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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.

In one line: More available lone pair means stronger base; resonance can hide the lone pair.

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Understand it

  • 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.

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+.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

Compare aniline and cyclohexylamine.

  1. 1In aniline, the lone pair is delocalised into the aromatic ring.
  2. 2In cyclohexylamine, the nitrogen lone pair is localised and the alkyl ring shows electron-releasing effect.
  3. 3Therefore cyclohexylamine is more basic.

Method you can reuse

  1. 1Check whether the amine is aliphatic or aromatic.
  2. 2Then judge lone-pair availability using inductive effect, resonance, solvation, and steric hindrance.
  3. 3Finally write the order with a reason, not only the order.

Visual tip

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

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why is aniline less basic than methylamine?

Think first, then show answer
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.

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.

Don't mix this up

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

Exam tip

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

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

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.
  1. 1

    Enough for 1 mark

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

  2. 2

    Add for 2 marks

    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.

  3. 3

    Add for 3 marks

    Amines are basic because nitrogen has a lone pair.

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