## 19.2 Preparations of Aldehydes and Ketones This section covers the synthesis of aldehydes and ketones through the oxidation of primary and secondary alcohols, hydration of alkynes, and other industrial methods. ### By the Oxidation of Primary and Secondary Alcohols The oxidation of alcohols is a common method for preparing aldehydes and ketones. The outcome of the reaction depends on whether the starting alcohol is primary or secondary. - **Oxidizing Agents:** Strong oxidizing agents like acidified potassium permanganate ($KMnO_4$) or potassium dichromate ($K_2Cr_2O_7$) are typically used. - **Characteristic Reaction:** The reaction of alcohols with acidified potassium dichromate ($K_2Cr_2O_7$) is easily observed by a color change in the solution from **orange** ($Cr_2O_7^{2-}$) to **green** ($Cr^{3+}$). > ***Do You Know?*** > Many fragrances and perfumes contain carbonyl compounds, which contribute to their aroma. For instance, vanillin is a carbonyl compound that imparts the characteristic flavor of vanilla. #### 1. Oxidation of Primary Alcohols A primary alcohol is first oxidized to an aldehyde. However, the aldehyde itself is susceptible to further oxidation, which converts it into a carboxylic acid. - **Controlling the Reaction:** To stop the reaction at the aldehyde stage, specific conditions are required. Since aldehydes have lower boiling points than their corresponding alcohols, the aldehyde can be distilled from the reaction mixture as soon as it is formed. - **Temperature Control:** The reaction temperature is typically kept slightly above the boiling point of the aldehyde (e.g., around $50^{\circ}C$ for acetaldehyde) to facilitate its immediate removal and prevent further oxidation. **Step 1: Alcohol to Aldehyde** The primary alcohol is oxidized to an aldehyde. $$ CH_3CH_2OH + [O] \xrightarrow{K_2Cr_2O_7 + H_2SO_4,\ 50^{\circ}C} \underset{\text{Acetaldehyde}}{CH_3CHO} + H_2O $$ **Step 2: Aldehyde to Carboxylic Acid (Further Oxidation)** If the aldehyde is not removed, it will be further oxidized to a carboxylic acid. $$ CH_3CHO + [O] \xrightarrow{K_2Cr_2O_7 + H_2SO_4} \underset{\text{Acetic Acid}}{CH_3COOH} $$ #### 2. Oxidation of Secondary Alcohols A secondary alcohol is oxidized under similar conditions to produce a **ketone**. Unlike aldehydes, ketones are resistant to further oxidation under these mild conditions. Therefore, controlling the reaction is much simpler. $$ \underset{\text{2-Propanol (a secondary alcohol)}}{\mathrm{CH_3-CH(OH)-CH_3}} + [O] \xrightarrow{K_2Cr_2O_7 + H_2SO_4} \underset{\text{Propanone (a ketone)}}{\mathrm{CH_3-C(=O)-CH_3}} + H_2O $$ ### By Hydration of Alkynes Alkynes react with water in the presence of mercuric sulfate ($HgSO_4$) and sulfuric acid ($H_2SO_4$) to produce carbonyl compounds. Ethyne produces acetaldehyde, while higher alkynes produce ketones. $$ HC \equiv CH + H_2O \xrightarrow{HgSO_4/H_2SO_4} [CH_2=CH-OH] \rightarrow CH_3CHO $$ ### From Calcium Salts of Carboxylic Acids Dry distillation of calcium salts of carboxylic acids is a traditional method for preparing aldehydes and ketones. - Distillation of calcium formate yields formaldehyde. - Distillation of a mixture of calcium formate and calcium acetate yields acetaldehyde. - Distillation of calcium acetate alone yields acetone. ### Rosenmund Reduction Aldehydes can be prepared by the reduction of acid chlorides with hydrogen in the presence of palladium catalyst supported on barium sulfate ($Pd/BaSO_4$), poisoned with quinoline or sulfur to prevent further reduction to alcohols. $$ R-COCl + H_2 \xrightarrow{Pd/BaSO_4} R-CHO + HCl $$ ### Possible Questions/Answers - **Q:** What is the product of the oxidation of a primary alcohol? **A:** A primary alcohol first oxidizes to an aldehyde. If the reaction is not controlled, the aldehyde will be further oxidized to a carboxylic acid. - **Q:** What is the product of the oxidation of a secondary alcohol? **A:** A secondary alcohol oxidizes to form a ketone. - **Q:** Why is temperature control and distillation important when preparing an aldehyde from a primary alcohol? **A:** To prevent the aldehyde from being further oxidized to a carboxylic acid. Since aldehydes have lower boiling points, they can be distilled off as they are formed.