Ch 3 · Amides · AS & A Level Chemistry 9701
Topic 3 of 4 in Nitrogen compounds — AS & A Level Chemistry 9701 — 58 lessons.
Introduction
Amides, RCONH_2, sit at the meeting point of the carbonyl and amine chemistry covered so far. This lesson brings together their preparation from acyl chlorides, their two modes of hydrolysis, their reduction to amines, and the key structural reason amides behave as neutral compounds in water rather than as bases, unlike the amines that so closely resemble them.
Key ideas — Forming amides from ammonia or an amine and an acyl chloride
An acyl chloride reacts with excess ammonia or with excess of a primary or secondary amine in a condensation reaction, releasing hydrogen chloride, which is neutralised by the excess nucleophile: CH_3COCl + 2NH_3 arrow CH_3CONH_2 + NH_4Cl CH_3COCl + 2CH_3NH_2 arrow CH_3CONHCH_3 + CH_3NH_3Cl
🧑🏫 Teacher's explanation
Think of an amide as a 'stabilized' version of an amine. In an amine, the nitrogen has a lone pair of electrons that is 'free' and ready to grab a proton (making it basic). In an amide, that lone pair is being 'pulled' toward the carbonyl group (C=O) through resonance. Imagine a tug-of-war: the nitrogen wants to use its electrons to act as a base, but the oxygen is pulling those electrons away to strengthen the C=O bond. Because the electrons are busy being shared across the N-C-O system, they aren't available to react with acids.
📖 Definitions
🔍 Worked example — Easy
Calculate the mass of ethanamide produced when 0.10 mol of ethanoyl chloride reacts completely with excess ammonia.
🔍 Worked example — Standard
An amide undergoes complete hydrolysis in aqueous HCl to produce ethanoic acid and ethanamine. If 0.50 mol of the amide is used, how many moles of ethanoic acid are produced?
🔍 Worked example — Exam-style
Compare the pK_b values: Methylamine (CH_3NH_2) has a pK_b of 3.8, while Acetamide (CH_3CONH_2) has a pK_b of 13.5. Which is a stronger base and why?
⚠️ Common errors
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🎯 Exam question
Key ideas — Hydrolysis with aqueous acid
Heating an amide under reflux with aqueous acid, such as dilute hydrochloric acid, hydrolyses it to a carboxylic acid and an ammonium salt: CH_3CONH_2 + H_2O + HCl arrow CH_3COOH + NH_4Cl
🧑🏫 Teacher's explanation
Think of an amide as a molecule where a nitrogen atom is 'stuck' to a carbonyl group (C=O). This bond is very stable, making amides much less reactive than acyl chlorides. However, if you add water and a strong acid, you can force the molecule to split apart. This process is called hydrolysis.
📖 Definitions
🔍 Worked example — Easy
Identify the products of the acid hydrolysis of ethanamide, ceCH3CONH2, using dilute ceHCl and write the balanced equation.
🔍 Worked example — Standard
Calculate the molar mass of the salt produced when 0.10 moles of benzamide, ceC6H5CONH2, undergoes acid hydrolysis with ceHCl. (Use C=12, H=1, N=14, O=16, Cl=35.5)
🔍 Worked example — Exam-style
A student heats 2.00 g of propanamide, ceCH3CH2CONH2, under reflux with 50 cm^3 of 2.0 mol dm^-3 ceHCl. Calculate the mass of the carboxylic acid produced. (Use C=12, H=1, N=14, O=16)
⚠️ Common errors
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🎯 Exam question
(a) Write the equation for the reaction of X with ceHCl and ceH2O. [2] (b) Identify the two organic products formed. [2] [4 marks]
Key ideas — Hydrolysis with aqueous alkali
Heating an amide under reflux with aqueous alkali, such as sodium hydroxide, instead hydrolyses it to a carboxylate salt and ammonia, which is driven off as a gas (turning damp red litmus paper blue): CH_3CONH_2 + NaOH arrow CH_3COONa + NH_3 An N-substituted amide is hydrolysed in the same way, releasing the corresponding amine in place of ammonia.
🧑🏫 Teacher's explanation
Think of an amide like a two-person team where the bond between the carbon and the nitrogen is very strong and stable. However, when you add a strong base like sodium hydroxide and heat it up, you are essentially forcing a 'divorce' between the carbonyl carbon and the nitrogen group. The hydroxide ion (OH^-) attacks the carbon, breaking the bond and splitting the molecule into two separate pieces.
📖 Definitions
🔍 Worked example — Easy
Write a balanced chemical equation for the hydrolysis of ethanamide (CH_3 extCONH_2) using aqueous sodium hydroxide (NaOH), including state symbols.
🔍 Worked example — Standard
Calculate the mass of CH_3 extCOONa produced when 1.00 g of CH_3 extCONH_2 is hydrolyzed with excess NaOH. (A_r: C=12, H=1, N=14, O=16, Na=23)
🔍 Worked example — Exam-style
An N-methylacetamide (CH_3 extCONHCH_3) is hydrolyzed with aqueous NaOH. Write the balanced equation for this reaction.
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🎯 Exam question
Key ideas — Reduction of amides to amines
Lithium aluminium hydride in dry ether reduces the carbonyl group of an amide, converting it into a CH_2 group and so converting the amide into an amine with the same number of carbon atoms: CH_3CONH_2 + 4[H] arrow CH_3CH_2NH_2 + H_2O An N-substituted amide is reduced in the same way to a secondary amine, since the group already on nitrogen is retained.
🧑🏫 Teacher's explanation
Imagine you are trying to transform a piece of furniture. An amide has a 'carbonyl group' (C=O) which acts like a heavy, bulky attachment on the carbon atom. To turn this into an amine, you need to strip away that oxygen and replace it with two hydrogen atoms, effectively turning the C=O into a CH_2 group.
📖 Definitions
🔍 Worked example — Easy
Calculate the molar mass of the amine produced when ethanamide (CH_3 extCONH_2) is reduced. (A_r: H=1, C=12, N=14, O=16)
🔍 Worked example — Standard
A sample of N-methylacetamide (CH_3 extCONHCH_3) is reduced using LiAlH_4 in dry ether. Write the balanced equation for the reduction of this secondary amide to its corresponding amine.
🔍 Worked example — Exam-style
How many moles of LiAlH_4 are required to reduce 0.50 moles of propanamide (CH_3 extCH_2 extCONH_2) to propan-1-amine (CH_3 extCH_2 extCH_2 extNH_2)? Assume 1 mole of LiAlH_4 provides 4 moles of [H].
⚠️ Common errors
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🎯 Exam question
(a) Identify the organic product, Y, formed in the reaction. [1] (b) Write the balanced chemical equation for the reduction of X to Y. [2] [3 marks]
Key ideas — Why amides are much weaker bases than amines
In an amine, the lone pair on nitrogen is fully available to accept a proton. In an amide, the lone pair on nitrogen instead overlaps with the adjacent carbonyl π system and becomes delocalised towards the electronegative oxygen atom, considerably reducing the electron density available on nitrogen. This delocalisation also gives the carbon-nitrogen bond of an amide partial double-bond character. Because the nitrogen lone pair is no longer readily available, amides do not behave as measurable bases in aqueous solution, unlike the amines from which they can be made or into which they can be reduced.
🧑🏫 Teacher's explanation
Imagine you have a handful of coins (electrons) that you want to use to buy something (a proton). In an amine, the coins are sitting loosely in your pocket, ready to be spent immediately. In an amide, you have taken those same coins and locked them inside a heavy, complex safe (the carbonyl group). While the coins are still technically there, they are so busy being part of the safe's structure that you can't easily pull them out to spend them.
📖 Definitions
🔍 Worked example — Easy
Compare the availability of the nitrogen lone pair in ethanamine (CH_3CH_2NH_2) and ethanamide (CH_3CONH_2).
🔍 Worked example — Standard
Predict the product of the complete hydrolysis of propanamide (CH_3CH_2CONH_2) in aqueous acid.
🔍 Worked example — Exam-style
Calculate the mass of an amide produced when 0.100 mol of ethanoyl chloride (CH_3COCl) reacts completely with excess ammonia (NH_3). Assume the product is ethanamide (CH_3CONH_2).
⚠️ Common errors
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🎯 Exam question
Worked example
Question. Ethanamide, CH_3CONH_2, and ethylamine, CH_3CH_2NH_2, are each shaken with dilute hydrochloric acid. (a) State and explain which compound reacts more readily with the acid. [3] (b) Ethanamide is then heated under reflux with aqueous sodium hydroxide. Name the two products formed. [2]
🔍 Worked example — Easy
Compare the basicity of methanamide, ceCH3CONH2, and methanamine, ceCH3NH2.
🔍 Worked example — Standard
Predict the products of the complete hydrolysis of propanamide, ceCH3CH2CONH2, using aqueous ceNaOH.
🔍 Worked example — Exam-style
A sample of ethanamide, ceCH3CONH2, is heated under reflux with dilute ceHCl. Write the balanced equation for the reaction and identify the products.
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Practice
Answers