# 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 on the carbon and a partial negative charge on the oxygen, makes the carbonyl carbon an electrophile. This allows it to be attacked by nucleophiles. There are two main types of nucleophilic addition reactions. ### I. Base Catalysed Nucleophilic Addition Reaction This reaction type occurs with *strong nucleophiles*. A base is often used to generate 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 $HCl$. $$ \mathrm{NaCN} + \mathrm{HCl} \longrightarrow \mathrm{HCN} + \mathrm{NaCl} $$ * **Mechanism:** This reaction is base-catalysed, where the cyanide ion ($CN^-$) is the nucleophile. 1. **Formation of Nucleophile:** A base (like $OH^-$) deprotonates $HCN$ to generate the stronger nucleophile, $CN^-$. $$ HCN + OH^- \rightleftharpoons H_2O + CN^- $$ 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** ($CH_3-CO-$) or compounds that can be oxidized to form this group (like ethanol and other secondary alcohols with a methyl group on the carbinol carbon). * **Reagents:** Iodine ($I_2$) and sodium hydroxide ($NaOH$). * **Positive Test:** Formation of a pale yellow precipitate of **iodoform** ($CHI_3$). * **Aldehydes:** Only **ethanal (acetaldehyde)** gives a positive iodoform test because it is the only aldehyde with a $CH_3-CO-$ group. $$ \underset{\text{Ethanal}}{CH_3CHO} + 3I_2 + 4NaOH \longrightarrow \underset{\text{Iodoform (yellow ppt)}}{CHI_3 \downarrow} + \underset{\text{Sodium methanoate}}{HCOONa} + 3NaI + 3H_2O $$ * **Ketones:** Only **methyl ketones** (ketones where the carbonyl group is attached to a $CH_3$ group) give a positive test. $$ \underset{\text{A methyl ketone}}{R-CO-CH_3} + 3I_2 + 4NaOH \longrightarrow \underset{\text{Iodoform}}{CHI_3 \downarrow} + \underset{\text{Sodium carboxylate}}{R-COONa} + 3NaI + 3H_2O $$ > **Note:** The iodoform test is crucial for distinguishing: > * Ethanal from all other aldehydes. > * Methyl ketones from other ketones. > * Ethanol from methanol and other primary alcohols. For more on distinguishing aldehydes and ketones, see . ### II. Acid Catalysed Nucleophilic Addition Reactions This reaction type occurs with *weak nucleophiles*. An acid catalyst is required to increase the electrophilicity of the carbonyl carbon by protonating the carbonyl oxygen. This makes the carbonyl carbon more susceptible to attack by a weak nucleophile. #### 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. #### 2. Reaction with Primary Nitrogen Nucleophiles (Imine Formation) Aldehydes and ketones react with primary amines ($R-NH_2$) in a condensation reaction to form an **imine** (also known as a Schiff base), which contains a carbon-nitrogen double bond. $$ \underset{\text{Aldehyde/Ketone}}{R-CO-R'} + \underset{\text{Primary Amine}}{R''-NH_2} \xrightarrow{H^+} \underset{\text{Imine}}{R-C(=NR'')-R'} + H_2O $$ ## Reduction of Aldehydes and Ketones ### 1. Reduction with Hydrides Aldehydes and ketones are readily reduced to alcohols using metal hydride reagents. * **Reducing Agents:** Sodium borohydride ($NaBH_4$) or Lithium aluminum hydride ($LiAlH_4$). * $NaBH_4$ is a milder, more selective reducing agent. * $LiAlH_4$ is a very strong and reactive reducing agent. * **Mechanism:** These reagents act as a source of the hydride ion ($H^-$), a strong nucleophile which attacks the carbonyl carbon. * **Products:** * **Aldehydes** are reduced to **primary alcohols**. * **Ketones** are reduced to **secondary alcohols**. ## Oxidation Reactions ### 1. Oxidation of Aldehydes Aldehydes are easily oxidized to **carboxylic acids**, even by mild oxidizing agents. This ease of oxidation is a key difference between aldehydes and ketones. * **Oxidizing Agents:** Strong agents like potassium permanganate ($KMnO_4 / H_2SO_4$), potassium dichromate ($K_2Cr_2O_7 / H_2SO_4$), and nitric acid ($HNO_3$). $$ \underset{\text{Aldehyde}}{R-CHO} \xrightarrow{[O]} \underset{\text{Carboxylic Acid}}{R-COOH} $$ **Example:** $$ \underset{\text{Ethanal}}{CH_3CHO} + [O] \xrightarrow{K_2Cr_2O_7 / H_2SO_4} \underset{\text{Ethanoic Acid}}{CH_3COOH} $$ ### 2. Oxidation of Ketones Ketones are resistant to oxidation by mild agents. They can only be oxidized under harsh conditions using strong oxidizing agents, which results in the cleavage of a carbon-carbon bond adjacent to the carbonyl group. * **Oxidizing Agents:** Hot, concentrated $KMnO_4 / H_2SO_4$, $K_2Cr_2O_7 / H_2SO_4$, or concentrated $HNO_3$. * **Product:** A mixture of carboxylic acids with fewer carbon atoms than the parent ketone. * **Rule (Popoff's Rule):** In unsymmetrical ketones, the cleavage occurs such that the carbonyl group remains with the smaller alkyl group. * **Symmetrical Ketone Example:** $$ \underset{\text{Propanone}}{CH_3-CO-CH_3} \xrightarrow{[O], \text{strong}} \underset{\text{Ethanoic Acid}}{CH_3COOH} + \underset{\text{Methanoic Acid}}{HCOOH} $$ * **Unsymmetrical Ketone Example:** $$ \underset{\text{Butan-2-one}}{CH_3CH_2-CO-CH_3} \xrightarrow{[O], \text{strong}} \underset{\text{Ethanoic Acid}}{2CH_3COOH} $$ *(The C-C bond between the carbonyl and ethyl group breaks, as the carbonyl group stays with the smaller methyl group).*