Nucleophilic properties of aminesAQA A-Level Chemistry: Revision notes
Section 1
Ammonia and amines as nucleophiles
A nucleophile is an electron pair donor. Ammonia and amines are nucleophiles because the nitrogen atom has a lone pair of electrons.
The carbon in a C–Br bond is δ+ because bromine is more electronegative. The lone pair on nitrogen attacks this carbon in a nucleophilic substitution, replacing the halogen.
Section 2
Mechanism: ammonia and a halogenoalkane
For CH₃CH₂Br + NH₃:
- The lone pair on N of NH₃ attacks the δ+ carbon, and at the same time the C–Br bond breaks, with both electrons going to Br. This forms Br⁻ and the intermediate CH₃CH₂NH₃⁺.
- A second NH₃ molecule removes H⁺ from the intermediate, forming CH₃CH₂NH₂ and NH₄⁺.
Overall: CH₃CH₂Br + 2NH₃ → CH₃CH₂NH₂ + NH₄Br.
The conditions are ammonia in ethanol, heated in a sealed tube.
In mechanism diagrams, the first arrow starts at the lone pair on N and ends at the δ+ carbon. The second starts at the C–Br bond and ends at Br.
Curly arrows show the movement of electron pairs, so each starts at a lone pair or a bond, never at an atom.
Section 3
Further substitution
The primary amine formed also has a lone pair on N, so it is also a nucleophile and can react with more halogenoalkane:
- primary amine → secondary amine (R₂NH)
- secondary amine → tertiary amine (R₃N)
- tertiary amine → quaternary ammonium salt (R₄N⁺ X⁻)
In the quaternary ion the nitrogen has four bonds and no lone pair, so it cannot react further.
To make mainly the primary amine use a large excess of ammonia. To make the quaternary salt use an excess of halogenoalkane.
Excess ammonia reduces but does not stop further substitution, so the product is still a mixture.
Section 4
Quaternary ammonium salts as surfactants
Quaternary ammonium salts with one or more long hydrocarbon chains are cationic surfactants, for example [CH₃(CH₂)₁₅N(CH₃)₃]⁺ Br⁻.
- the long non-polar chain is hydrophobic
- the charged head N⁺ is hydrophilic
The ion is made by reacting a tertiary amine with a halogenoalkane. It is used in fabric softeners and hair conditioners, where the positive head is attracted to negatively charged surfaces and the chain coats the surface.
Must know
- Amines and ammonia are nucleophiles because of the lone pair on N
- Mechanism: lone pair attacks δ+ carbon, Br⁻ leaves, a second NH₃ removes H⁺
- Further substitution gives secondary, tertiary and quaternary ammonium salts
- Excess ammonia favours the primary amine
- Quaternary ammonium salts with long chains are cationic surfactants
- Reactions with acyl chlorides and acid anhydrides are covered under acylation
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Nucleophilic properties of amines
- A chemist heats bromoethane, CH₃CH₂Br, with a large excess of ammonia dissolved in ethanol. The reaction is carried out in a sealed tube and ethylamine is formed.Explain why ammonia can act as a nucleophile in this reaction.2 marks
- A student heats an excess of 1-bromopropane, CH₃CH₂CH₂Br, with a small amount of ethanolic ammonia in a sealed tube. Analysis shows a mixture of amines and an ionic compound.Write an equation for the reaction of propylamine with 1-bromopropane to form a secondary amine, and explain why propylamine can react in this way.2 marks
- Hexadecyltrimethylammonium bromide, [CH₃(CH₂)₁₅N(CH₃)₃]⁺ Br⁻, is a cationic surfactant used in hair conditioners. It is manufactured from the tertiary amine hexadecyldimethylamine, CH₃(CH₂)₁₅N(CH₃)₂.Suggest how this surfactant is formed from the tertiary amine, giving the reagent, an equation and the type of mechanism.3 marks
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).