19.4 Tests Used to Distinguishing Aldehydes and Ketones

Aldehydes and ketones are two classes of organic compounds containing the carbonyl group ($>C=O$). While they share some similarities, their reactivity towards oxidation differs significantly. This difference forms the basis for simple chemical tests to distinguish between them.

Mild oxidizing agents can easily oxidize aldehydes to carboxylic acids (or their corresponding carboxylate salts in basic conditions). Ketones, however, are resistant to oxidation by these same weak oxidizing agents because it would require breaking a strong carbon-carbon bond. Common reagents used for this purpose include Tollen's reagent, Fehling's solution, and Benedict's solution.

Test 1: Fehling's Solution Test

This test uses a mild oxidizing agent to differentiate aldehydes from ketones.

• Reagent: Fehling's solution is a freshly prepared alkaline solution containing copper(II) ions ($C{u}^{2+}$) complexed with tartrate ions.

• Principle: Aldehydes are oxidized to carboxylate anions, while the $C{u}^{2+}$ ions in the Fehling's solution are reduced to copper(I) ions ($C{u}^{+}$).

• Observation: When an aldehyde is warmed with Fehling's solution, a brick-red precipitate of copper(I) oxide ($C{u}_{2}O$) is formed.

• Result for Ketones: Ketones do not react with Fehling's solution, so the blue color of the solution remains unchanged.

• Note: Aromatic aldehydes (e.g., benzaldehyde) also do not give a positive Fehling's test — only aliphatic aldehydes react.

The reaction with an aldehyde (e.g., acetaldehyde) is as follows:

$C{H}_{3}CHO+2C{u}^{2+}+5O{H}^{-}\to \underset{\text{(Brick-Red Precipitate)}}{C{u}_{2}O\downarrow }+C{H}_{3}CO{O}^{-}+3H_{2}O$

In this reaction:

• Oxidation: Acetaldehyde ($C{H}_{3}CHO$) is oxidized to the acetate ion ($C{H}_{3}CO{O}^{-}$).
• Reduction: The copper(II) ion ($C{u}^{2+}$) is reduced to the copper(I) ion ($C{u}^{+}$) in $C{u}_{2}O$.

Test 2: Tollen's Test (Silver Mirror Test)

This is another classic test used to detect the presence of an aldehyde functional group.

• Reagent: Tollen's reagent is an ammoniacal solution of silver nitrate. It is prepared by adding aqueous ammonia to silver nitrate solution until the initial precipitate of silver oxide dissolves, forming the diamminesilver(I) complex ion, $[Ag(N{H}_3{)}_2{]}^{+}$.

• Principle: The aldehyde is oxidized to a carboxylate anion, while the silver(I) ions ($A{g}^{+}$) in the Tollen's reagent are reduced to metallic silver ($Ag$).

• Observation: When an aldehyde is warmed with Tollen's reagent in a clean glass test tube, a shiny silver mirror is deposited on the inner surface of the test tube.

• Result for Ketones: Ketones do not give a positive Tollen's test.

The reaction with an aldehyde (e.g., acetaldehyde) is:

$C{H}_{3}CH{O}_{(l)}+2[Ag(N{H}_3{)}_2{]}_{(aq)}^{+}+3O{H}_{(aq)}^{-}\to C{H}_{3}CO{O}_{(aq)}^{-}+\underset{\text{(Silver Mirror)}}{2A{g}_{(s)}\downarrow }+4N{H}_{3(aq)}+2H_2{O}_{(l)}$

Test 3: 2,4-Dinitrophenylhydrazine (2,4-DNPH) Test

This test is used to detect the presence of the carbonyl group ($C=O$) in both aldehydes and ketones. Unlike Fehling's and Tollen's tests, it does not distinguish between the two classes.

• Reagent: 2,4-Dinitrophenylhydrazine (Brady's reagent) in acidic solution.
• Reaction: The carbonyl compound undergoes a condensation reaction with 2,4-DNPH to form a 2,4-dinitrophenylhydrazone derivative.
• Observation: A positive result is indicated by the formation of a yellow, orange, or red crystalline precipitate.
• Significance: The melting point of the precipitate can be measured to help identify the specific aldehyde or ketone.

General reaction: $R_{2}C=O+(O_{2}N{)}_2{C}_6{H}_{3}NHN{H}_2\to R_{2}C=N-NH-C_6{H}_3(N{O}_2{)}_2\downarrow +H_{2}O$

Test 4: Iodoform Test (Haloform Reaction)

This test identifies the presence of a methyl ketone group ($C{H}_3-C=O$). It can also be applied to certain alcohols like ethanol and secondary alcohols with a methyl group on the carbinol carbon (which are first oxidized to methyl ketones).

• Reagent: Iodine in the presence of Sodium Hydroxide ($I_2/NaOH$).
• Observation: Formation of a yellow crystalline precipitate of iodoform ($CH{I}_3$) with a characteristic medicinal smell.
• Compounds giving positive iodoform test: Acetaldehyde ($C{H}_{3}CHO$), acetone ($C{H}_{3}COC{H}_3$), ethanol ($C_2{H}_{5}OH$), and any methyl ketone ($C{H}_{3}COR$).
• Compounds NOT giving positive iodoform test: Ketones without a $C{H}_{3}CO-$ group (e.g., 3-pentanone), and most other aldehydes.

The overall reaction: $C{H}_{3}COR+3I_2+4NaOH\to \underset{\text{(Yellow precipitate)}}{CH{I}_3\downarrow }+RCOONa+3NaI+3H_{2}O$

Summary Table

Significance: These tests are fundamental in qualitative organic analysis for identifying the presence of an aldehyde functional group in an unknown compound.