# 19.6 Preparation of Carboxylic Acids This section outlines the primary methods for synthesizing carboxylic acids from various organic starting materials. ## 1. By the Hydrolysis of Nitriles Organic compounds containing a cyanide group ($-C \equiv N$) are known as **nitriles** or *cyanides*. Carboxylic acids can be prepared by hydrolyzing an alkyl nitrile, which is typically done by boiling it with a dilute mineral acid (e.g., $HCl$, $H_2SO_4$) or an alkali (e.g., $NaOH$). **Reaction Pathway:** The process starts from an alkyl halide, which is converted to a nitrile, and then hydrolyzed to the carboxylic acid. $$ R-X \xrightarrow{KCN} R-C \equiv N \xrightarrow{H_3O^+ \text{ or } OH^-, \Delta} R-COOH $$ **Acidic Hydrolysis:** $$ R-C \equiv N + 2H_2O + H^+ \xrightarrow{\Delta} R-COOH + NH_4^+ $$ **Basic Hydrolysis:** $$ R-C \equiv N + OH^- + H_2O \xrightarrow{\Delta} R-COO^- + NH_3 $$ The resulting carboxylate salt is then acidified to yield the carboxylic acid: $$ R-COO^- + H_3O^+ \rightarrow R-COOH + H_2O $$ **Key Feature:** This method is useful for increasing the length of a carbon chain, as the final carboxylic acid contains **one more carbon atom** than the original alkyl halide. --- ## 2. By the Oxidation of Primary Alcohols Primary alcohols can be oxidized to carboxylic acids using strong oxidizing agents. The reaction proceeds through an intermediate aldehyde, which is further oxidized. **Oxidizing Agents:** Common agents include potassium permanganate ($KMnO_4$) or potassium dichromate ($K_2Cr_2O_7$) in an acidic medium (e.g., dilute $H_2SO_4$). **General Reaction:** $$ R-CH_2OH \xrightarrow{[O]} [R-CHO] \xrightarrow{[O]} R-COOH $$ *Where [O] represents the oxidizing agent.* **Example:** Oxidation of ethanol to acetic acid. $$ CH_3CH_2OH \xrightarrow{K_2Cr_2O_7 / H^+} CH_3COOH $$ --- ## 3. By the Hydrolysis of Esters Esters undergo hydrolysis to produce a carboxylic acid and an alcohol. This reaction can be catalyzed by either an acid or a base. **Acidic Hydrolysis:** An ester is refluxed with a dilute mineral acid like $HCl$ or $H_2SO_4$. This is a reversible reaction. $$ R-COOR' + H_2O \rightleftharpoons R-COOH + R'OH $$ **Basic Hydrolysis (Saponification):** An ester is heated with a dilute alkali, such as $NaOH$. This process is called **saponification** because it is used to make soap (alkali salts of fatty acids). 1. The ester is hydrolyzed to form a carboxylate salt. $$ R-COOR' + NaOH \rightarrow R-COONa + R'OH $$ 2. The salt is then acidified to produce the final carboxylic acid. $$ R-COONa + HCl \rightarrow R-COOH + NaCl $$ --- ## 4. From Grignard Reagents (Carbonation) Carboxylic acids can be prepared by reacting Grignard reagents with dry ice (solid $CO_2$) followed by acid hydrolysis. $$ R-MgX + O=C=O \rightarrow R-COOMgX \xrightarrow{H_3O^+} R-COOH + Mg(OH)X $$ --- ## 5. Oxidative Cleavage of Alkenes Alkenes react with hot, concentrated alkaline $KMnO_4$ to undergo cleavage at the double bond, forming carboxylic acids (if the double bond carbon is attached to a hydrogen). $$ R-CH=CH-R' \xrightarrow{KMnO_4/OH^-, \Delta} R-COOH + R'-COOH $$ --- ## Concept Assessment Exercise 19.1 **Q:** How can you convert $CH_3Cl$ into $CH_3COOH$? **A:** This conversion involves increasing the carbon chain by one atom (from chloromethane to acetic acid). The nitrile hydrolysis method is suitable for this. 1. **Step 1: Formation of the Nitrile** React chloromethane ($CH_3Cl$) with potassium cyanide ($KCN$) in an alcoholic solution. This is a nucleophilic substitution reaction where the chloride is replaced by a cyanide group. $$ CH_3Cl + KCN \xrightarrow{\text{Alcohol}} CH_3CN + KCl $$ The product is methyl cyanide, also known as ethanenitrile or acetonitrile. 2. **Step 2: Hydrolysis of the Nitrile** Hydrolyze the methyl cyanide ($CH_3CN$) by boiling it with a dilute mineral acid (e.g., $H_3O^+$). $$ CH_3CN + 2H_2O \xrightarrow{H^+, \Delta} CH_3COOH + NH_3 $$ The overall two-step conversion is an effective way to synthesize a carboxylic acid from an alkyl halide with an additional carbon atom.