JEE Mains Chemistry · Alcohols, Phenols and Ethers

Acidity of Alcohols and Phenols

Phenol is about a million times more acidic than ethanol because the phenoxide ion spreads its charge over the ring; groups that pull electrons, above all nitro groups at ortho and para, make a phenol stronger, and the NaOH, NaHCO₃ and FeCl₃ screens sort compounds by that acidity.

All Alcohols, Phenols and Ethers notes JEE Mains Chemistry strategy

Why this matters

Twenty PYQs, eighteen of them multiple choice, and two from 2026. Six compare phenols with alcohols, including two numerical questions that count active hydrogens with a Grignard reagent. Ten rank substituted phenols or alcohols by pKa, most of them with nitro, methoxy, methyl or halogen groups. Four are solubility and colour screens with NaOH, NaHCO₃ and neutral FeCl₃.

Concept 1 of 3: Why phenol is a stronger acid than an alcohol

An acid is strong when the ion it leaves behind is stable. The phenoxide ion spreads its negative charge from oxygen onto the ortho and para carbons of the ring. The alkoxide ion keeps the charge on oxygen, and its alkyl group pushes more electron density there. So phenol gives up its proton far more easily.

Definition

pKa (lower means stronger): carboxylic acids about 4–5, phenol 10.0, water 15.7, ethanol 15.9.
Among alcohols: $\mathrm{CH_3OH}$ > 1° > 2° > 3°, because each extra alkyl group (+I) destabilises the alkoxide.
Alcohols react with Na, K and Al to give alkoxides and $\mathrm{H_2}$ , but not with aqueous NaOH. Phenol reacts with both, giving sodium phenoxide with NaOH.
An alcohol acts as a nucleophile (its O lone pair attacks) and as an electrophile (the protonated C–O carbon is attacked). The reaction with sodium shows its acidity, not either of these roles.
Active hydrogen: every O–H destroys one Grignard reagent, $\mathrm{ROH + CH_3MgI \to ROMgI + CH_4}$ . Moles of $\mathrm{CH_4}$ = moles of active H.
A phenolic O–H is removed by the first equivalent of Grignard reagent before any adds to a C=O in the same molecule.

Counting active hydrogens with a Grignard reagent

\mathrm{ROH + CH_3MgI \to ROMgI + CH_4\uparrow} \qquad M = \dfrac{m}{n(\mathrm{CH_4})} \ \ (\text{one O–H per molecule})

Worked example

Arrange water, ethanol, phenol and 2-methylpropan-2-ol in increasing order of acidity, and say which of them dissolve in aqueous NaOH as a salt.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2024 · 31 January 2024 · Q44Moderate

Example 1 · Alcohols, Phenols and Ethers · Acidity of Alcohols and Phenols

Given below are two statements: One is labelled as Assertion A and the other is labelled as Reason R: Assertion A:

pK_{a}

value of phenol is 10.0 while that of ethanol is 15.9. Reason R: Ethanol is stronger acid than phenol. In the light of the above statements, choose the correct answer from the options given below:

Lower pKa means the stronger acid

Phenol (10.0) is the stronger acid and ethanol (15.9) the weaker. A statement that ethanol is the stronger acid reverses this, even when it quotes the right numbers.

A phenolic O–H uses up a Grignard reagent

With one equivalent of Grignard reagent, a hydroxy aldehyde or ketone is only deprotonated. After work-up you get the starting compound back, not an alcohol.

Use the molar volume the question gives

Older papers use 22.4 L mol

^{-1}

at STP and newer data 22.7 L mol

^{-1}

. Take whichever is stated; the nearest-integer answer usually survives either.

Drill 5 more on why phenol is a stronger acid than an alcohol

Concept 2 of 3: Ranking substituted phenols and alcohols by pKa

A group that pulls electrons stabilises the phenoxide and lowers the pKa. At ortho or para, a nitro group takes the negative charge onto its own oxygens (−R); at meta it can only pull through the bonds (−I). A group that pushes electrons does the opposite.

Definition

NCERT values: p-nitrophenol 7.1, o-nitrophenol 7.2, m-nitrophenol 8.3, phenol 10.0, cresols 10.1–10.2, ethanol 15.9.
More nitro groups at ortho and para: 2,4-dinitrophenol about 4, 2,4,6-trinitrophenol (picric acid) about 0.4.
Methoxy flips with position: at para its +R wins, so p-methoxyphenol (10.2) is weaker than phenol; at meta only its −I acts, so m-methoxyphenol (9.65) is stronger.
Halogens act by −I: chlorophenols are stronger acids than phenol (m-chlorophenol about 9.1).
Alkyl groups (+I, hyperconjugation) and para $\mathrm{NMe_2}$ (+R) weaken a phenol.
In alcohols the −I effect fades with distance: a chlorine on the carbon next to the carbinol carbon raises acidity much more than a fluorine four carbons away.

The pKa ladder (NCERT values)

p\text{-NO}_2\,(7.1) < o\text{-NO}_2\,(7.2) < m\text{-NO}_2\,(8.3) < \text{phenol}\,(10.0) < p\text{-CH}_3\,(10.2) < \text{ethanol}\,(15.9)

Worked example

Arrange phenol, p-cresol, m-nitrophenol, p-nitrophenol and m-chlorophenol in increasing order of pKa.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2023 · 29 January 2023 · Q36Moderate

Example 2 · Alcohols, Phenols and Ethers · Acidity of Alcohols and Phenols

The increasing order of

pK_{a}

for the following phenols is (A) 2,4-Dinitrophenol (B) 4-Nitrophenol (C) 2,4,5-Trimethylphenol (D) Phenol (E) 3-Chlorophenol Choose the correct answer from the option given below:

Ortho-nitro is not the strongest

The intramolecular hydrogen bond between OH and

\mathrm{NO_2}

in o-nitrophenol holds the proton back a little. So p-nitrophenol (7.1) is slightly stronger than o-nitrophenol (7.2), and m-nitrophenol (8.3) is the weakest of the three.

Methoxy at meta makes phenol stronger

Do not treat

\mathrm{OCH_3}

as always electron-releasing. At meta its +R cannot reach the oxygen, so its −I effect makes m-methoxyphenol more acidic than phenol.

Distance beats electronegativity for −I

2-Chlorocyclohexanol is more acidic than 4-fluorocyclohexanol even though F is more electronegative than Cl. The inductive pull dies away within two or three bonds.

Drill 9 more on ranking substituted phenols and alcohols by pka

Concept 3 of 3: Solubility and colour screens with NaOH, NaHCO₃ and FeCl₃

Each screen is a pKa threshold. Hydroxide takes a proton from anything more acidic than water, so NaOH dissolves phenols and acids but not alcohols. Bicarbonate only takes a proton from acids stronger than carbonic acid (pKa about 6.4), so it separates carboxylic acids from ordinary phenols. Neutral FeCl₃ gives a colour with a phenolic OH.

Definition

Na metal gives $\mathrm{H_2}$ with any O–H compound: alcohols, phenols and acids, but not ethers.
Aqueous NaOH dissolves phenols and carboxylic acids as their sodium salts; alcohols stay undissolved.
Aqueous $\mathrm{NaHCO_3}$ gives $\mathrm{CO_2}$ only with acids of pKa below about 6.4: carboxylic acids, picric acid, 2,4-dinitrophenol.
Neutral $\mathrm{FeCl_3}$ gives a violet colour with phenol (and a colour with other phenols and enols); alcohols give none.

Screen	Passes	Fails	Why
Sodium metal ( $\mathrm{H_2}$ evolved)	Alcohols, phenols, carboxylic acids	Ethers and alkanes	Sodium replaces the hydrogen of an O–H group
Aqueous NaOH (dissolves as a salt)	Phenols, cresols, nitrophenols, carboxylic acids	Alcohols such as cyclohexanol and benzyl alcohol; ethers	Hydroxide removes a proton only from acids stronger than water
Aqueous $\mathrm{NaHCO_3}$ ( $\mathrm{CO_2}$ evolved)	Carboxylic acids, picric acid, 2,4-dinitrophenol	Phenol, cresols, m- and p-nitrophenol, alcohols	Needs an acid stronger than carbonic acid, pKa below about 6.4
Neutral $\mathrm{FeCl_3}$ (colour)	Phenol (violet), salicylic acid, other phenols and enols	Alcohols, benzyl alcohol, anisole	Iron(III) forms a coloured complex with an OH on the ring

Read the screens in order: NaOH tells a phenol from an alcohol, and NaHCO₃ tells a carboxylic acid (or a strongly nitrated phenol) from a phenol.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2022 · 29 July 2022 · Q50Moderate

Example 3 · Alcohols, Phenols and Ethers · Acidity of Alcohols and Phenols

A compound '

X

' is acidic and it is soluble in

NaOH

solution, but insoluble in

NaHCO_{3}

solution. Compound '

X

' also gives violet colour with neutral

FeCl_{3}

solution. The compound '

X

' is:

Benzyl alcohol is not a phenol

\mathrm{C_6H_5CH_2OH}

the OH sits on a

\mathrm{CH_2}

, not on the ring. It behaves as an alcohol: no salt with NaOH and no colour with

\mathrm{FeCl_3}

One nitro group is not enough for bicarbonate

m- and p-Nitrophenol are still weaker acids than carbonic acid. It takes two or three nitro groups at ortho and para (2,4-dinitrophenol, picric acid) to release

\mathrm{CO_2}

Drill 3 more on solubility and colour screens with naoh, nahco₃ and fecl₃

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (2)

Why phenol is a stronger acid than an alcohol
Counting active hydrogens with a Grignard reagent
$\mathrm{ROH + CH_3MgI \to ROMgI + CH_4\uparrow} \qquad M = \dfrac{m}{n(\mathrm{CH_4})} \ \ (\text{one O–H per molecule})$
Ranking substituted phenols and alcohols by pKa
The pKa ladder (NCERT values)
$p\text{-NO}_2\,(7.1) < o\text{-NO}_2\,(7.2) < m\text{-NO}_2\,(8.3) < \text{phenol}\,(10.0) < p\text{-CH}_3\,(10.2) < \text{ethanol}\,(15.9)$

Reference tables (1)

Solubility and colour screens with NaOH, NaHCO₃ and FeCl₃4 rows

Screen	Passes	Fails	Why
Sodium metal ( $\mathrm{H_2}$ evolved)	Alcohols, phenols, carboxylic acids	Ethers and alkanes	Sodium replaces the hydrogen of an O–H group
Aqueous NaOH (dissolves as a salt)	Phenols, cresols, nitrophenols, carboxylic acids	Alcohols such as cyclohexanol and benzyl alcohol; ethers	Hydroxide removes a proton only from acids stronger than water
Aqueous $\mathrm{NaHCO_3}$ ( $\mathrm{CO_2}$ evolved)	Carboxylic acids, picric acid, 2,4-dinitrophenol	Phenol, cresols, m- and p-nitrophenol, alcohols	Needs an acid stronger than carbonic acid, pKa below about 6.4
Neutral $\mathrm{FeCl_3}$ (colour)	Phenol (violet), salicylic acid, other phenols and enols	Alcohols, benzyl alcohol, anisole	Iron(III) forms a coloured complex with an OH on the ring

Read the screens in order: NaOH tells a phenol from an alcohol, and NaHCO₃ tells a carboxylic acid (or a strongly nitrated phenol) from a phenol.

Watch out for (8)

Lower pKa means the stronger acid
→ Why phenol is a stronger acid than an alcohol
A phenolic O–H uses up a Grignard reagent
→ Why phenol is a stronger acid than an alcohol
Use the molar volume the question gives
→ Why phenol is a stronger acid than an alcohol
Ortho-nitro is not the strongest
→ Ranking substituted phenols and alcohols by pKa
Methoxy at meta makes phenol stronger
→ Ranking substituted phenols and alcohols by pKa
Distance beats electronegativity for −I
→ Ranking substituted phenols and alcohols by pKa
Benzyl alcohol is not a phenol
→ Solubility and colour screens with NaOH, NaHCO₃ and FeCl₃
One nitro group is not enough for bicarbonate
→ Solubility and colour screens with NaOH, NaHCO₃ and FeCl₃

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