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Basicity of Amines

Basicity of amines is the tendency of nitrogen to donate its lone pair to a proton, forming a substituted ammonium ion.

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Student-friendly explanation

Amines are basic because nitrogen has a lone pair. Alkyl groups generally increase basicity through the +I effect by increasing electron density on nitrogen. Aromatic amines such as aniline are less basic because the lone pair on nitrogen is delocalised into the benzene ring by resonance, making it less available for protonation. In aqueous solution, solvation and steric effects also influence the order among primary, secondary, and tertiary aliphatic amines.

How to write this in exams

  1. 1

    Start with the exact idea

    Basicity of amines is the tendency of nitrogen to donate its lone pair to a proton, forming a substituted ammonium ion.

  2. 2

    Then show how to use it

    Check whether the amine is aliphatic or aromatic. Then judge lone-pair availability using inductive effect, resonance, solvation, and steric hindrance. Finally write the order with a reason, not only the order.

  3. 3

    Add one concrete example

    Ethylamine is more basic than ammonia due to the electron-releasing alkyl group, while aniline is less basic than ammonia because its lone pair participates in resonance with the benzene ring.

  4. 4

    Avoid this incomplete answer

    Saying aniline is more basic than ammonia because it has a larger carbon group is wrong; resonance decreases the availability of nitrogen's lone pair.

Definition

Basicity of amines is the tendency of nitrogen to donate its lone pair to a proton, forming a substituted ammonium ion.

Example

Ethylamine is more basic than ammonia due to the electron-releasing alkyl group, while aniline is less basic than ammonia because its lone pair participates in resonance with the benzene ring.

Rule to remember

Basicity pattern support: availability of lone pair controls basicity. Alkyl +I effect increases electron density; aryl resonance decreases lone-pair availability. Protonation pattern: RNH2 + H+ gives RNH3+.

Memory hook

More available lone pair means stronger base; resonance can hide the lone pair.

Examples and method

Worked example

Compare aniline and cyclohexylamine. In aniline, the lone pair is delocalised into the aromatic ring. In cyclohexylamine, the nitrogen lone pair is localised and the alkyl ring shows electron-releasing effect. Therefore cyclohexylamine is more basic.

Method to apply

Check whether the amine is aliphatic or aromatic. Then judge lone-pair availability using inductive effect, resonance, solvation, and steric hindrance. Finally write the order with a reason, not only the order.

Diagram support

A resonance structure sketch for aniline is useful to show why the lone pair is less available, but no apparatus diagram is needed.

How CBSE asks it

It appears as order-of-basicity questions, reason-based short answers, assertion-reason items, and comparison of aniline with ammonia or aliphatic amines.

Avoid common mistakes

Common confusion

Students often apply only the inductive effect and conclude that all tertiary amines must be most basic in water, ignoring solvation and steric effects.

Common wrong answer

Saying aniline is more basic than ammonia because it has a larger carbon group is wrong; resonance decreases the availability of nitrogen's lone pair.

Exam tip

For aromatic amines, mention resonance delocalisation of the nitrogen lone pair; for aliphatic amines, discuss +I effect along with solvation when needed.

Quick check

Why is aniline less basic than methylamine?

In aniline, the nitrogen lone pair is delocalised into the benzene ring by resonance, so it is less available for protonation; in methylamine, the methyl group increases electron density on nitrogen.

Answer writing and exam use

1-mark answer

Basicity of amines is the tendency of nitrogen to donate its lone pair to a proton, forming a substituted ammonium ion.

2-mark answer

Basicity of amines is the tendency of nitrogen to donate its lone pair to a proton, forming a substituted ammonium ion. Basicity pattern support: availability of lone pair controls basicity. Alkyl +I effect increases electron density; aryl resonance decreases lone-pair availability. Protonation pattern: RNH2 + H+ gives RNH3+. Ethylamine is more basic than ammonia due to the electron-releasing alkyl group, while aniline is less basic than ammonia because its lone pair participates in resonance with the benzene ring.

3-mark answer

Amines are basic because nitrogen has a lone pair. Alkyl groups generally increase basicity through the +I effect by increasing electron density on nitrogen. Aromatic amines such as aniline are less basic because the lone pair on nitrogen is delocalised into the benzene ring by resonance, making it less available for protonation. In aqueous solution, solvation and steric effects also influence the order among primary, secondary, and tertiary aliphatic amines. Basicity pattern support: availability of lone pair controls basicity. Alkyl +I effect increases electron density; aryl resonance decreases lone-pair availability. Protonation pattern: RNH2 + H+ gives RNH3+. Compare aniline and cyclohexylamine. In aniline, the lone pair is delocalised into the aromatic ring. In cyclohexylamine, the nitrogen lone pair is localised and the alkyl ring shows electron-releasing effect. Therefore cyclohexylamine is more basic. It appears as order-of-basicity questions, reason-based short answers, assertion-reason items, and comparison of aniline with ammonia or aliphatic amines. Saying aniline is more basic than ammonia because it has a larger carbon group is wrong; resonance decreases the availability of nitrogen's lone pair.
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