Arrange water, ethanol and phenol in increasing order of acidity and give reason for your answer.

NCERT Exemplar · Q51

Q. Arrange water, ethanol and phenol in increasing order of acidity and give reason for your answer.

Sikkim CbseShort· 2mImportance★★★★★

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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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Acidity increases with the stability of the conjugate base after losing a proton. Phenol is the most acidic because its phenoxide ion is resonance-stabilised, ethanol is the least acidic because its alkoxide ion has no such stabilisation, and water lies in between. The increasing order is: ethanol < water < phenol.

Why Acidity Depends on the Conjugate Base

Acidity is the ability to donate a proton ( $H^+$ ). When an acid $HA$ loses $H^+$ , it forms its conjugate base $A^-$ . The more stable this conjugate base is, the more willing the acid is to give up its proton — so the stronger the acid.

For alcohols, water, and phenols, the acidic proton is the one attached to oxygen ( $O-H$ ). The conjugate base is an alkoxide ion ( $RO^-$ ), hydroxide ion ( $OH^-$ ), or phenoxide ion ( $C_6H_5O^-$ ). The key question: how well does each conjugate base handle the negative charge?

A negative charge is stabilised when it can be spread out (delocalised) over a larger volume or onto more electronegative atoms. The more the charge is dispersed, the more stable the anion, and the stronger the original acid.

Step-by-step comparison

1. Ethanol ( $C_2H_5OH$ ) — the weakest acid

When ethanol loses $H^+$ , we get the ethoxide ion: $C_2H_5O^-$ .

The negative charge is localised entirely on the oxygen atom. There is no resonance — the alkyl group ( $-C_2H_5$ ) is electron-donating (inductive effect), which actually pushes electron density toward the oxygen, making the negative charge even more concentrated and less stable.

Watch out

A common mistake is to think that because ethanol is an "alcohol", it must be more acidic than water. In fact, the electron-donating alkyl group makes the alkoxide ion less stable than hydroxide, so ethanol is actually a weaker acid than water.

Result: Ethoxide ion is poorly stabilised. Ethanol is the weakest acid among the three.

2. Water ( $H_2O$ ) — intermediate acidity

Water loses $H^+$ to give the hydroxide ion: $OH^-$ .

The negative charge is again on oxygen, but there is no electron-donating alkyl group pushing charge onto it. The hydroxide ion is more stable than ethoxide because the charge is not further destabilised by an inductive effect.

However, there is still no resonance delocalisation. The charge is localised on a single oxygen atom.

Result: Hydroxide is moderately stable. Water is a stronger acid than ethanol, but weaker than phenol.

3. Phenol ( $C_6H_5OH$ ) — the strongest acid

Phenol loses $H^+$ to give the phenoxide ion: $C_6H_5O^-$ .

Here, the negative charge on oxygen can be delocalised into the aromatic ring. The lone pair on oxygen overlaps with the $\pi$ -electron system of the benzene ring, allowing the charge to spread onto the ortho and para carbon atoms (and partially onto the ring itself).

C_6H_5O^- \leftrightarrow \text{(structures with negative charge on ortho/para carbons)}

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Method: Relative Acidity via Conjugate Base Stability

This method compares the stability of the conjugate base (the anion formed after losing $\ce{H+}$ ) — the more stable the conjugate base, the stronger the acid.

Step 1: Write the conjugate bases

Acid	Conjugate base (anion)
Water ( $\ce{H2O}$ )	$\ce{OH-}$ (hydroxide ion)
Ethanol ( $\ce{C2H5OH}$ )	$\ce{C2H5O-}$ (ethoxide ion)
Phenol ( $\ce{C6H5OH}$ )	$\ce{C6H5O-}$ (phenoxide ion)

Step 2: Analyse stability of each conjugate base

Ethoxide ion ( $\ce{C2H5O-}$ )

The negative charge is localised on oxygen. The ethyl group is electron-donating (inductive effect), which destabilises the negative charge.

→ Least stable conjugate base.
Hydroxide ion ( $\ce{OH-}$ )

Negative charge is on oxygen, but no electron-donating alkyl group. Slightly more stable than ethoxide.
Phenoxide ion ( $\ce{C6H5O-}$ )

The negative charge is delocalised into the benzene ring via resonance. This spreads the charge over multiple atoms, greatly stabilising the anion.

→ Most stable conjugate base.

Step 3: Relate stability to acidity

Stronger acid → more stable conjugate base.

Phenol gives the most stable conjugate base → strongest acid among the three. …

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Here is a breakdown of the common mistakes students make on this specific question, along with the correct conceptual approach to avoid them.

The Correct Answer (For Reference)

Increasing order of acidity: Ethanol < Water < Phenol

Reason: Acidity is determined by the stability of the conjugate base after losing a proton ( $H^+$ ). The more stable the conjugate base (alkoxide/phenoxide ion), the stronger the acid.

Ethanol ( $C_2H_5OH$ ): The ethoxide ion ( $C_2H_5O^-$ ) has the negative charge localized on the oxygen atom. There is no resonance stabilization. It is the least stable conjugate base, hence ethanol is the weakest acid.
Water ( $H_2O$ ): The hydroxide ion ( $OH^-$ ) also has the negative charge localized on oxygen. However, $OH^-$ is slightly more stable than $C_2H_5O^-$ because the ethyl group is an electron-donating group (+I effect) which destabilizes the negative charge in ethoxide. Water is a stronger acid than ethanol.
Phenol ( $C_6H_5OH$ ): The phenoxide ion ( $C_6H_5O^-$ ) is the most stable conjugate base because the negative charge is delocalized into the benzene ring via resonance. This extensive charge delocalization makes phenol the strongest acid among the three.

Common Mistake #1: Ordering by "Reactivity" or "Polarity" Instead of Acidity

The Mistake: Students often rank ethanol as the most acidic because they think "alcohols are reactive" or because the $O-H$ bond in ethanol is "more polar" than in phenol.

Why it's wrong: Acidity is not about bond polarity or general reactivity. It is specifically about the ease of losing $H^+$ and the stability of the resulting anion. A more polar bond does not automatically mean easier proton loss.

How to Avoid: Always shift your focus from the bond itself to the conjugate base. Ask: "After losing $H^+$ , which negative ion is most stable?" The answer is always the one with the most resonance or inductive stabilization.

Common Mistake #2: Forgetting the +I Effect of the Ethyl Group

The Mistake: Students correctly identify that phenol is the most acidic, but then incorrectly state that water and ethanol have the same acidity, or that ethanol is more acidic than water.

Why it's wrong: The ethyl group ( $-C_2H_5$ ) is an electron-donating group (+I effect). It pushes electron density towards the oxygen in the ethoxide ion, making the negative charge even more concentrated and unstable. Water has no such group, so $OH^-$ is slightly more stable than $C_2H_5O^-$ .

How to Avoid: Remember the inductive effect rule:

Electron-donating groups (+I) destabilize a negative charge → decrease acidity.
Electron-withdrawing groups (-I) stabilize a negative charge → increase acidity.

Since the ethyl group donates electrons, ethanol is a weaker acid than water.

Common Mistake #3: Describing Phenol's Resonance Incorrectly

The Mistake: Students say "Phenol is acidic because the benzene ring has resonance." This is vague and often leads to losing marks. They might also draw resonance structures incorrectly (e.g., showing the negative charge on carbon atoms in the phenol molecule itself, not in the phenoxide ion).

Why it's wrong: The resonance stabilization is in the phenoxide ion, not in phenol. The phenol molecule itself has resonance, but that resonance is broken when the $O-H$ bond is intact. The key is that the negative charge on oxygen can be delocalized onto the ortho and para positions of the ring.

How to Avoid: Be precise in your language. …

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Q.Arrange water, ethanol and phenol in increasing order of acidity and give reason for your answer.

Why Acidity Depends on the Conjugate Base

Step-by-step comparison

1. Ethanol (C2H5OHC_2H_5OHC2​H5​OH) — the weakest acid

2. Water (H2OH_2OH2​O) — intermediate acidity

3. Phenol (C6H5OHC_6H_5OHC6​H5​OH) — the strongest acid

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1. Ethanol ( $C_2H_5OH$ ) — the weakest acid

2. Water ( $H_2O$ ) — intermediate acidity

3. Phenol ( $C_6H_5OH$ ) — the strongest acid

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