Ch 1 · Primary amines · AS & A Level Chemistry 9701
Topic 1 of 2 in Nitrogen compounds — AS & A Level Chemistry 9701 — 57 lessons.
Introduction
Primary amines, R-NH_2, contain a nitrogen atom with a lone pair, making them both weak bases and effective nucleophiles. This lesson covers their preparation from halogenoalkanes and the reactivity that follows directly from that lone pair.
Key ideas — Preparation from halogenoalkanes
Heating a halogenoalkane with a large excess of concentrated ammonia dissolved in ethanol, in a sealed tube, substitutes the halogen with an amino group by nucleophilic substitution: CH_3CH_2Br + NH_3 arrow CH_3CH_2NH_2 + HBr The hydrogen bromide formed is neutralised by further ammonia, giving ammonium bromide as a by-product.
🧑🏫 Teacher's explanation
Think of a halogenoalkane as a house where a specific person (the halogen atom) is about to move out, and a new person (the ammonia molecule) wants to move in. However, once the new person moves in, they might accidentally bump into the person who just left, causing a chain reaction that leads to even more people moving in. To make sure we only get one person in the house, we flood the neighborhood with the new person so they don't have time to react with the others.
📖 Definitions
🔍 Worked example — Easy
Identify the structure of the primary amine produced when chloroethane, CH_3CH_2Cl, reacts with ammonia, NH_3.
🔍 Worked example — Standard
Calculate the molar mass (M_r) of ethanamine, CH_3CH_2NH_2. (H=1, C=12, N=14).
🔍 Worked example — Exam-style
A student reacts 0.10 mol of bromoethane with ammonia in ethanol. If the yield of ethanamine is 0.08 mol, calculate the percentage yield.
⚠️ Common errors
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🎯 Exam question
Key ideas — Why excess ammonia favours the primary amine
The primary amine product still has a lone pair on nitrogen and is itself a nucleophile, so it can react further with unreacted halogenoalkane to give a secondary amine, then a tertiary amine and eventually a quaternary ammonium salt. Using a large excess of ammonia makes a collision between a halogenoalkane molecule and an ammonia molecule far more probable than a collision between a halogenoalkane molecule and an amine molecule, so substitution stops predominantly at the primary amine and further substitution is minimised.
🧑🏫 Teacher's explanation
Imagine you are at a crowded party where everyone is trying to hand out flyers. The 'flyers' are the halogenoalkane molecules, and the 'people' are the ammonia molecules. If the room is packed with people (excess ammonia), a person is much more likely to grab a flyer before the flyer can be grabbed by someone who has already received one (an amine).
📖 Definitions
🔍 Worked example — Easy
Write the balanced chemical equation for the reaction between bromoethane and ammonia in ethanol to produce the primary amine, including state symbols.
🔍 Worked example — Standard
Calculate the molar mass of ethanamine (C_2H_5NH_2).
🔍 Worked example — Exam-style
A student reacts bromoethane with a 1:1 molar ratio of ammonia in ethanol. Explain why the yield of ethanamine is low.
⚠️ Common errors
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🎯 Exam question
(a) Explain why an excess of ammonia is required to maximise the yield of ethanamine. [2] (b) Draw the structure of the secondary amine formed if ethanamine reacts with more chloroethane. [1] [3 marks]
Key ideas — Naming and structure of primary amines
A primary amine is named by adding "-amine" to the alkyl group's name, or by using the prefix "amino-" with the appropriate locant on the parent chain. Ethylamine, CH_3CH_2NH_2, has the amino group on the end carbon of a two-carbon chain; propan-2-amine, CH_3CH(NH_2)CH_3, has the amino group on the central carbon of a three-carbon chain.
🧑🏫 Teacher's explanation
Think of an amine as a modified hydrocarbon. Imagine a long chain of carbon atoms (an alkane) where one of the hydrogen atoms has been swapped out for a nitrogen-based group (-NH_2). This nitrogen atom is the 'star' of the molecule because it has a lone pair of electrons looking for a positive target to attack.
📖 Definitions
🔍 Worked example — Easy
Draw the structural formula for propan-1-amine.
🔍 Worked example — Standard
A halogenoalkane, CH_3CH_2Cl, reacts with excess ammonia (NH_3) in ethanol to form a primary amine. Write the balanced equation for this reaction.
🔍 Worked example — Exam-style
Explain why excess ammonia is used when reacting a halogenoalkane with ammonia to produce a primary amine.
⚠️ Common errors
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🎯 Exam question
Key ideas — Amines as nucleophiles
The nitrogen atom of a primary amine carries a lone pair of electrons, just as ammonia does, and can donate this pair to an electron-deficient carbon atom, such as the delta^+ carbon of a halogenoalkane. This lone pair makes primary amines effective nucleophiles, capable of undergoing the same type of nucleophilic substitution reaction used to prepare them in the first place.
🧑🏫 Teacher's explanation
Think of a nucleophile as a person with a spare key looking for a lock. In chemistry, the 'spare key' is a lone pair of electrons, and the 'lock' is an atom that is missing electrons (an electrophile). Because the nitrogen in a primary amine has a lone pair of electrons sitting out in the open, it is very 'eager' to find a positive charge to share those electrons with.
📖 Definitions
🔍 Worked example — Standard
A sample of 2.50 g of ethanamine (C_2H_5NH_2) is reacted with excess bromoethane (C_2H_5Br). State the type of reaction occurring and write the balanced equation for the formation of the primary amine.
🔍 Worked example — Exam-style
In the reaction between chloroethane and ammonia, why is it necessary to use an excess of ammonia to maximize the yield of ethanamine?
⚠️ Common errors
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🎯 Exam question
Key ideas — Basicity of primary amines
The same lone pair that makes a primary amine a nucleophile also makes it a weak Brønsted–Lowry base: it accepts a hydrogen ion from water or from an acid to form a substituted ammonium ion, for example CH_3CH_2NH_2 + HCl arrow CH_3CH_2NH_3^+Cl^-. Alkyl groups are electron-donating, so they increase the electron density on nitrogen relative to ammonia, making most primary amines slightly stronger bases than ammonia itself.
🧑🏫 Teacher's explanation
Think of a primary amine as a person holding a very attractive, high-energy magnet (the lone pair of electrons on the nitrogen). Because this magnet is so concentrated, the nitrogen is looking for something to grab. If it grabs a positive nucleus like a proton (H^+), it acts as a Brønsted-Lowry base. If it uses that same magnet to attack a carbon atom that is 'starving' for electrons, it acts as a nucleophile.
📖 Definitions
🔍 Worked example — Easy
Predict the structure of propan-1-amine and state its role when reacting with HCl.
🔍 Worked example — Standard
Explain why ethanamine (CH_3CH_2NH_2) is a stronger base than ammonia (NH_3).
🔍 Worked example — Exam-style
Complete and balance the equation for the reaction between ethanoic acid and ethanamine: CH_3COOH + CH_3CH_2NH_2 arrow ? (Assume acid-base reaction).
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Worked example
Question. 1-bromopropane is heated with a large excess of concentrated ethanolic ammonia in a sealed tube. (a) Name the mechanism of this reaction and identify the role of ammonia. [2] (b) Name the organic product. [1] (c) Explain why a large excess of ammonia, rather than an equimolar amount, is used. [2]
🔍 Worked example — Easy
Identify the structure of propan-1-amine and state the number of carbon atoms present.
🔍 Worked example — Standard
Calculate the molar mass (M_r) of propan-1-amine (C_3H_9N).
🔍 Worked example — Exam-style
A student reacts chloroethane with ammonia in ethanol. The yield of ethanamine is low because secondary amines are formed. Write the balanced equation for the formation of the secondary amine product.
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Practice
Answers