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Chemistry · Practice book · Senior 6 · Unit 4 of 14

Derivatives of benzene

Senior 6 Chemistry · 14 questions with answers. Try each question on paper, then tap it to see the answer.

Unit 4 Derivatives of benzene

Key unit competence: Relate aromatic ketones, aldehydes, carboxylic acids, phenols and amines to their chemical reactivity and uses.

Key ideas

  • Substituents change the reactivity of the ring. Activating groups that release electrons (–OH, –NH2, –CH3) make substitution easier and direct to positions 2 and 4 (ortho/para). Deactivating groups that withdraw electrons (–NO2, –COOH, –CHO, –CN) make it harder and direct to position 3 (meta). Halogens deactivate but direct 2,4.
  • Phenol C6H5OH: weakly acidic (pKa 10; stronger than ethanol, weaker than carboxylic acids): reacts with NaOH but not with Na2CO3 or NaHCO3. With bromine water it gives a white precipitate of 2,4,6-tribromophenol at once; with neutral FeCl3 a violet colour.
  • Methylbenzene: side-chain oxidation by hot alkaline KMnO4 gives benzoic acid; with Cl2 in UV light the side chain is substituted (C6H5CH2Cl); with Cl2/AlCl3 the ring is substituted at 2 and 4.
  • Aromatic carbonyls: benzaldehyde C6H5CHO gives a silver mirror with Tollens' reagent (but no reaction with Fehling's), undergoes the Cannizzaro reaction (no α-hydrogen). Phenylethanone C6H5COCH3 gives the iodoform test (yellow CHI3). Both give an orange precipitate with 2,4-DNPH.
  • Benzoic acid C6H5COOH: white crystalline solid, slightly soluble in cold water; gives CO2 with NaHCO3; forms esters (e.g. methyl benzoate); slightly stronger acid than ethanoic acid. Sodium benzoate is a food preservative.
  • Phenylamine (aniline) C6H5NH2: made by reducing nitrobenzene with Sn and conc. HCl, then NaOH. A weaker base than ammonia (lone pair delocalised into the ring). With NaNO2/HCl at 0–5 °C it forms benzenediazonium chloride, which couples with phenol in alkali to give an orange azo dye; with bromine water: white 2,4,6-tribromophenylamine.

Common misconceptions

  • "Phenol is an alcohol, so it is neutral." No: The ring stabilises the phenoxide ion, so phenol is weakly acidic and reacts with NaOH.
  • "Phenylamine is a stronger base than ammonia because it is bigger." No: Its nitrogen lone pair is delocalised into the ring and less available, so it is a WEAKER base.
  • "Every aldehyde gives a red precipitate with Fehling's solution." No: Aromatic aldehydes such as benzaldehyde do not reduce Fehling's solution, though they give Tollens' test.

Worked examples

Example 1. Show how phenylamine is converted into an azo dye.

Solution

Dissolve phenylamine in excess conc. HCl and cool to 0–5 °C in ice.

Add cold sodium nitrite solution slowly: HCl and NaNO2 give nitrous acid, which forms benzenediazonium chloride C6H5N2+Cl−.

Add this cold solution to phenol dissolved in NaOH(aq).

An orange-yellow precipitate forms: 4-hydroxyazobenzene, C6H5–N=N–C6H4OH; the –N=N– link in a long delocalised system absorbs visible light, giving colour.

Example 2. Explain the relative acidity: ethanol < water < phenol < benzoic acid.

Solution

Ethanol: the ethyl group releases electrons, destabilising the C2H5O− ion.

Phenol: the negative charge of C6H5O− is spread into the ring (delocalised), so the ion is more stable.

Benzoic acid: the charge of C6H5COO− is delocalised over two oxygen atoms, more stable still.

The more stable the anion, the stronger the acid.

Practice questions

  1. Write the equation for phenol with sodium hydroxide.

    Show answer

    C6H5OH + NaOH → C6H5ONa + H2O

    Common mistake: Writing CO2 as a product.

  2. Give a chemical test to distinguish phenol from ethanol.

    Show answer

    Neutral FeCl3: violet with phenol, no change with ethanol (or bromine water: white ppt with phenol).

    Common mistake: Using sodium metal (both fizz).

  3. Name the reagent that oxidises methylbenzene to benzoic acid.

    Show answer

    Hot alkaline (or acidified) potassium manganate(VII), then acidify.

    Common mistake: Using Tollens' reagent.

  4. Classify –CH3, –NO2 and –OH as activating or deactivating.

    Show answer

    –CH3 activating; –NO2 deactivating; –OH activating.

    Common mistake: Calling –NO2 activating.

  5. Write the equation for the Cannizzaro reaction of benzaldehyde.

    Show answer

    2C6H5CHO + NaOH → C6H5CH2OH + C6H5COONa

    Common mistake: Forgetting that half is oxidised and half reduced.

  6. Give one use of phenol.

    Show answer

    Making antiseptics, plastics (phenol–methanal resins, bakelite), drugs (aspirin), dyes (any one).

    Common mistake: Saying it is a food.

  7. Why is phenylamine slightly soluble in water but very soluble in HCl?

    Show answer

    In HCl it forms the ionic salt phenylammonium chloride, which dissolves.

    Common mistake: Saying HCl reduces it.

  8. Name the product of benzoic acid with methanol and conc. H2SO4.

    Show answer

    Methyl benzoate, C6H5COOCH3.

    Common mistake: Calling it benzyl methanoate.

  9. What is seen when phenylamine is added to bromine water?

    Show answer

    Orange colour disappears and a white precipitate (2,4,6-tribromophenylamine) forms.

    Common mistake: Saying no reaction.

  10. Give a test that is positive for phenylethanone but negative for benzaldehyde.

    Show answer

    Iodoform test: I2/NaOH gives a yellow precipitate with phenylethanone.

    Common mistake: Using 2,4-DNPH (both positive).

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. 2,4-DNPH gives an orange precipitate with:a. its lone pair is delocalised into the ring
    2. Phenylamine is a weaker base than ammonia because:b. C6H5OH
    3. Phenol reacts with sodium hydroxide because it is:c. weakly acidic
    4. The formula of phenol is:d. benzaldehyde and phenylethanone
    Show answer

    1 → d (benzaldehyde and phenylethanone) · 2 → a (its lone pair is delocalised into the ring) · 3 → c (weakly acidic) · 4 → b (C6H5OH)

    • Test for carbonyl compounds.
    • Less available for H+.
    • Forms sodium phenoxide.
    • OH directly on the ring.

  2. Drag and drop

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

    nowhere  ·  2-nitro- and 4-nitromethylbenzene  ·  in the side chain  ·  benzoic acid
    1. With Cl2 in UV light, methylbenzene is chlorinated .
    2. Oxidising methylbenzene with hot alkaline KMnO4, then acidifying, gives .
    3. Nitration of methylbenzene gives mainly .
    Show answer

    1: in the side chain · 2: benzoic acid · 3: 2-nitro- and 4-nitromethylbenzene

    • Free-radical substitution of the CH3 group.
    • The side chain becomes –COOH.
    • Step 1. CH3 is activating.
      Step 2. It directs to positions 2 and 4.

  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. Phenylethanone gives a yellow precipitate with iodine and NaOH because it contains:a. carbon dioxide gas
    2. Benzoic acid reacts with sodium hydrogencarbonate to give:b. phenol
    3. Benzaldehyde gives a positive result with:c. Tollens' reagent
    4. Neutral iron(III) chloride gives a violet colour with:d. a CH3CO– group
    Show answer

    1 → d (a CH3CO– group) · 2 → a (carbon dioxide gas) · 3 → c (Tollens' reagent) · 4 → b (phenol)

    • Iodoform test.
    • It is a carboxylic acid.
    • Silver mirror.
    • Test for phenols.

  4. Drag and drop

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

    3  ·  food preservative  ·  0–5 °C  ·  room temperature with no acid
    1. Sodium benzoate is used as a .
    2. Diazotisation of phenylamine is carried out at .
    3. The –NO2 group on a benzene ring directs incoming groups to position .
    Show answer

    1: food preservative · 2: 0–5 °C · 3: 3

    • E211.
    • Diazonium salts decompose when warm.
    • It is deactivating.

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