Grade by Grade
B. Organic Chemistry · Organic Chemistry

Amines and Amides: Nitrogen Compounds

B2.2 B3.1 B3.2 B3.3 B1.1About 70 minutes
Learning goals
  • I can name and draw amines and classify them as primary, secondary or tertiary.
  • I can explain the boiling points, solubility, odour and basic properties of amines.
  • I can name and draw amides and write equations for their formation by condensation and their hydrolysis.
  • I can identify amide links in proteins, nylon and medicines.

Words to know

amine
An organic compound in which one or more H atoms of ammonia (NH₃) are replaced by carbon groups.
primary, secondary, tertiary amine
An amine with one, two or three carbon groups bonded to the nitrogen atom.
amide
A compound with a carbonyl group bonded to a nitrogen atom: R–CO–NR′₂ (R′ may be H).
amide (peptide) link
The –CO–NH– group that joins units in proteins and nylon.
base (Brønsted-Lowry)
A substance that accepts H⁺. Amines are weak bases because the N lone pair can accept H⁺.
ammonium salt
An ionic compound formed when an amine (or ammonia) accepts H⁺ from an acid, such as methylammonium chloride, CH₃NH₃⁺Cl⁻.

The nitrogen families

The fishy smell of old fish comes mostly from trimethylamine, an amine. The proteins in that same fish are held together by amide links, and so is the nylon in a tent or a backpack. Both families contain nitrogen.

Amines are relatives of ammonia, NH₃. Replace one, two or three H atoms of ammonia with carbon groups and you get a primary (1°), secondary (2°) or tertiary (3°) amine.

Careful: the classification is different from alcohols. For amines, count the carbons bonded to the nitrogen. For alcohols, you counted the carbons bonded to the C–OH carbon.

NHHHammoniaNH₃NHHCH₃primary (1°)methanamineNCH₃HCH₃secondary (2°)N-methylmethanamineNCH₃CH₃CH₃tertiary (3°)N,N-dimethylmethanaminecount the carbons bonded to N: 1, 2 or 3
Amines are classified by the number of carbon groups bonded to nitrogen. Each N also has a lone pair (not drawn), which makes amines basic.

Naming amines

Primary amines: replace the final -e of the alkane with -amine, and give the carbon bonded to N the lowest number: CH₃NH₂ is methanamine; CH₃CH₂CH₂NH₂ is propan-1-amine; CH₃CH(NH₂)CH₃ is propan-2-amine.

Secondary and tertiary amines: the largest carbon group attached to N is the parent amine. The other groups are named as substituents with the locant N- (meaning "on the nitrogen"): CH₃CH₂–NH–CH₃ is N-methylethanamine; (CH₃)₂N–CH₂CH₃ is N,N-dimethylethanamine.

Common names list the groups alphabetically plus -amine, as one word: methylamine, ethylmethylamine, trimethylamine. When the –NH₂ is a substituent on a chain with a higher-priority group, it is called amino-, as in the amino acids. Aminobenzene, C₆H₅NH₂, is usually called aniline.

Worked example

Example 1: Naming amines

Name and classify (a) CH₃CH₂CH₂CH₂NH₂, (b) CH₃CH₂NHCH₂CH₃, (c) (CH₃)₂NCH₂CH₂CH₃.

  1. (a) 4 C chain, N on C1, one carbon group on N → butan-1-amine, primary.
  2. (b) Two ethyl groups on N. Parent: ethanamine; the other ethyl is an N-substituent → N-ethylethanamine (diethylamine), secondary.
  3. (c) Groups on N: propyl (largest, parent) and two methyls → N,N-dimethylpropan-1-amine, tertiary.

(a) butan-1-amine (1°), (b) N-ethylethanamine (2°), (c) N,N-dimethylpropan-1-amine (3°).

Properties of amines

  • Hydrogen bonding: N–H bonds are polar, so primary and secondary amines hydrogen-bond to each other. Nitrogen is less electronegative than oxygen, so these hydrogen bonds are weaker than in alcohols, and amines boil lower than alcohols of similar mass but higher than alkanes.
  • Tertiary amines have no N–H, so they cannot hydrogen-bond to each other and boil lower still.
  • Solubility: the N lone pair accepts hydrogen bonds from water, so small amines (up to about 5 or 6 carbons) dissolve well.
  • Odour: small amines smell fishy or like ammonia; some larger ones smell of rotting.
  • Basicity: the lone pair on N can accept H⁺, so amines are weak bases, like ammonia:

CH₃NH₂ + H₂O ⇌ CH₃NH₃⁺ + OH⁻

With acids, amines form ammonium salts: CH₃NH₂ + HCl → CH₃NH₃⁺Cl⁻ (methylammonium chloride). The salts are ionic, odourless and water-soluble. Many medicines that contain amines are sold as hydrochloride salts for this reason (look for "hydrochloride" on cold-medicine labels).

0 °C−42 °Cpropane44 g/mol3 °Ctrimethylamine59 g/mol, 3°48 °Cpropan-1-amine59 g/mol, 1°97 °Cpropan-1-ol60 g/mol
Boiling points of molecules with similar masses. The primary amine hydrogen-bonds through N–H, so it boils far above propane, but N–H···N bonds are weaker than O–H···O, so it boils below the alcohol. The tertiary amine has no N–H and boils lowest of the three nitrogen and oxygen compounds.

Amides

An amide has a carbonyl group bonded to a nitrogen: R–CO–NH₂, or R–CO–NHR′ or R–CO–NR′R″.

Naming: the part with the C=O is named from the acid, changing -oic acid to -amide: CH₃CONH₂ is ethanamide. Groups on the nitrogen get the locant N-: CH₃CONHCH₃ is N-methylethanamide; HCON(CH₃)₂ is N,N-dimethylmethanamide (DMF, an industrial solvent).

Formation (condensation): a carboxylic acid and an amine (or ammonia), heated together, join and release water:

CH₃COOH + CH₃NH₂ → CH₃CONHCH₃ + H₂O

The new C–N bond links the acid part and the amine part. This amide link, –CO–NH–, is called a peptide bond in proteins.

Properties:

  • Amides with N–H bonds hydrogen-bond very strongly, so most amides are solids at room temperature: ethanamide melts at about 80 °C and boils at 221 °C.
  • Small amides dissolve well in water.
  • Unlike amines, amides are not basic: the N lone pair is drawn toward the neighbouring C=O and is not available to accept H⁺.

Hydrolysis: heating an amide with acid or base breaks the C–N bond and gives back the carboxylic acid (or its salt) and the amine (or its ammonium salt). Digestion of proteins in your stomach and intestines is hydrolysis of amide links, helped by enzymes.

CH₃COOHethanoic acid+HNHCH₃methanamineheatCH₃CONHCH₃N-methylethanamide + H₂Oamide link
Forming an amide. The OH of the acid and an H of the amine leave as water, and a new C–N bond forms. The highlighted –CO–NH– group is the amide link.
Worked example

Example 2: Amide formation and naming

Write the equation for the reaction of propanoic acid with ethanamine, and name the product.

  1. Acid: CH₃CH₂COOH. Amine: CH₃CH₂NH₂.
  2. Remove OH from the acid and H from the N; join C to N.
  3. CH₃CH₂COOH + H₂NCH₂CH₃ → CH₃CH₂CONHCH₂CH₃ + H₂O.
  4. Name: acid part propanamide; ethyl on N.

N-ethylpropanamide, CH₃CH₂CONHCH₂CH₃ (+ water).

Worked example

Example 3: Hydrolysis

Give the products of hydrolysing N-methylbutanamide (a) with aqueous NaOH, (b) with aqueous HCl.

  1. N-methylbutanamide: CH₃CH₂CH₂CONHCH₃. Breaking the C–N bond gives butanoic acid and methanamine.
  2. (a) In base, the acid becomes its salt: CH₃CH₂CH₂COONa + CH₃NH₂.
  3. (b) In acid, the amine becomes its ammonium salt: CH₃CH₂CH₂COOH + CH₃NH₃⁺Cl⁻.

(a) sodium butanoate + methanamine; (b) butanoic acid + methylammonium chloride.

Amides and amines in your life

  • Proteins are long chains of amino acids joined by amide (peptide) links. Insulin, discovered at the University of Toronto in 1921–1922 by Frederick Banting, Charles Best, James Collip and John Macleod, is a small protein of 51 amino acids.
  • Nylon and Kevlar are synthetic polyamides (Lesson 15).
  • Urea, (NH₂)₂CO, the compound Wöhler made in 1828, is a diamide and one of the world's most widely used nitrogen fertilizers.
  • Medicines: acetaminophen (Tylenol) contains an amide group; many others, such as the decongestant pseudoephedrine, contain amine groups and are sold as hydrochloride salts.
  • Some amines are toxic or addictive, and some, such as the neurotransmitters dopamine and serotonin, are made by your own body.
Hands-on activity

Why lemon goes with fish

You need: a small can of tuna or a piece of raw fish, two small bowls, a lemon (or bottled lemon juice), a knife and cutting board, and a grown-up. Grown-up note: an adult should cut the lemon. Wash hands after handling raw fish and refrigerate or cook the fish afterwards.

  1. Put a spoonful of fish in each bowl. Waft each one and rate the fishy smell from 0 (none) to 5 (strong).
  2. Squeeze lemon juice over the fish in one bowl only. Wait 2 minutes.
  3. Waft both bowls again and rate them. Ask another person to rate them too, without telling them which bowl had lemon (a "blind" test reduces bias, A1.8).
  4. Write the equation for trimethylamine, (CH₃)₃N, reacting with H⁺ from citric acid.
  5. Explain the change in smell.

What to look for: the lemon bowl should smell less fishy. The acid gives H⁺ to the amine: (CH₃)₃N + H⁺ → (CH₃)₃NH⁺. The trimethylammonium ion is part of an ionic salt that does not evaporate, so less of it reaches your nose.

Just for fun

Lemon wedges served with fish are not just decoration: they are a portable acid-base neutralization kit for smelly amines.

Your turn

1

Name CH₃CH₂NH₂ and classify it.

2

Name (CH₃)₃N.

3

Draw (condensed) N-methylpropan-1-amine.

4

Name CH₃CH₂CONH₂.

5

Write the equation for methanamine + hydrochloric acid.

6

Which acid and amine make N-ethylethanamide?

7

Why does propan-1-amine boil higher than trimethylamine, even though they have the same formula (C₃H₉N)?

More practice: warm-up

1

What element do all amines and amides contain?

2

What ending is used for an amine? For an amide?

3

Are amines acidic, basic or neutral?

4

What is the amide link in proteins called?

5

What small molecule forms when an acid and an amine make an amide?

More practice: on your own

1

Name CH₃CH₂CH(NH₂)CH₃ and classify it.

2

Name HCONHCH₂CH₃.

3

Rank by boiling point: ethanamide, ethanamine, ethanol. Explain.

4

Write the equation for the hydrolysis of ethanamide with NaOH.

5

Classify each as amine, amide, or both: (a) acetaminophen has –NH–CO–CH₃ on a benzene ring; (b) the amino acid glycine, H₂NCH₂COOH.

Challenge

1

Two glycine molecules (H₂NCH₂COOH) join to form a dipeptide. Write the condensed formula of the product, circle (describe) the amide link, and state the other product.

2

Explain why amides are neutral while amines are basic, even though both have a nitrogen with a lone pair.

3

An unknown C₂H₇N compound boils at 7 °C. Its isomer boils at 17 °C. Identify both and explain.

Check your understanding

Quiz

1.

Which compound is a secondary amine?

2.

What is the IUPAC name of CH₃CONH₂?

3.

Which product forms when CH₃NH₂ reacts with HCl?

4.

Why does trimethylamine have a lower boiling point than propan-1-amine?

5.

What type of reaction joins a carboxylic acid and an amine into an amide?

6.

Which link holds amino acids together in proteins?

For parents and teachers

Big ideas (B2.2, B3.2, B3.3): amines (R–NH₂ and relatives) are weak bases named with -amine and classified by the number of carbons on N. Amides (R–CO–NH–) form by condensation of acids and amines, are named with -amide, are neutral, and form the links in proteins and nylon.

How to help: Ask your teen to explain the lemon-and-fish result using acid-base chemistry. Look at a medicine label together for the word "hydrochloride" and ask what it means.

Common difficulties:

  • Classifying amines like alcohols (count carbons on the N, not on a carbon).
  • Confusing amines and amides (an amide's N is bonded to a C=O).
  • Forgetting the N- locant for groups on nitrogen.
  • Expecting amides to be basic.

Safety: wash hands after handling raw fish.