Explain why $MeNH_2$ is stronger base than $MeOH$?

Explain why MeNH₂ is stronger base than MeOH?

NCERT Exemplar · Q36

MeNH_2

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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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The key idea is that the conjugate acid of a base determines its strength — a more stable conjugate acid means a stronger base. $MeNH_2$ is a stronger base than $MeOH$ because its conjugate acid ( $MeNH_3^+$ ) is stabilised by the lower electronegativity of nitrogen, which better accommodates the positive charge, whereas oxygen in $MeOH$ destabilises the positive charge on its conjugate acid ( $MeOH_2^+$ ).

Why This Question Matters

This is a classic comparison that tests your understanding of basicity — not just memorising pKa values, but the reason behind them. In Indian exams (JEE, NEET, CUET), they love asking why one compound is more basic than another, and the answer always traces back to the stability of the conjugate acid.

Let’s break it down properly.

1. What does "stronger base" actually mean?

A base is a species that accepts a proton ( $H^+$ ). When $MeNH_2$ (methylamine) accepts a proton, it becomes $MeNH_3^+$ (methylammonium ion). When $MeOH$ (methanol) accepts a proton, it becomes $MeOH_2^+$ (methyloxonium ion).

The stronger base is the one that holds onto that proton more tightly — or equivalently, the one whose conjugate acid is more stable (less eager to give the proton back).

B + H^+ \rightleftharpoons BH^+

So the question becomes: Which conjugate acid is more stable — $MeNH_3^+$ or $MeOH_2^+$ ?

2. Compare the atoms bearing the positive charge

In $MeNH_3^+$ , the positive charge sits on nitrogen.

In $MeOH_2^+$ , the positive charge sits on oxygen.

Now, think about electronegativity:

Oxygen is more electronegative (3.44) than nitrogen (3.04).
A more electronegative atom pulls electron density towards itself.
When a positive charge is on a highly electronegative atom, that atom wants electrons even more — but it already has a strong pull. The positive charge becomes less stable because the atom is "unhappy" with the electron deficiency.

Tip

Think of it like this: An electronegative atom is like a miser — it hoards electrons. If you take one away (positive charge), it's very unhappy. A less electronegative atom is more generous — it doesn't mind the positive charge as much.

So:

$MeOH_2^+$ : positive charge on O (highly electronegative) → unstable conjugate acid.
$MeNH_3^+$ : positive charge on N (less electronegative) → more stable conjugate acid.

Since $MeNH_3^+$ is more stable, $MeNH_2$ is the stronger base.

3. A second angle: inductive effects

Both molecules have a methyl group ( $-CH_3$ ). The methyl group is electron-donating (through hyperconjugation and inductive effect). This helps stabilise a positive charge.

But here’s the catch: In $MeOH_2^+$ , the oxygen is so electronegative that it overpowers the electron donation from the methyl group. The positive charge remains poorly stabilised.

In $MeNH_3^+$ , nitrogen is less electronegative, so the electron donation from the methyl group is more effective at spreading out the positive charge. …

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Concept: Acidity of Phenol

Method: Resonance Stabilisation of Conjugate Base (Phenoxide Ion)

Why this method?

The acidity of a compound is determined by the stability of its conjugate base after losing a proton ( $H^+$ ). For phenol, the key is to compare the resonance stabilisation of the phenoxide ion ( $C_6H_5O^-$ ) versus the alkoxide ion (e.g., $CH_3O^-$ from methanol).

Steps:

Write the dissociation equilibrium Phenol donates a proton:

C_6H_5OH \rightleftharpoons C_6H_5O^- + H^+

Draw resonance structures of the conjugate base

The negative charge on oxygen in phenoxide can be delocalised into the benzene ring via resonance. Draw the contributing structures:
- The lone pair on oxygen moves to form a double bond with the ring carbon.
- The π-electrons shift, placing the negative charge on ortho and para positions of the ring.
(Key result: The negative charge is spread over multiple atoms, not localised on oxygen alone.)
Compare with a non-aromatic conjugate base

For methanol ( $CH_3OH$ ), the conjugate base is methoxide ( $CH_3O^-$ ). The negative charge is localised on oxygen — no resonance delocalisation is possible.
Conclude

The phenoxide ion is more stable than methoxide because resonance spreads the negative charge, reducing electron density on oxygen. Greater stability of the conjugate base means phenol is more acidic than methanol.

Final takeaway:

Phenol is acidic because its conjugate base (phenoxide) is resonance-stabilised. Methanol lacks this stabilisation, so it is far less acidic.

Why $MeNH_2$ is a stronger base than $MeOH$

Method: Inductive Effect and Lone Pair Availability

Steps:

Identify the basic site

Both molecules have a lone pair that can accept a proton ( $H^+$ ). In methylamine ( $CH_3NH_2$ ), the lone pair is on nitrogen. In methanol ( $CH_3OH$ ), it is on oxygen.
Compare electronegativity
- Nitrogen is less electronegative (3.0) than oxygen (3.5). …

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Common Mistakes: Acidity of Phenol & Basicity of Amines vs Alcohols

Students often confuse acidity and basicity trends. The question asks about basicity of $MeNH_2$ vs $MeOH$ , but many mistakenly apply acidity logic. Here are the most frequent errors and how to avoid them.

✗ Mistake 1: Confusing Acidity with Basicity

What students do wrong:

They think: "Since phenol is more acidic than alcohol, its conjugate base is more stable — so the amine must be less basic." This is irrelevant here because we are comparing $MeNH_2$ (an amine) with $MeOH$ (an alcohol), not phenol.

Why it’s wrong:

Acidity and basicity are reverse concepts:

Acid strength ∝ stability of conjugate base
Base strength ∝ availability of lone pair to accept $H^+$

How to avoid:

Always check: Are we comparing acids or bases? For basicity, focus on the lone pair availability on nitrogen vs oxygen.

✗ Mistake 2: Ignoring Electronegativity

What students do wrong:

They say: "Both have lone pairs, so they should be similar."

Why it’s wrong:

Oxygen is more electronegative (3.44) than nitrogen (3.04). This means:

Oxygen holds its lone pair more tightly
Nitrogen’s lone pair is more available for protonation

Correct reasoning:

$MeNH_2$ is a stronger base because N is less electronegative → lone pair is more loosely held → easier to donate to $H^+$ .

How to avoid:

Memorise the trend: Basicity decreases across a period (as electronegativity increases).

$NH_3 > H_2O > HF$ (in water).

✗ Mistake 3: Forgetting the Role of the Alkyl Group

What students do wrong:

They compare $NH_3$ vs $H_2O$ but forget that $Me$ (methyl) is an electron-donating group (+I effect).

Why it matters:

The methyl group pushes electron density toward the atom with the lone pair, making it even more basic.

$MeNH_2$ is more basic than $NH_3$
$MeOH$ is slightly more basic than $H_2O$ , but still far weaker than $MeNH_2$

How to avoid:

Always consider inductive effects of substituents. Alkyl groups increase basicity of amines.

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Q.Explain why MeNH2MeNH_2MeNH2​ is stronger base than MeOHMeOHMeOH?

Why This Question Matters

1. What does "stronger base" actually mean?

2. Compare the atoms bearing the positive charge

3. A second angle: inductive effects

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Q.Explain why $MeNH_2$ is stronger base than $MeOH$ ?

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