Amides
Chemistry· 9701 A2 Unit 26: Carboxylic acid derivatives· 12 min read
1. Structure and Naming of Amides★★☆☆☆⏱ 2 min
Amides are classified as primary, secondary or tertiary based on the number of alkyl groups bonded directly to the nitrogen atom. Primary amides have no alkyl substituents on N, secondary have one, and tertiary have two.
IUPAC Naming Rule for Amides
The parent name comes from the longest carbon chain containing the amide functional group, replacing the -oic acid suffix of the corresponding carboxylic acid with -amide. Alkyl groups attached to nitrogen are prefixed with N- to indicate their position.
Primary amide from ethanoic acid = ethanamide
Secondary amide with one methyl on N = N-methylethanamide
Tertiary amide with two methyl groups on N = N,N-dimethylethanamide
Name the amide with structure CH₃CH₂C(=O)NHCH₃
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Step 1: Identify the longest carbon chain containing the amide group. This is 3 carbons, derived from propanoic acid.
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Step 2: The suffix for a 3-carbon amide is -propanamide.
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Step 3: There is one methyl group attached to the nitrogen atom, so add the N-methyl prefix.
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Final name: N-methylpropanamide
Which of the following is the correct name for CH₃C(=O)N(CH₃)₂?
N,N-dimethylethanamide
2-dimethylethanamide
di-N-methylmethanamide
Reveal answer
N,N-dimethylethanamide —The parent chain is 2 carbons (ethanamide), with two methyl groups bonded to nitrogen.
2. Physical Properties of Amides★★☆☆☆⏱ 2 min
Primary amides have two N-H bonds that can form strong hydrogen bonds between adjacent molecules, leading to much higher boiling points than comparable carboxylic acids, alcohols or amines. Low molecular weight amides are fully miscible with water due to hydrogen bonding with water molecules.
Compound | Relative molecular mass | Boiling point / °C |
|---|---|---|
Propan-1-ol | 60 | 97 |
Ethanoic acid | 60 | 118 |
Ethanamide | 59 | 221 |
Explain why ethanamide has a higher boiling point than ethanoic acid of similar molecular mass
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Step 1: Both molecules form hydrogen bonds between their functional groups.
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Step 2: Ethanamide has two polar N-H bonds that can act as hydrogen bond donors, and the carbonyl oxygen acts as an acceptor, forming a more extensive intermolecular hydrogen bond network.
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Step 3: The partial double bond character of the C-N bond in amides increases the polarity of the functional group, further strengthening intermolecular forces.
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Conclusion: More energy is required to separate amide molecules, leading to a higher boiling point.
3. Hydrolysis of Amides★★★☆☆⏱ 4 min
Amide hydrolysis is the reverse of amide formation, and proceeds via nucleophilic acyl substitution. It requires heating under reflux with either strong aqueous acid or strong aqueous alkali.
State the products of full hydrolysis of propanamide under (i) acidic conditions with dilute HCl, heat and (ii) alkaline conditions with aqueous NaOH, heat
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Part (i): Acidic hydrolysis
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The amide bond breaks, the -OH from water adds to the acyl group to form propanoic acid, and the NH₂ group is protonated by excess HCl to form ammonium chloride.
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Part (ii): Alkaline hydrolysis
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The amide bond breaks, the Na⁺ from the alkali forms a propanoate salt, and ammonia gas is released. Free propanoic acid is only formed if you add a subsequent acidification step.
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4. Reduction and Dehydration of Amides★★★☆☆⏱ 3 min
Two key synthetic reactions of primary amides are tested regularly in CIE A-Level exams:
Reduction with LiAlH₄
Lithium aluminium hydride in dry ether reduces the carbonyl C=O group to a CH₂ group, leaving the C-N bond intact.
+ Pros: Produces an amine with exactly the same number of carbon atoms as the starting amide
− Cons: Requires strictly anhydrous conditions
Dehydration with P₄O₁₀
Strong heating with the dehydrating agent phosphorus(V) oxide removes one molecule of water from the primary amide to form a nitrile.
+ Pros: High yield, one step conversion of amide to nitrile
− Cons: Requires very high temperatures
Predict the organic products of (i) reduction of benzamide with LiAlH₄ in dry ether, and (ii) dehydration of benzamide with P₄O₁₀
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Step 1: For LiAlH₄ reduction, convert the C=O group of benzamide (C₆H₅CONH₂) to CH₂, giving C₆H₅CH₂NH₂ (phenylmethanamine). No carbon atoms are lost.
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Step 2: For dehydration, remove one H₂O molecule from benzamide, giving C₆H₅CN (benzonitrile). The nitrile functional group has the same number of carbon atoms as the starting amide.
5. Common Pitfalls
Wrong move:
Stating alkaline amide hydrolysis directly produces a free carboxylic acid
Why:
Students incorrectly mix up amide hydrolysis with neutral acyl chloride hydrolysis
Correct move:
Explicitly note that alkaline hydrolysis first produces a carboxylate salt, and a separate acidification step is required to get the free carboxylic acid
Wrong move:
Naming secondary amides as N-acyl derivatives of the amine parent
Why:
Misapplying amine naming rules instead of amide naming conventions
Correct move:
Always select the longest carbon chain containing the amide functional group as the parent, with N-alkyl substituents listed first
Wrong move:
Claiming LiAlH₄ reduction of amides produces a primary alcohol
Why:
Confusing amide reduction with reduction of carboxylic acids or esters
Correct move:
Amide reduction converts C=O to CH₂, producing an amine with the same total number of carbon atoms as the starting amide
Wrong move:
Drawing the amide functional group as R-CO-NH₃⁺
Why:
Mixing up neutral amide structure with ammonium salt ions
Correct move:
The neutral amide has a covalent C-N single bond with partial double bond character, no full positive charge on the nitrogen atom
Wrong move:
Stating all amides are fully miscible with non-polar organic solvents
Why:
Ignoring the high polarity of the amide functional group
Correct move:
Only low molecular weight amides (up to 3 carbons) are miscible with water, higher molecular weight amides are insoluble in both water and non-polar solvents
6. Quick Reference Cheatsheet
Reaction | Reagents & Conditions | Organic Products |
|---|---|---|
Acid hydrolysis | Dilute strong acid, heat under reflux | Carboxylic acid + ammonium salt |
Alkaline hydrolysis | Aqueous NaOH, heat under reflux | Carboxylate salt + ammonia |
Reduction | LiAlH₄, dry ether, warm | Primary/secondary/tertiary amine |
Dehydration | P₄O₁₀, strong heat | Nitrile (from primary amide) |
When this came up on past exams
AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2023 · Paper 4
Amide hydrolysis reaction pathway
- 2022 · Paper 3
Practical amide preparation and purification
- 2021 · Paper 2
IUPAC naming of substituted amides
What's Next
Mastering amide chemistry completes your foundational knowledge of core carboxylic acid derivatives, and builds directly to high-weight CIE A-Level Paper 4 topics including amino acids, peptides, and condensation polymerisation. Amides are a very common intermediate in synthetic pathway questions, where you will be expected to interconvert between nitriles, carboxylic acids, and amines using the reactions covered in this module. To reinforce your understanding, practice applying these reaction rules to multi-step synthesis problems before moving to the related topics below.