19.3 REACTIONS OF ALDEHYDES AND KETONES

Aldehydes and ketones undergo several types of reactions, with the most characteristic being nucleophilic addition to the carbon-oxygen double bond.

Nucleophilic Addition Reactions

The polar nature of the carbonyl group ($>C=O$), with a partial positive charge ($\delta +$) on the carbon and a partial negative charge ($\delta -$) on the oxygen, makes the carbonyl carbon an electrophile. This allows it to be attacked by nucleophiles.

Reactivity Order: Formaldehyde > Aldehydes > Ketones

Aldehydes are more reactive than ketones because:

1. Less steric hindrance — only one alkyl group is attached to the carbonyl carbon.
2. Greater electrophilicity — fewer electron-donating alkyl groups means a higher $\delta +$ on the carbonyl carbon.

There are two main types of nucleophilic addition reactions.

I. Base Catalysed Nucleophilic Addition Reaction

This reaction type occurs with strong nucleophiles. A base generates the nucleophile from its conjugate acid. The reaction is initiated by the attack of the strong nucleophile on the electrophilic carbonyl carbon.

1. Addition of Hydrogen Cyanide (Cyanohydrin Formation)

Hydrogen cyanide ($HCN$) adds to aldehydes and ketones to form products called cyanohydrins. This reaction is synthetically useful as it increases the carbon chain length by one and introduces both a hydroxyl ($-OH$) and a nitrile ($-CN$) group.

• $HCN$ is highly toxic and is usually generated in situ from sodium cyanide ($NaCN$) and a strong acid like $HCl$: $\mathrm{NaCN}+\mathrm{HCl}⟶\mathrm{HCN}+\mathrm{NaCl}$

• Mechanism: This reaction is base-catalysed, where the cyanide ion ($C{N}^{-}$) is the nucleophile.

  1. Formation of Nucleophile: A base (like $O{H}^{-}$) deprotonates $HCN$ to generate the stronger nucleophile, $C{N}^{-}$: $HCN+O{H}^{-}\rightleftharpoons H_{2}O+C{N}^{-}$
  2. Nucleophilic Attack: The cyanide ion attacks the carbonyl carbon to form a tetrahedral alkoxide intermediate.
  3. Protonation: The alkoxide intermediate is protonated by undissociated $HCN$ (or another proton source) to yield the cyanohydrin.

2. Haloform Reaction (Iodoform Test)

The iodoform test is a specific type of haloform reaction used to identify the presence of a methyl keto group ($C{H}_3-CO-$) or compounds that can be oxidised to form this group (e.g., ethanol).

• Reagents: Iodine ($I_2$) and sodium hydroxide ($NaOH$).

• Positive Test: Formation of a pale yellow precipitate of iodoform ($CH{I}_3$).

• Aldehydes: Only ethanal (acetaldehyde) gives a positive iodoform test because it is the only aldehyde with a $C{H}_{3}CO-$ group: $\underset{\text{Ethanal}}{C{H}_{3}CHO}+3I_2+4NaOH⟶\underset{\text{Iodoform (yellow ppt)}}{CH{I}_3\downarrow }+\underset{\text{Sodium methanoate}}{\mathrm{HCOONa}}+3NaI+3H_{2}O$

• Ketones: Only methyl ketones (ketones where the carbonyl group is attached to a $C{H}_3$ group) give a positive test: $\underset{\text{A methyl ketone}}{R-CO-C{H}_3}+3I_2+4NaOH⟶\underset{\text{Iodoform}}{CH{I}_3\downarrow }+\underset{\text{Sodium carboxylate}}{R-COONa}+3NaI+3H_{2}O$

For more on distinguishing aldehydes and ketones, see Tests Used to Distinguish Aldehydes and Ketones→.

II. Acid Catalysed Nucleophilic Addition Reactions

This reaction type occurs with weak nucleophiles. An acid catalyst protonates the carbonyl oxygen, increasing the electrophilicity of the carbonyl carbon and making it susceptible to attack by weak nucleophiles.

1. Reaction with 2,4-Dinitrophenylhydrazine (2,4-DNPH)

This is a classic qualitative test for identifying aldehydes and ketones.

• Reaction: Aldehydes and ketones react with 2,4-DNPH in an acidic medium to form 2,4-dinitrophenylhydrazones.
• Observation: A bright yellow, orange, or red precipitate is formed, confirming the presence of a carbonyl group ($>C=O$).

2. Reaction with Primary Nitrogen Nucleophiles (Imine Formation)

Aldehydes and ketones react with primary amines ($R-N{H}_2$) in a condensation reaction to form an imine (Schiff base), which contains a carbon-nitrogen double bond ($C=N$):

$\underset{\text{Aldehyde/Ketone}}{R-CO-R^{\prime }}+\underset{\text{Primary Amine}}{R^{\prime \prime }-N{H}_2}\stackrel{H^{+}}{\to }\underset{\text{Imine (Schiff Base)}}{R-C(=N{R}^{\prime \prime })-R^{\prime }}+H_{2}O$

Reduction of Aldehydes and Ketones

Reduction with Metal Hydrides

Aldehydes and ketones are readily reduced to alcohols using metal hydride reagents.

• Reducing Agents:

  • Sodium borohydride ($NaB{H}_4$) — milder, more selective; safe to use in protic solvents.
  • Lithium aluminium hydride ($LiAl{H}_4$) — very strong and reactive; must be used in anhydrous conditions.

• Mechanism: These reagents act as a source of the hydride ion ($H^{-}$), a strong nucleophile which attacks the carbonyl carbon.

• Products:

  • Aldehydes $⟶$ Primary alcohols ($RCHO\stackrel{[H]}{\to }RC{H}_{2}OH$)
  • Ketones $⟶$ Secondary alcohols ($RCO{R}^{\prime }\stackrel{[H]}{\to }RCHOH{R}^{\prime }$)

Oxidation Reactions

Oxidation of Aldehydes

Aldehydes are easily oxidised to carboxylic acids, even by mild oxidising agents. This ease of oxidation is a key difference between aldehydes and ketones — ketones resist oxidation under normal conditions.

• Oxidising Agents:

  • Strong: Acidified potassium permanganate ($KMn{O}_4/H^{+}$) or acidified potassium dichromate ($K_{2}C{r}_2{O}_7/H^{+}$).
  • Mild (specific to aldehydes): Tollen's reagent (ammoniacal silver nitrate) — gives a silver mirror; Fehling's/Benedict's solution — gives a brick-red precipitate.

• General equation: $RCHO+[O]⟶RCOOH$

• Example (ethanal to ethanoic acid): $C{H}_{3}CHO+[O]\stackrel{K_{2}C{r}_2{O}_7/H^{+}}{\to }C{H}_{3}COOH$