DONATSciences and Maths
Chemistry · Practice book · Senior 5 · Unit 9 of 15

Amines and amino acids

Senior 5 Chemistry · 14 questions with answers. Try each question on paper, then tap it to see the answer.

Unit 9 Amines and amino acids

Key unit competence: Evaluate the relationship between the molecular structure of amines and amino acids, their physical and chemical properties and their uses.

Key ideas

  • Amines are derivatives of ammonia: primary (RNH2), secondary (R2NH), tertiary (R3N) and quaternary ammonium salts (R4N+). Named as alkylamines (ethylamine) or aminoalkanes; phenylamine (C6H5NH2) is aromatic.
  • Physical properties: small amines are gases or volatile liquids with fishy smells, soluble in water (hydrogen bonding); boiling points lower than alcohols of similar mass (N–H...N bonds are weaker).
  • Amines are bases (lone pair on N): CH3NH2 + H2O ⇌ CH3NH3+ + OH−. Basic strength: secondary ≈ primary > ammonia > phenylamine (alkyl groups donate electrons; in phenylamine the lone pair is delocalised into the ring).
  • Preparation: halogenoalkane + excess ammonia (heat, sealed tube); reduction of nitriles or amides (LiAlH4); reduction of nitrobenzene (Sn/conc. HCl) for phenylamine. Reactions: with acids → salts; with acyl chlorides → amides; with halogenoalkanes → higher amines; phenylamine + nitrous acid (below 10 °C) → diazonium salt → azo dyes.
  • Amino acids contain NH2 and COOH on the same carbon (H2N-CHR-COOH); glycine (R = H) is the simplest; all others are chiral (optically active, L-form in nature). In water and in the solid they exist as zwitterions (H3N+-CHR-COO−), giving high melting points and water solubility; they are amphoteric; the pH at which they have no net charge is the isoelectric point.
  • Two amino acids join by a condensation reaction forming a peptide (amide) bond (–CO–NH–) and water: dipeptide, polypeptide, protein. Proteins are hydrolysed by acid or enzymes back to amino acids (separated by chromatography and detected with ninhydrin). Uses: amines in dyes, drugs and nylon; amino acids as building blocks of proteins, food supplements.

Common misconceptions

  • "Phenylamine is a stronger base than ammonia because it is bigger." No: Phenylamine is weaker: its N lone pair is delocalised into the benzene ring.
  • "Amino acids exist mainly as neutral molecules in the solid." No: They exist as zwitterions, which is why they have high melting points.
  • "Peptide bonds form by addition with no by-product." No: Peptide bonds form by condensation, eliminating water.

Worked examples

Example 1. Explain the relative basic strengths of ethylamine, ammonia and phenylamine.

Solution

Basicity depends on how available the N lone pair is to accept H+.

Ethylamine: the ethyl group donates electrons (+I effect), increasing electron density on N: stronger base than ammonia.

Phenylamine: the lone pair is delocalised into the benzene ring π system: less available, weaker base than ammonia.

Order: phenylamine < ammonia < ethylamine.

Example 2. Describe the formation and hydrolysis of the dipeptide alanylglycine.

Solution

Formation: the –COOH of alanine reacts with the –NH2 of glycine; water is eliminated (condensation): H2NCH(CH3)CONHCH2COOH.

The –CO–NH– link is a peptide (amide) bond.

Hydrolysis: heat with 6 mol/dm3 HCl (or use enzymes such as proteases); water adds across the peptide bond, giving back alanine and glycine.

Practice questions

  1. Classify: CH3NH2, (CH3)2NH, (CH3)3N.

    Show answer

    Primary, secondary, tertiary.

    Common mistake: Classifying by number of carbons.

  2. Write the equation for methylamine with water.

    Show answer

    CH3NH2 + H2O ⇌ CH3NH3+ + OH−.

    Common mistake: Writing CH3NH2 → CH3NH− + H+.

  3. Why are small amines soluble in water?

    Show answer

    They form hydrogen bonds with water.

    Common mistake: Saying they are ionic.

  4. Write the zwitterion of glycine.

    Show answer

    H3N+CH2COO−.

    Common mistake: Writing H2NCH2COOH only.

  5. What is a peptide bond?

    Show answer

    The amide link –CO–NH– between amino acids.

    Common mistake: Saying an ester link.

  6. Give one use of amines.

    Show answer

    Dyes, drugs, nylon manufacture, surfactants (any one).

    Common mistake: Saying fertiliser only.

  7. Write the product of ethylamine with ethanoyl chloride.

    Show answer

    N-ethylethanamide.

    Common mistake: Writing ethyl ethanoate.

  8. Why is alanine optically active?

    Show answer

    Its α-carbon has four different groups (H, CH3, NH2, COOH).

    Common mistake: Saying it contains nitrogen.

  9. How are amino acids from a protein separated and identified?

    Show answer

    Hydrolyse; paper chromatography; spray ninhydrin; compare Rf values.

    Common mistake: Using distillation.

  10. State why amino acids are amphoteric.

    Show answer

    They have a basic –NH2 group and an acidic –COOH group.

    Common mistake: Saying they are neutral.

Matching and drag-and-drop

  1. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Amines are basic because:a. detect amino acids on chromatograms (purple spots)
    2. Amines smell:b. the nitrogen has a lone pair that accepts H+
    3. The isoelectric point of an amino acid is the pH at which it:c. fishy
    4. Ninhydrin is used to:d. has no net charge
    Show answer

    1 → b (the nitrogen has a lone pair that accepts H+) · 2 → c (fishy) · 3 → d (has no net charge) · 4 → a (detect amino acids on chromatograms (purple spots))

    • Lewis/Brønsted base.
    • Rotting fish contains amines.
    • Exists mainly as the zwitterion.
    • Locating reagent.

  2. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    water  ·  H3N+CH2COOH  ·  peptide (amide) bond  ·  ammonia
    1. Joining two amino acids eliminates .
    2. The bond joining amino acids in proteins is the .
    3. In acid solution (low pH), glycine exists mainly as .
    Show answer

    1: water · 2: peptide (amide) bond · 3: H3N+CH2COOH

    • Condensation.
    • –CO–NH–.
    • Step 1. Acid protonates –COO−.
      Step 2. Net positive charge.

  3. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. The simplest amino acid is:a. Ethylamine
    2. Amino acids have high melting points because:b. they exist as zwitterions with ionic attractions
    3. Which is the strongest base?c. secondary amine
    4. CH3NHCH3 is a:d. glycine
    Show answer

    1 → d (glycine) · 2 → b (they exist as zwitterions with ionic attractions) · 3 → a (Ethylamine) · 4 → c (secondary amine)

    • R = H.
    • Strong electrostatic forces.
    • Electron-donating ethyl group.
    • Two alkyl groups on N.

  4. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    ethylammonium chloride  ·  ethanamide  ·  tin and concentrated HCl  ·  phenols or aromatic amines
    1. Phenylamine is prepared by reducing nitrobenzene with .
    2. Azo dyes are formed by coupling diazonium salts with .
    3. Ethylamine reacts with hydrochloric acid to form .
    Show answer

    1: tin and concentrated HCl · 2: phenols or aromatic amines · 3: ethylammonium chloride

    • Then add NaOH.
    • Coloured –N=N– compounds.
    • Salt formation.

Take the Unit 9 quiz → · Back to contents