Why are amines bases?

The nitrogen atom has a lone pair of electrons that can accept a proton.

Write the equation for methylamine acting as a base in water.

CH₃NH₂ + H₂O ⇌ CH₃NH₃⁺ + OH⁻

Why are amines weak bases?

They only partially accept protons, so the equilibrium lies well to the left.

What is the order of base strength of ammonia, phenylamine and a primary aliphatic amine?

Primary aliphatic amine ammonia phenylamine.

Why is ethylamine a stronger base than ammonia?

The ethyl group pushes electron density towards N, so the lone pair is more available to accept H⁺.

What is the inductive effect of an alkyl group?

It releases (pushes) electron density towards the atom it is attached to, here nitrogen.

Why is phenylamine a weaker base than ammonia?

Its lone pair is delocalised into the benzene ring, so it is less available to accept H⁺.

What is formed when ethylamine reacts with HCl?

Ethylammonium chloride, CH₃CH₂NH₃⁺Cl⁻.

How can a free amine be released from its ammonium salt?

Add a stronger base such as NaOH, which removes H⁺.

Base properties of aminesAQA A-Level Chemistry: Flashcards

Card 1 of 120 of 12 known

Question

What is a base?

Tap or press Space to reveal

Tap card or press Space to flip

See all 12 cards

What is a base?: A proton (H⁺) acceptor.
Why are amines bases?: The nitrogen atom has a lone pair of electrons that can accept a proton.
Write the equation for methylamine acting as a base in water.: CH₃NH₂ + H₂O ⇌ CH₃NH₃⁺ + OH⁻
Why are amines weak bases?: They only partially accept protons, so the equilibrium lies well to the left.
What is the order of base strength of ammonia, phenylamine and a primary aliphatic amine?: Primary aliphatic amine > ammonia > phenylamine.
Why is ethylamine a stronger base than ammonia?: The ethyl group pushes electron density towards N, so the lone pair is more available to accept H⁺.
What is the inductive effect of an alkyl group?: It releases (pushes) electron density towards the atom it is attached to, here nitrogen.
Why is phenylamine a weaker base than ammonia?: Its lone pair is delocalised into the benzene ring, so it is less available to accept H⁺.
What is formed when ethylamine reacts with HCl?: Ethylammonium chloride, CH₃CH₂NH₃⁺Cl⁻.
How can a free amine be released from its ammonium salt?: Add a stronger base such as NaOH, which removes H⁺.
Which of two amine solutions of the same concentration is the stronger base?: The one with the higher pH.
In what ratio does a primary amine react with HCl?: 1 : 1.

Exam questions on Base properties of amines

A technician measures the pH of three aqueous solutions, each of concentration 0.100 mol dm⁻³, at 298 K. The solution of methylamine has a pH of 11.8, the solution of ammonia has a pH of 11.1 and the solution of phenylamine has a pH of 8.8.
Write an equation for the reaction of phenylamine with water and explain why phenylamine is described as a weak base.2 marks
A technician has separate 0.100 mol dm⁻³ solutions of ethylamine, CH₃CH₂NH₂, and phenylamine, C₆H₅NH₂, and plans to titrate samples of each against hydrochloric acid.
A 25.0 cm³ sample of 0.100 mol dm⁻³ ethylamine reacts with 0.125 mol dm⁻³ hydrochloric acid. Calculate the volume of acid needed to neutralise it.2 marks
A student measures the pH of a 0.100 mol dm⁻³ solution of ammonia at 298 K and finds it is 11.1. At this temperature the ionic product of water, Kw, is 1.00 × 10⁻¹⁴ mol² dm⁻⁶.
Write an equation for the reaction of ammonia with water, and calculate the concentration of hydroxide ions in the solution.3 marks

See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).