Alkanals (Aldehydes) and Alkanones (Ketones) are two homologous series in the class of oxycarbons.
Functional Group
- Both alkanals and alkanones contain the carbonyl group (C=O).
Nomenclature of Alkanals
- The name of an alkanal is derived from the parent alkane by replacing the –e ending with –al.
- Examples:
- 1 Carbon: Methanal
- 2 Carbons: Ethanal
- 3 Carbons: Propanal
- 4 Carbons: Butanal
- 5 Carbons: Pentanal
General Formula of Alkanals
- The general formula for alkanals is: CnH2n+1CHO
Example Calculations:
- For n=0n = 0n=0: Methanal (HCHO)
- For n=1n = 1n=1: Ethanal (CH₃CHO)
- For n=4n = 4n=4: Butanal (C₄H₉CHO)
Molecular and Condensed Formulae of Alkanals
- Methanal: HCHO
- Ethanal: CH₃CHO
- Propanal: C₂H₅CHO
- Butanal: C₃H₇CHO
- Pentanal: C₄H₉CHO
Structural Formula of Alkanals
- In alkanals, at least one hydrogen atom is bonded to the carbonyl group. The other group is an alkyl group.
- General structure:
- Where RRR represents an alkyl group (e.g., CH₃−, CH₃CH₂−).
Structures of the First Five Alkanals:
- Methanal: HCHO
- Ethanal: CH₃CHO
- Propanal: C₂H₅CHO
- Butanal: C₃H₇CHO
- Pentanal: C₄H₉CHO
Nomenclature of Alkanones
- Alkanones are named by removing the –e ending from the parent alkane and replacing it with –one.
- The smallest alkanone is propanone.
Examples:
- 3 Carbons: Propanone
- 4 Carbons: Butanone
- 5 Carbons: Pentanone
- 6 Carbons: Hexanone
- 7 Carbons: Heptanone
General Formula of Alkanones
- The general formula for alkanones is: CnH2nO
Molecular Formulae of the First Five Alkanones:
- Propanone: C₃H₆O
- Butanone: C₄H₈O
- Pentanone: C₅H₁₀O
- Hexanone: C₆H₁₂O
- Heptanone: C₇H₁₄O
Structures of Alkanones
In alkanones, the carbonyl group (C=O) is bonded to two alkyl groups, which may be similar or different.
- General Structure:
- Where RRR and R′R’R′ represent alkyl groups.
Examples:
- Propanone:
- Structure: CH3COCH3
- Butanone:
- Structure: CH3COC2H5
- Pentan-2-one (or 2-Pentanone):
- Structure: CH3C(O)C2H5
- Hexan-2-one (or 2-Hexanone):
- Structure: CH3C(O)C3H7
- Heptan-2-one (or 2-Heptanone):
- Structure: CH3C(O)C4H9
Sources of Alkanals and Alkanones
Oxidation of Alkanols is the primary source of both alkanals and alkanones. Common oxidizing agents include acidified potassium permanganate (VII) and acidified potassium dichromate (VI).
- a. Oxidation of Primary Alkanols:
- Produces alkanals.
- b. Oxidation of Secondary Alkanols:
- Produces alkanones.
Physical Properties of Alkanals
- State: All alkanals are liquids except methanal, which is a gas.
- Appearance: They are colorless with characteristic smells.
- Solubility: Solubility in water decreases along the series; smaller alkanals are more soluble than larger ones.
- Melting and Boiling Points: Increase with relative molecular mass.
Chemical Properties of Alkanals
- Combustion: They burn in air to produce carbon dioxide and water.
Example:
2CH3CHO(l)+5O2(g)→4CO2(g)+4H2O(l) - Oxidation: Easily oxidized to alkanoic acids.
- Reduction: Can be reduced to primary alkanols.
- Litmus Test: Neutral to litmus paper.
- Addition Polymerization: They can undergo addition polymerization.
Physical Properties of Alkanones
- State: All alkanones are liquids.
- Polarity: They are polar due to the carbonyl group.
- Solubility: Solubility in water decreases along the series.
- Melting and Boiling Points: Increase with molecular mass.
Chemical Properties of Alkanones
- Combustion: They burn in air to produce carbon dioxide and water.
- Oxidation Resistance: Not easily oxidized due to the absence of hydrogen on the carbonyl group.
- Litmus Test: Neutral to litmus paper.
- Reduction: Can be reduced to secondary alkanols.
Distinguishing Alkanals and Alkanones Using Chemical Tests
- Fehling’s Test
Purpose: Confirms that alkanals are easily oxidized to alkanoic acids.
Procedure:
- Add 15 drops of Fehling’s solution to a clean test tube.
- Add about 15 drops of the test liquid.
- Heat the mixture gently for about 5 minutes.
Results:
- Positive Result: Blue Fehling’s solution turns to red precipitate.
- Interpretation: The test substance is either an alkanal or a reducing sugar.
- Negative Result: Blue color remains unchanged.
- Interpretation: The test liquid is an alkanone.
- Brady’s Test
Purpose: Detects the presence of the carbonyl group (–C=O) in a compound.
Reagent: 2,4-Dinitrophenylhydrazine (2,4-DNPH).
Procedure:
- Add 2 cm³ of the test liquid to 2 cm³ of 2,4-DNPH reagent.
- Shake the mixture well.
Results:
- Positive Result: Formation of an orange precipitate.
- Interpretation: The liquid could be either an alkanal or an alkanone since both contain the carbonyl group.
- Negative Result: No precipitate forms.
- Interpretation: The test indicates the absence of a carbonyl compound.
- Tollen’s Test
Purpose: Distinguishes between alkanals and alkanones based on their oxidation behavior.
Reagent: Tollen’s reagent (mixture of excess ammonia solution and silver nitrate solution).
Procedure:
- To 3 cm³ of Tollen’s reagent, add 2-3 drops of the test liquid.
Results:
- Positive Result: Silver mirror forms inside the test tube.
- Interpretation: The unknown liquid is an alkanal, as it reduces Tollen’s reagent to metallic silver.
- Negative Result: No silver mirror forms.
- Interpretation: The test liquid is an alkanone, which does not reduce Tollen’s reagent.
Summary of Distinguishing Tests
- Fehling’s Test: Tests for oxidation; blue to red indicates alkanal.
- Brady’s Test: Tests for carbonyl group; orange precipitate indicates presence but not differentiation.
- Tollen’s Test: Tests for reducing ability; silver mirror indicates alkanal.