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9476 / 2027
Organic Chemistry

Lesson 28 of 34

Basicity depends on how available the nitrogen lone pair is

Gas-phase alkyl donation and aqueous solvation answer different comparisons.

A-Level 9476 (2026-2027)

  • I can explain primary, secondary and tertiary amine gas-phase basicity.Syllabus

    Syllabus 9476, 11.9(c). Explain primary, secondary and tertiary amine gas-phase basicity.

  • I can compare aqueous ammonia, ethylamine and phenylamine basicity.Syllabus

    Syllabus 9476, 11.9(d). Compare aqueous ammonia, ethylamine and phenylamine basicity.

  • I can describe phenylamine with aqueous bromine.Syllabus

    Syllabus 9476, 11.9(e). Describe phenylamine with aqueous bromine.

Basicity depends on how available the nitrogen lone pair is

Gas-phase alkyl donation and aqueous solvation answer different comparisons.

Make a guess. You are not marked.

Which is the stronger base in water, ethylamine or phenylamine?

  1. Phenylamine, as the ring pushes electrons onto N
  2. Phenylamine, as the ring has many electrons
  3. Ethylamine, as its lone pair stays on N
Show the answer

Ethylamine, as its lone pair stays on N

In phenylamine the nitrogen lone pair spreads into the benzene ring, so it is less available to accept a proton. The ethyl group pushes electrons towards nitrogen.

Amines are Lewis bases, because nitrogen can donate its lone pair to an electron-pair acceptor such as H+. In the gas phase, more alkyl groups make a stronger base: tertiary > secondary > primary > ammonia. Alkyl groups push electron density onto nitrogen and stabilise the protonated ion. In the gas phase there is no water to complicate this.

In water, the order you need is ethylamine > ammonia > phenylamine. The ethyl group pushes electron density onto nitrogen, so its lone pair is more available than in ammonia. In phenylamine, the lone pair spreads into the benzene ring, so it is less available to accept a proton. In water, how well the ions are surrounded by water molecules also matters. So do not use the gas-phase order of primary, secondary and tertiary amines for water.

The same lone-pair donation that makes phenylamine a weak base also makes its ring more reactive towards electrophiles. Aqueous bromine gives a white precipitate of 2,4,6-tribromophenylamine and loses its colour. No AlBr3 catalyst is needed. The NH2 group sends the bromine to the two positions next to it and the position opposite it.

Check your understanding

Phenylamine is a weaker base than ethylamine, yet its ring reacts very readily with electrophiles. How can both be true?

  1. Its N lone pair is delocalised into the ring: less available at N, more density in the ring.
  2. The two facts conflict, so one of them must have been measured at a different pH.
  3. The ring donates electrons to nitrogen, which makes N weaker but the ring stronger.
Show the answer

Its N lone pair is delocalised into the ring: less available at N, more density in the ring.

Delocalisation moves electron density from nitrogen into the ring. Nitrogen is then a poorer proton acceptor, while the ring becomes richer in electrons and more open to electrophilic attack.