Phenol can be distinguished from ethanol by the reactions with ______. (Two or more than two options may be co

NCERT Exemplar · Q59

Q. Phenol can be distinguished from ethanol by the reactions with ____________. (Two or more than two options may be correct.)

(i)

Br_2

/water

(ii) Na

(iii) neutral

FeCl_3

(iv) all of these

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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 and ethanol differ in acidity and aromatic reactivity. Phenol reacts with bromine water (electrophilic substitution) and neutral FeCl3 (colour complex); both react with Na (acidic H). So the correct options are (i) and (iii).

Reasoning

Option (i), Br2/water: phenol's activated aromatic ring undergoes rapid electrophilic substitution with bromine water at room temperature, giving a white precipitate of 2,4,6-tribromophenol. Ethanol has no aromatic ring and does not react. This distinguishes them - correct. …

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Here are the common mistakes students make on this question, along with how to avoid each.

Mistake 1: Forgetting that Na reacts with both phenol and ethanol

Many students see "Na" and immediately think of the classic phenol reaction (evolution of $H_2$ gas). They forget that ethanol also has a labile $-OH$ hydrogen and reacts with sodium to produce hydrogen gas.

The Trap: Option (B) Na is a distinguishing test only if you are looking for a difference in vigour (phenol reacts slower due to weaker acidity in practice, but both still react). In a multiple-choice exam, "distinguishing" usually means one reacts and the other does not.
How to Avoid: Memorise the reactivity order: Na metal reacts with any compound having an acidic hydrogen (alcohols, phenols, carboxylic acids, water). If a question asks for a distinguishing test, Na is not a reliable choice because both give the same observation (bubbles of $H_2$ ).

Mistake 2: Confusing the $Br_2$ /water test with the $Br_2$ / $CCl_4$ test

Students often think bromine water tests only for unsaturation (alkenes/alkynes). They forget that phenol undergoes electrophilic substitution with bromine water, even at room temperature, to give a white precipitate of 2,4,6-tribromophenol.

The Trap: Ethanol does not react with $Br_2$ /water at all. Phenol does. So this is a valid distinguishing test.
How to Avoid: Remember the specific reagent: $Br_2$ /water (aqueous bromine) is a test for phenol (substitution). $Br_2$ in $CCl_4$ (non-polar) is a test for unsaturation (addition). Write this distinction in your notes.

Mistake 3: Thinking neutral $FeCl_3$ gives a colour with all alcohols

Neutral $FeCl_3$ is a specific test for enols (compounds with a $-OH$ group attached to an $sp^2$ hybridised carbon). Phenol is an enol; ethanol is not.

The Trap: Students assume any $-OH$ compound gives a colour. They forget that aliphatic alcohols (like ethanol) do not give a violet/blue colour with neutral $FeCl_3$ .
How to Avoid: Link the $FeCl_3$ test to aromatic -OH (phenols) and enolisable $\beta$ -diketones only. Ethanol gives no colour change.

Mistake 4: Selecting "all of these" without checking each option independently …

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Q.Phenol can be distinguished from ethanol by the reactions with ____________. (Two or more than two options may be correct.)

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)

Reasoning

Method: Reagent-Based Distinction Test for Phenol vs. Ethanol

Steps:

Mistake 1: Forgetting that Na reacts with both phenol and ethanol

Mistake 2: Confusing the $Br_2$ /water test with the $Br_2$ / $CCl_4$ test

Mistake 3: Thinking neutral $FeCl_3$ gives a colour with all alcohols

Mistake 4: Selecting "all of these" without checking each option independently …

Q.Phenol can be distinguished from ethanol by the reactions with ____________. (Two or more than two options may be correct.)

Reasoning

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