Amides
A concise revision guide to amides: the structures of primary and secondary (N-substituted) amides, their preparation from acyl chlorides with ammonia or amines, and the planar amide link that appears in polyamides and proteins.
Before You Start
Amides are made from acyl chlorides, so the chemistry of the acyl chloride is the starting point.
- An acyl chloride, RCOCl, has a very δ+ carbonyl carbon and reacts vigorously with nucleophiles at room temperature: Acyl Chlorides.
- The reaction of butylamine and phenylamine with ethanoyl chloride to give N-substituted (secondary) amides: Amines as Nucleophiles.
- The names ethanamide and N-methylethanamide and the amide group drawn displayed: Amines, Amides and Amino Acids: Structure and Naming.
- A condensation reaction joins two molecules and releases a small molecule, here HCl.
The Amide Group
An amide contains a carbonyl group bonded directly to a nitrogen atom: the –CONH– or –CONH₂ group. The nitrogen may carry hydrogen atoms or carbon groups.
| Class | General formula | Example | Condensed formula |
|---|---|---|---|
| primary amide | RCONH₂ | ethanamide | CH₃CONH₂ |
| N-substituted (secondary) amide | RCONHR′ | N-methylethanamide | CH₃CONHCH₃ |
| tertiary amide (one line only) | RCONR′R″ | N,N-dimethylethanamide | CH₃CON(CH₃)₂ |
- The “N-” in a name means the group is on the nitrogen, not on the carbon chain: N-methylethanamide has a two-carbon acid part and a methyl group on N.
- N-phenylethanamide, CH₃CONHC₆H₅, is the N-substituted (secondary) amide made from phenylamine and ethanoyl chloride.
- A nitrogen bonded to a carbonyl carbon is an amide; a nitrogen bonded only to alkyl or aryl groups and hydrogen is an amine.
Ethanamide and N-methylethanamide drawn displayed, with the nitrogen lone pair spread over the N–C=O group and the planar amide link labelled; propanamide in skeletal form.
Remember: Primary, secondary and tertiary for amides count the carbon groups on the nitrogen, exactly as for amines. Ethanamide is primary even though its carbon chain is two carbons long.
Quick Check: Amide or Not?
Check what each nitrogen atom is bonded to in these new formulae.
Why the Amide Link Is Planar and Not Basic
The lone pair on the amide nitrogen is not free. It is delocalised over the N–C=O group, so the C–N bond has partial double bond character.
- The C, O, N and the atoms attached to N lie in one plane, and rotation about the C–N bond is restricted.
- The lone pair cannot accept a proton, so amides are not basic like amines: an amide solution is almost neutral.
- The C=O oxygen is δ− and the N–H hydrogens are δ+, so amide molecules hydrogen bond strongly with each other and with water.
| Property | Ethylamine, CH₃CH₂NH₂ | Ethanamide, CH₃CONH₂ |
|---|---|---|
| state at room temperature | gas, boiling point 17 °C | white solid, melting point 82 °C |
| solution in water | alkaline | almost neutral |
| with dilute HCl | forms a salt | no salt at room temperature |
| hydrogen bonding | N–H···N, weak | N–H···O=C, two N–H bonds and a C=O |
Amide or amine? The lone pair, basicity, hydrogen bonding, state and solubility of ethanamide and ethylamine set side by side.
Exam focus: Primary amides are solids with high melting points because each molecule has two N–H bonds and a C=O to hydrogen bond with; small amines are gases or volatile liquids.
Check: Recognising and Naming Amides
Pick out the amide group in condensed formulae, classify amides as primary or N-substituted, and name them from the acid and the group on nitrogen.
Making Amides from Acyl Chlorides
An acyl chloride reacts with ammonia or with a primary amine at room temperature. The nitrogen lone pair attacks the δ+ carbonyl carbon and HCl is lost.
With ammonia: a primary amide
- CH₃COCl + NH₃ → CH₃CONH₂ + HCl: ethanoyl chloride gives ethanamide.
- The reaction is vigorous at room temperature and gives white fumes of HCl, which react with more ammonia to form white solid ammonium chloride.
- With excess ammonia the overall equation is CH₃COCl + 2NH₃ → CH₃CONH₂ + NH₄Cl.
With a primary amine: an N-substituted amide
- CH₃COCl + CH₃NH₂ → CH₃CONHCH₃ + HCl: methylamine gives N-methylethanamide.
- With butylamine: CH₃COCl + CH₃CH₂CH₂CH₂NH₂ → CH₃CONHCH₂CH₂CH₂CH₃ + HCl, an N-substituted amide named N-butylethanamide.
- With phenylamine: CH₃COCl + C₆H₅NH₂ → CH₃CONHC₆H₅ + HCl, N-phenylethanamide, a white solid.
- Excess amine takes the HCl as the ammonium salt: CH₃COCl + 2RNH₂ → CH₃CONHR + RNH₃⁺Cl⁻.
Ethanoyl chloride with ammonia and with methylamine: the lone pair attacks the carbonyl carbon, HCl is lost and the amide link (boxed) is formed.
- Naming: the acid part gives the parent (ethan-, propan-), the ending is -amide, and any group on nitrogen takes the prefix N-.
- A carboxylic acid and an amine do not give an amide at room temperature: an acid-base reaction gives the ammonium salt instead (extension).
Exam focus: Write the small molecule lost: HCl with one mole of ammonia or amine, NH₄Cl or RNH₃⁺Cl⁻ when the base is in excess. State “room temperature” as the condition.
Quick Check: Product and Maximum Amount
Name the product, then decide which reagent runs out first.
The Amide Link in Polymers and Proteins
The same –CONH– group joins the units of a polyamide such as nylon and the amino acids in a protein, where it is called the peptide link.
- Nylon-6,6 forms when a diamine and a dicarboxylic acid (or dioyl chloride) join through amide links with loss of water (or HCl): Condensation Polymers: Polyamides and Polyesters.
- A dipeptide forms when the –COOH of one amino acid and the –NH₂ of another join through an amide (peptide) link: Peptides and Proteins.
- Hydrogen bonding between the N–H of one chain and the C=O of another gives polyamides their strength and high melting temperatures.
Amides at a glance: the three classes, how each is made from an acyl chloride, and where the amide link appears in nylon and in proteins.
Worked Example: Spot the Amide
Answer. A is N-methylpropanamide, an N-substituted (secondary) amide, made from propanoyl chloride and methylamine with loss of HCl.
A second compound
Check: Preparing Amides
Give the reagents, conditions and products for making primary and N-substituted amides from acyl chlorides, and write the balanced equations.
Common Exam Points
- An amide has a carbonyl group bonded directly to nitrogen: RCONH₂ (primary) or RCONHR′ (N-substituted (secondary) amide).
- Acyl chloride + ammonia → primary amide + HCl; acyl chloride + primary amine → N-substituted (secondary) amide + HCl; room temperature, vigorous.
- With excess ammonia or amine the HCl is taken as the ammonium salt: CH₃COCl + 2NH₃ → CH₃CONH₂ + NH₄Cl.
- The amide nitrogen lone pair is delocalised over N–C=O, so the link is planar and the amide is not basic like an amine.
- Primary amides are solids with high melting points because of hydrogen bonding through two N–H bonds and a C=O.
- Name from the acid with the ending -amide; a group on nitrogen takes the prefix N- (N-methylethanamide).
FAQs
Use these quick answers to check the structure of primary and N-substituted amides, why they are not basic, how they are made from acyl chlorides and where the amide link appears.
What is the difference between a primary and an N-substituted amide?
A primary amide, RCONH₂, has two hydrogens on the nitrogen: ethanamide is CH₃CONH₂. In an N-substituted (secondary) amide one of those hydrogens is replaced by a carbon group: N-methylethanamide is CH₃CONHCH₃. Both contain the amide group, a C=O bonded directly to N.
Why are amides not basic like amines?
The nitrogen lone pair is delocalised over the N–C=O system, giving the C–N bond partial double bond character and making the amide link planar. The lone pair is therefore not available to accept a proton, so amides are effectively neutral in water.
How do I make a primary amide?
React an acyl chloride with ammonia at room temperature: CH₃COCl + NH₃ → CH₃CONH₂ + HCl. The reaction is vigorous and gives white fumes; with excess ammonia the HCl is captured as ammonium chloride, CH₃COCl + 2NH₃ → CH₃CONH₂ + NH₄Cl.
Why do amides have high melting points?
Each primary amide molecule has two N–H bonds and a C=O group, so it forms several hydrogen bonds to its neighbours. Ethanamide melts at 82 °C and is a solid at room temperature, while ethylamine is a gas; amides also dissolve in water.
Where does the amide link appear outside simple amides?
The –CONH– link joins the monomers in polyamides such as nylon-6,6 and joins amino acids in peptides and proteins, where it is called the peptide link.
Spotting a C=O bonded directly to an N–H in a long chain tells you the polymer is a polyamide.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.
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