## 19.8 Reactions of Carboxylic Acids Carboxylic acids undergo several types of reactions, which can be categorized based on the part of the functional group involved: 1. Reactions involving the acidic **hydrogen atom** of the hydroxyl group ($R-COO\underline{H}$). 2. Reactions involving the replacement of the entire **hydroxyl group** ($R-C(O)\underline{OH}$). 3. Reactions involving the entire **carboxyl group** ($R-\underline{COOH}$). ### 1. Reactions Involving the Acidic Hydrogen Atom (Acidic Properties) Carboxylic acids are weak acids compared to mineral acids. They ionize in water to produce a carboxylate ion and a hydronium ion ($H_3O^+$), establishing an equilibrium. $$ R-COOH + H_2O \rightleftharpoons R-COO^- + H_3O^+ $$ This acidic nature allows them to react with bases, carbonates, and active metals. #### a) Reactions with Bases Carboxylic acids neutralize strong bases like sodium hydroxide ($NaOH$) and potassium hydroxide ($KOH$) to form a salt and water. $$ CH_3COOH \, (\text{Acetic acid}) + NaOH \rightarrow CH_3COONa \, (\text{Sodium acetate}) + H_2O $$ #### b) Reaction with Carbonates and Bicarbonates Carboxylic acids are strong enough to react with carbonates ($Na_2CO_3$) and bicarbonates ($NaHCO_3$), liberating carbon dioxide gas with a characteristic effervescence. This reaction is often used as a test for the carboxyl group. $$ 2CH_3COOH + Na_2CO_3 \rightarrow 2CH_3COONa + H_2O + CO_2 \uparrow $$ $$ CH_3COOH + NaHCO_3 \rightarrow CH_3COONa + H_2O + CO_2 \uparrow $$ #### c) Reactions with Metals Active metals such as $Na$, $K$, $Ca$, and $Mg$ react with carboxylic acids to produce a metal carboxylate salt and hydrogen gas. $$ 2CH_3COOH + 2Na \rightarrow 2CH_3COONa + H_2 \uparrow $$ ### 2. Reactions Involving the -OH Group #### Reaction with Alcohol (Esterification) When carboxylic acids are heated with alcohols in the presence of a strong acid catalyst (like concentrated $H_2SO_4$), they form **esters**. This process is known as *Fischer esterification*. This is a type of **condensation reaction**, where two molecules combine with the elimination of a small molecule, such as water. **General Reaction:** $$ R-COOH \, (\text{Carboxylic Acid}) + R'-OH \, (\text{Alcohol}) \xrightarrow{H^+} R-COOR' \, (\text{Ester}) + H_2O $$ **Example:** Acetic acid reacts with ethanol to form ethyl acetate. $$ CH_3COOH \, (\text{Acetic Acid}) + C_2H_5OH \, (\text{Ethanol}) \xrightarrow{H_2SO_4} CH_3COOC_2H_5 \, (\text{Ethyl Acetate}) + H_2O $$ #### Mechanism of Acid-Catalyzed Esterification The reaction proceeds through a multi-step mechanism: **Step 1: Protonation of the Carbonyl Oxygen** The acid catalyst protonates the carbonyl oxygen, making the carbonyl carbon more electrophilic and susceptible to nucleophilic attack. **Step 2: Nucleophilic Attack by Alcohol** The alcohol's hydroxyl oxygen acts as a nucleophile, attacking the electrophilic carbonyl carbon to form a tetrahedral intermediate. **Step 3: Proton Transfer** A proton is transferred from the newly added hydroxyl group to one of the original hydroxyl groups, converting it into a good leaving group (water). **Step 4: Elimination of Water** The intermediate eliminates a molecule of water. The resulting species is deprotonated (often by a water molecule or the conjugate base of the acid catalyst) to regenerate the acid catalyst and form the final ester product. ### 3. Reactions Involving the Carboxyl Group (-COOH) #### Reduction to Alcohols - Carboxylic acids can be reduced to form **primary alcohols**. - Due to the low reactivity of the carboxyl group towards reduction, a very strong reducing agent is required. **Lithium aluminum hydride ($LiAlH_4$)** is typically used. - Carboxylic acids are generally less reactive to hydride reduction than aldehydes, ketones, or esters. **Example:** Reduction of acetic acid to ethanol. $$ CH_3COOH + 4[H] \xrightarrow{LiAlH_4} CH_3CH_2OH + H_2O $$ #### Decarboxylation Sodium salts of carboxylic acids undergo decarboxylation when heated with soda lime ($NaOH$ and $CaO$ mixture), resulting in the formation of alkanes. #### Reaction with Thionyl Chloride Carboxylic acids react with $SOCl_2$ to form acid chlorides. This is preferred over $PCl_5$ because the by-products ($SO_2$ and $HCl$) are gases and escape easily.