KETONES at a glance

 

Definition

Ketones are organic compounds that contain the carbonyl group (C=O) bonded to two alkyl or aryl groups.
The carbonyl carbon is not attached to a hydrogen atom.

General Formula

R–CO–R′

Where R and R′ are alkyl or aryl groups.

📌 Important point: Ketones must have at least three carbon atoms.

Functional Group

• Carbonyl group (C=O)

• Located within the carbon chain, not at the end.

Nomenclature (IUPAC Naming)

Ketones are named by:

• Replacing the ending –e in the corresponding alkane name with –one

• Indicating the position of the carbonyl group if necessary

Examples:

• Propanone (acetone) → CH₃COCH₃

• Butanone → CH₃COCH₂CH₃

• Pentan-2-one

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Preparation of Ketones

1. Oxidation of Secondary Alcohols
   Secondary alcohol + [O] → Ketone

Example:

        Propan-2-ol → Propanone

2. Dry Distillation of Calcium Salts of Carboxylic Acids

3. Hydration of Alkynes (acid-catalysed)

Physical Properties

• Colourless liquids (lower members)

• Pleasant or sharp smell

• Boiling points higher than alkanes but lower than alcohols

• Soluble in water (lower members like propanone)

• Polar due to the C=O group

Chemical Properties

1. Resistance to Oxidation

• Ketones are not easily oxidized

• Do not react with:

  • Tollens’ reagent

  • Fehling’s solution

📌 Important distinction from aldehydes

2. Reduction

• Reduced to secondary alcohols

3. Addition Reactions

• React with hydrogen cyanide (HCN)

• React with sodium hydrogen sulphite

Laboratory Tests for Ketones

1. 2,4-Dinitrophenylhydrazine (2,4-DNPH) Test

• Yellow or orange precipitate

• Confirms presence of carbonyl group

2. Iodoform Test

• Positive for methyl ketones

• Yellow precipitate of iodoform (CHI₃)

Example:

• Propanone gives a positive iodoform test

Uses of Ketones

• Solvents (e.g. acetone)

• Nail polish remover

• Paint and varnish industries

• Pharmaceutical production

• Plastic and fibre manufacture

Differences Between Aldehydes and Ketones

Aldehydes	Ketones
–CHO group	–CO– group
Easily oxidized	Not easily oxidized
Positive Tollens’ test	Negative Tollens’ test
At end of chain	In the middle of chain

Examples of Ketones

• Propanone (acetone)

• Butanone

• Cyclohexanone

WAEC / NECO EXAM TIPS

• Ketones do not reduce Tollens’ or Fehling’s solution

• Oxidation of secondary alcohols gives ketones

• Methyl ketones give a positive iodoform test

OBJECTIVE QUESTIONS

1. Ketones are organic compounds that contain the functional group
   A. –OH
   B. –COOH
   C. –CHO
   D. –CO–

2. The general formula of ketones is
   A. R–CHO
   B. R–COOH
   C. R–CO–R′
   D. R–OH

3. Which of the following is the simplest ketone?
   A. Methanone
   B. Ethanal
   C. Propanone
   D. Ethanoic acid

4. Ketones differ from aldehydes because ketones
   A. are easily oxidized
   B. reduce Fehling’s solution
   C. have the carbonyl group at the end of the chain
   D. have the carbonyl group within the chain

5. Which reagent can be used to distinguish between an aldehyde and a ketone?
   A. Bromine water
   B. Tollens’ reagent
   C. Sodium hydroxide
   D. Dilute acid

6. Ketones do NOT react with
   A. Hydrogen cyanide
   B. Sodium hydrogen sulphite
   C. 2,4-dinitrophenylhydrazine
   D. Tollens’ reagent

7. The oxidation of secondary alcohols produces
   A. aldehydes
   B. ketones
   C. alkanes
   D. carboxylic acids

8. Which of the following ketones gives a positive iodoform test?
   A. Butanone
   B. Pentanone
   C. Cyclohexanone
   D. Benzophenone

9. The yellow precipitate formed in the iodoform test is
   A. CHCl₃
   B. CHBr₃
   C. CHI₃
   D. C₂H₅I

10. Ketones can be reduced to form
    A. primary alcohols
    B. secondary alcohols
    C. tertiary alcohols
    D. carboxylic acids

11. Which of the following reagents gives an orange precipitate with ketones?
    A. Fehling’s solution
    B. Tollens’ reagent
    C. 2,4-dinitrophenylhydrazine
    D. Benedict’s solution

12. The boiling points of ketones are generally
    A. lower than alkanes
    B. equal to alcohols
    C. higher than alkanes but lower than alcohols
    D. higher than carboxylic acids

13. Propanone is commonly known as
    A. formaldehyde
    B. acetaldehyde
    C. acetone
    D. acetic acid

14. Which of the following is NOT a use of ketones?
    A. Solvent
    B. Nail polish remover
    C. Paint manufacture
    D. Fuel for engines

15. Ketones are best described as
    A. non-polar compounds
    B. ionic compounds
    C. polar compounds
    D. basic compounds

THEORY QUESTIONS