethanol: The ethoxide ion has the negative charge localized on oxygen — no resonance — so it is less stable, making ethanol a weaker acid.

Comparison table (exam-ready):

Property	Phenol	Ethanol
Conjugate base	Phenoxide ion ( $\ce{C6H5O-}$ )	Ethoxide ion ( $\ce{C2H5O-}$ )
Stabilization of conjugate base	Resonance with benzene ring	No resonance
Acidity	More acidic (pKa ≈ 10)	Less acidic (pKa ≈ 16)

Mistake 3: Confusing pKa values or writing them incorrectly

The error: Students sometimes write pKa of phenol as 16 and ethanol as 10 (reversed), or forget that lower pKa = stronger acid.

How to avoid:

Memorize the order:

Phenol: pKa ≈ 10 (stronger acid)
Ethanol: pKa ≈ 16 (weaker acid) …

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Q.Give two reactions that show the acidic nature of phenol. Compare acidity of phenol with that of ethanol.

Concept understanding — Acidity Of Phenol

Inductive Effect on Acidity – From Intuition to Precision

The Core Intuition

The Precise Statement

Why Distance Matters

Why this formula?

Acidity of Phenol: Why It's More Acidic Than Alcohols

1. The Core Observation

2. The Key: Stability of the Conjugate Base

The Resonance Explanation

3. Why Alcohols Can't Do This

4. The Inductive Effect Also Helps (But Resonance Dominates)

5. The Quantitative Picture (pKa Values)

Why Phenol Is Acidic — The Core Idea

Two Reactions That Show Phenol Is Acidic

1. Reaction with Sodium Metal

2. Reaction with Aqueous Sodium Hydroxide

Comparing Acidity: Phenol vs Ethanol

Why the Difference? The Resonance Argument …

Concept: Acidity of Phenol

Method: Acid-Base Reaction & Indicator Test

Step-by-Step Reasoning

Comparison: Acidity of Phenol vs. Ethanol

Mistake 1: Writing the wrong reactions to show acidic nature

Mistake 2: Saying phenol is more acidic than ethanol because of the –OH group

Mistake 3: Confusing pKa values or writing them incorrectly

Q.Give two reactions that show the acidic nature of phenol. Compare acidity of phenol with that of ethanol.

Why Phenol Is Acidic — The Core Idea

Two Reactions That Show Phenol Is Acidic

1. Reaction with Sodium Metal

2. Reaction with Aqueous Sodium Hydroxide

Comparing Acidity: Phenol vs Ethanol

Why the Difference? The Resonance Argument …

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