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Chemistry · Practice book · Senior 5 · Unit 6 of 15

Carbonyl compounds: aldehydes and ketones

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

Unit 6 Carbonyl compounds: aldehydes and ketones

Key unit competence: Relate the chemical and physical properties of carbonyl compounds to their structures and uses.

Key ideas

  • The carbonyl group C=O is planar (sp2 carbon) and polar (Cδ+ = Oδ−). Aldehydes (RCHO, suffix -al: ethanal) have C=O at the end of the chain; ketones (RCOR', suffix -one: propan-2-one/propanone) have C=O within the chain.
  • Physical properties: dipole–dipole forces give higher boiling points than alkanes but lower than alcohols (no O–H hydrogen bonding); small members are water-soluble because C=O accepts hydrogen bonds from water.
  • Preparation: oxidation of primary alcohols (aldehydes, distil off) and secondary alcohols (ketones) with acidified K2Cr2O7; ozonolysis of alkenes; hydration of alkynes; Friedel–Crafts acylation for aromatic ketones.
  • Nucleophilic addition: HCN (with KCN catalyst) gives hydroxynitriles (cyanohydrins), adding a carbon; NaBH4 or LiAlH4 reduce aldehydes to primary and ketones to secondary alcohols; sodium hydrogensulfite gives crystalline addition products.
  • Condensation (addition–elimination) with 2,4-dinitrophenylhydrazine (Brady's reagent) gives orange/yellow precipitates (test for C=O); melting points of the derivatives identify the compound. Hydroxylamine gives oximes; hydrazine gives hydrazones.
  • Distinguishing tests (aldehydes are oxidised, ketones are not): Tollens' reagent ([Ag(NH3)2]+) gives a silver mirror; Fehling's/Benedict's solution gives a brick-red Cu2O precipitate; acidified dichromate turns green. Iodoform test (yellow CHI3) for CH3CO– (ethanal and methyl ketones). Uses: methanal (formalin, resins), propanone (solvent, nail-polish remover), flavourings (vanillin, benzaldehyde).

Common misconceptions

  • "Ketones give a silver mirror with Tollens' reagent." No: Ketones are not easily oxidised; only aldehydes reduce Tollens' reagent.
  • "2,4-DNPH distinguishes aldehydes from ketones." No: Both give orange precipitates; 2,4-DNPH tests for the C=O group in general.
  • "Carbonyl compounds undergo electrophilic addition like alkenes." No: The Cδ+ of C=O is attacked by nucleophiles: nucleophilic addition.

Worked examples

Example 1. Describe tests to distinguish between propanal, propanone and propan-1-ol.

Solution

2,4-DNPH: orange precipitate with propanal and propanone (carbonyls), none with propan-1-ol.

Tollens' reagent: silver mirror with propanal only (aldehyde).

Iodoform test: yellow precipitate with propanone (methyl ketone).

Propan-1-ol reacts with sodium (fizzing) and turns acidified dichromate green.

Example 2. Describe the mechanism of the addition of HCN to propanone.

Solution

CN− (from KCN catalyst) attacks the δ+ carbon of C=O; the π electrons move to oxygen, forming an alkoxide ion intermediate.

The negative oxygen takes H+ from HCN, regenerating CN−.

Product: 2-hydroxy-2-methylpropanenitrile, (CH3)2C(OH)CN.

Type: nucleophilic addition; the carbon chain is lengthened by one.

Practice questions

  1. Name CH3CH2COCH3.

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    Butanone (butan-2-one).

    Common mistake: Writing butanal.

  2. Give the reagent and observation for an aldehyde with Fehling's solution.

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    Warm with Fehling's: blue solution gives a brick-red precipitate.

    Common mistake: Saying silver mirror.

  3. Write the product of reducing propanone with NaBH4.

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    Propan-2-ol.

    Common mistake: Writing propan-1-ol.

  4. Why are small aldehydes and ketones soluble in water?

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    The C=O oxygen forms hydrogen bonds with water.

    Common mistake: Saying they are ionic.

  5. Name the type of reaction between carbonyls and 2,4-DNPH.

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    Condensation (addition–elimination).

    Common mistake: Saying oxidation.

  6. Give one use of methanal and one of propanone.

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    Methanal: formalin/resins; propanone: solvent.

    Common mistake: Swapping the uses.

  7. What is Tollens' reagent?

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    Ammoniacal silver nitrate, [Ag(NH3)2]+.

    Common mistake: Saying copper sulfate.

  8. Which carbonyl compounds give the iodoform test?

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    Ethanal and methyl ketones (CH3CO–).

    Common mistake: Saying all aldehydes.

  9. Write the formula of the product of HCN with methanal.

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    HOCH2CN (hydroxyethanenitrile).

    Common mistake: Writing CH3CN.

  10. Explain why ketones are not oxidised by Tollens' reagent.

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    There is no H on the carbonyl carbon; oxidation would need C–C bond breaking.

    Common mistake: Saying ketones are acidic.

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. Tollens' reagent gives a silver mirror with:a. a secondary alcohol
    2. Reduction of a ketone gives:b. preserve biological specimens
    3. HCN adds to carbonyl compounds by:c. aldehydes
    4. Methanal solution (formalin) is used to:d. nucleophilic addition
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    1 → c (aldehydes) · 2 → a (a secondary alcohol) · 3 → d (nucleophilic addition) · 4 → b (preserve biological specimens)

    • Aldehydes reduce Ag+ to Ag.
    • Two R groups remain.
    • CN− attacks Cδ+.
    • It kills microorganisms.

  2. Drag and drop

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

    benzaldehyde  ·  a brick-red precipitate  ·  ethanal and propanone  ·  ketone
    1. The iodoform test is positive for .
    2. Fehling's solution with an aldehyde gives .
    3. Propanone is a .
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    1: ethanal and propanone · 2: a brick-red precipitate · 3: ketone

    • CH3CO– group needed.
    • Copper(I) oxide forms.
    • C=O in the middle of the chain.

  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 functional group of aldehydes is:a. 2-hydroxypropanenitrile
    2. The product of HCN with ethanal is:b. sp2
    3. The carbon of the C=O group is hybridised:c. –CHO
    4. Propanone is commonly used as:d. a solvent such as nail-polish remover
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    1 → c (–CHO) · 2 → a (2-hydroxypropanenitrile) · 3 → b (sp2) · 4 → d (a solvent such as nail-polish remover)

    • Formyl group at the chain end.
    • CH3CH(OH)CN.
    • Planar, 120°.
    • Good organic solvent.

  4. Drag and drop

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

    oxygen is more electronegative than carbon  ·  ethanal  ·  bananas  ·  almonds
    1. Benzaldehyde gives the smell of .
    2. The IUPAC name of CH3CHO is .
    3. The C=O bond is polar because .
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    1: almonds · 2: ethanal · 3: oxygen is more electronegative than carbon

    • Aromatic aldehyde flavouring.
    • Two carbons, aldehyde.
    • Electrons pulled to O.

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