In simple terms
A friendly intro before the formal notes — no formulas yet.
Amines: Nitrogen's Nucleophilic Nature
Amines contain a nitrogen atom with a lone pair of electrons, making them both basic and nucleophilic. This allows them to react with acids, halogenoalkanes, and acyl chlorides to form a variety of important products.
Think of the nitrogen atom's lone pair of electrons as a 'free hand'. This hand is ready to grab a passing proton (H⁺), which is acting as a base. Alternatively, the hand can reach out and form a bond with an electron-poor carbon atom, which is acting as a nucleophile. This dual capability is central to all amine chemistry.
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R–NH₂ or R₂NH — lone pair on N is nucleophilic. Basicity order is typically alkyl amines > NH₃ > aryl amines due to electron-donating/withdrawing effects.
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With a halogenoalkane, the amine's lone pair attacks the δ+ carbon in a nucleophilic substitution, leading to further alkylation to form 2°/3° amines. Using excess ammonia favours the 1° amine product.
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With an acyl chloride, the amine attacks the carbonyl carbon in a nucleophilic addition-elimination reaction. This forms a stable N-substituted amide and hydrogen chloride gas.
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As bases, amines react with acids in a neutralisation reaction to form water-soluble ionic salts, called ammonium salts, such as ethylammonium chloride, RNH₃⁺Cl⁻.
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Full topic notes
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Classification of Amines
Amines are classified as primary (1°), secondary (2°), or tertiary (3°) based on how many organic groups are attached to the nitrogen atom. It's crucial not to confuse this with the classification of alcohols or halogenoalkanes, which is based on the carbon atom the functional group is attached to.
Primary (1°) amines: One alkyl/aryl group is attached to the nitrogen atom (e.g., ethylamine, CH₃CH₂NH₂). General formula RNH₂.
Secondary (2°) amines: Two alkyl/aryl groups are attached to the nitrogen atom (e.g., diethylamine, (CH₃CH₂)₂NH). General formula R₂NH.
Tertiary (3°) amines: Three alkyl/aryl groups are attached to the nitrogen atom (e.g., triethylamine, (CH₃CH₂)₃N). General formula R₃N.
The Basicity of Amines
The lone pair of electrons on the nitrogen atom allows amines to act as Brønsted-Lowry bases by accepting a proton. They are weak bases, meaning they establish an equilibrium in water. The strength of an amine as a base depends on the availability of this lone pair.
RNH₂(aq) + H₂O(l) ⇌ RNH₃⁺(aq) + OH⁻(aq)
Alkylamines vs Ammonia: Alkyl groups are electron-donating due to the positive inductive effect (+I effect). They 'push' electron density towards the nitrogen atom, making the lone pair more available and the amine a stronger base than ammonia.
Aromatic amines vs Ammonia: In aromatic amines like phenylamine (C₆H₅NH₂), the nitrogen's lone pair is delocalised into the benzene ring's delocalised π-system. This makes the lone pair less available to accept a proton, so phenylamine is a significantly weaker base than ammonia.
General order of basicity: Secondary alkylamine > Primary alkylamine > Ammonia > Phenylamine. (Note: Tertiary amines are often slightly weaker bases than secondary amines in aqueous solution due to factors like steric hindrance and solvation, but are still stronger than primary amines).
Reactions of Amines as Nucleophiles
The lone pair on the nitrogen atom also allows amines to act as nucleophiles, attacking an electron-deficient (δ+) atom, typically carbon. We will focus on two key reactions: nucleophilic substitution with halogenoalkanes and nucleophilic addition-elimination with acyl chlorides.
1. Nucleophilic Substitution with Halogenoalkanes
Ammonia and amines react with halogenoalkanes to form a more substituted amine. For example, ammonia reacts with a primary halogenoalkane to form a primary amine. However, the primary amine product is also a nucleophile and can react further with the halogenoalkane, leading to a mixture of primary, secondary, tertiary amines, and even a quaternary ammonium salt. This makes it a poor method for preparing a pure amine.
CH₃CH₂Br + NH₃ → CH₃CH₂NH₃⁺Br⁻ → CH₃CH₂NH₂ + HBr
2. Nucleophilic Addition-Elimination with Acyl Chlorides
Primary and secondary amines undergo a vigorous reaction with acyl chlorides at room temperature. The amine acts as a nucleophile, attacking the electron-deficient carbonyl carbon. The reaction mechanism is nucleophilic addition-elimination. The products are an N-substituted amide and misty white fumes of hydrogen chloride gas. This is an important reaction for forming the amide linkage.
R'COCl + 2RNH₂ → R'CONHR + RNH₃⁺Cl⁻
Note that two moles of the amine are required per mole of acyl chloride. One mole acts as the nucleophile, and the second acts as a base to react with the HCl produced, forming an ammonium salt. This prevents the acidic HCl from protonating the amine reactant, which would deactivate it as a nucleophile.
To maximise the yield of the primary amine, use a large excess of ammonia in a sealed tube under heat and pressure (using ethanol as a solvent). The high concentration of ammonia makes it statistically more likely that the halogenoalkane will collide with an ammonia molecule rather than the amine product.
Worked examples
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Explain, in terms of electronic effects, why ethylamine is a stronger base than phenylamine.
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In ethylamine (CH₃CH₂NH₂), the ethyl group is an electron-donating group. It exerts a positive inductive effect, pushing electron density onto the nitrogen atom. [1 mark]
Propylamine (CH₃CH₂CH₂NH₂) is warmed with an excess of bromoethane (CH₃CH₂Br). (a) Write an equation for the first step of this reaction, forming a secondary amine. (b) Name the secondary amine formed. (c) Draw the structure of the quaternary ammonium salt that is the final product of the reaction sequence.
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(a) The amine acts as a nucleophile. The initial reaction forms the salt, which is then deprotonated by another amine molecule. Overall equation for the formation of the secondary amine: CH₃CH₂CH₂NH₂ + CH₃CH₂Br → CH₃CH₂CH₂NH(CH₂CH₃) + HBr [1 mark] Note: Often shown with a second amine molecule reacting with HBr: CH₃CH₂CH₂NH₂ + CH₃CH₂Br → [CH₃CH₂CH₂NH₂(CH₂CH₃)]⁺Br⁻, which then reacts with another CH₃CH₂CH₂NH₂ to give the final products.
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Glossary
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What is a primary (1°) amine?
An organic compound where one hydrogen atom in an ammonia molecule has been replaced by an alkyl or aryl group. General formula: RNH₂.
Key takeaways
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Primary (1°) amines: One alkyl/aryl group is attached to the nitrogen atom (e.g., ethylamine, CH₃CH₂NH₂). General formula RNH₂.
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Secondary (2°) amines: Two alkyl/aryl groups are attached to the nitrogen atom (e.g., diethylamine, (CH₃CH₂)₂NH). General formula R₂NH.
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Tertiary (3°) amines: Three alkyl/aryl groups are attached to the nitrogen atom (e.g., triethylamine, (CH₃CH₂)₃N). General formula R₃N.
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Test Your Knowledge on Amines
Test Your Knowledge on Amines
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