# 19.7 Reactivity of Carboxylic Acids This section explores the chemical behavior of carboxylic acids, focusing on the unique reactivity imparted by the carboxyl functional group. ### The Carboxyl Group: A Hybrid Functional Group The defining feature of a carboxylic acid is the **carboxyl group** ($-COOH$), which is a composite of two functional groups: * A **carbonyl group** ($>C=O$) * A **hydroxyl group** ($-OH$) The close proximity of these two groups on the same carbon atom creates a unique chemical reactivity that is different from that of simple ketones/aldehydes or alcohols. $$ \Large \underset{\text{Carboxylic Acid}}{R-\overset{\large\overset{\Large O}{||}}{C}-OH} \quad \equiv \quad \underset{\text{Carbonyl Group}}{\Large R-\overset{\large\overset{\Large O}{||}}{C}-} \quad + \quad \underset{\text{Hydroxyl Group}}{\Large -OH} $$ The overall reactivity of carboxylic acids is primarily dictated by the electrophilic nature of the carbonyl carbon and the acidic nature of the hydroxyl proton. In most reactions, the core carboxyl group is retained but modified. ### Types of Reactions The reactivity of carboxylic acids can be categorized based on which part of the functional group is involved in the reaction. #### 1. Reactions Involving Cleavage of the O-H Bond (Acidity) Carboxylic acids are acidic because the hydrogen atom of the hydroxyl group can be donated as a proton ($H^+$). The resulting carboxylate anion is stabilized by resonance, which delocalizes the negative charge over both oxygen atoms. * **Reaction:** Deprotonation in the presence of a base. * **Equation:** $$ R-\mathrm{COOH} + \mathrm{H_2O} \rightleftharpoons R-\mathrm{COO^-} + \mathrm{H_3O^+} $$ The stability of the carboxylate ion ($R-\mathrm{COO^-}$) makes the carboxylic acid significantly more acidic than an alcohol ($R-\mathrm{OH}$). #### 2. Reactions Involving Cleavage of the C-OH Bond (Nucleophilic Acyl Substitution) The carbonyl carbon is electrophilic and can be attacked by nucleophiles. This often leads to the replacement of the entire $-OH$ group with the incoming nucleophile. This is a characteristic reaction of carboxylic acid derivatives. * **Mechanism:** Nucleophilic Acyl Substitution. * **Key Feature:** The $-OH$ group is converted into a better leaving group (like $-OH_2^+$ in acidic conditions) before being displaced. **Common Nucleophilic Acyl Substitution Reactions:** | Reaction Type | Reagent Used | Product Formed | General Reaction | | :--- | :--- | :--- | :--- | | **Esterification** | Alcohol ($R'-\mathrm{OH}$) / Acid Catalyst ($H^+$) | Ester | $$ R-\mathrm{COOH} + R'-\mathrm{OH} \stackrel{H^+}{\rightleftharpoons} R-\mathrm{COOR'} + \mathrm{H_2O} $$ | | **Amide Formation** | Ammonia ($\mathrm{NH_3}$) or Amine ($R'\mathrm{NH_2}$) | Amide | $$ R-\mathrm{COOH} + R'\mathrm{NH_2} \rightarrow R-\mathrm{CONHR'} + \mathrm{H_2O} $$ | | **Acyl Halide Formation** | Thionyl Chloride ($\mathrm{SOCl_2}$) or $\mathrm{PCl_5}$ | Acyl Chloride | $$ R-\mathrm{COOH} + \mathrm{SOCl_2} \rightarrow R-\mathrm{COCl} + \mathrm{SO_2} \uparrow + \mathrm{HCl} \uparrow $$ | #### 3. Reactions Involving the Entire Carboxyl Group (Reduction) The carboxyl group is resistant to reduction but can be reduced to a primary alcohol using a strong reducing agent like Lithium Aluminum Hydride ($\mathrm{LiAlH_4}$). * **Reagent:** Lithium Aluminum Hydride ($\mathrm{LiAlH_4}$), followed by an aqueous workup ($\mathrm{H_3O^+}$). * **Reaction:** $$ R-\mathrm{COOH} \xrightarrow{1. \, LiAlH_4, \, \text{Ether}} \xrightarrow{2. \, H_3O^+} R-\mathrm{CH_2OH} $$ *Note: Milder reducing agents like $\mathrm{NaBH_4}$ do not reduce carboxylic acids.* #### 4. Decarboxylation Carboxylic acids or their salts can undergo decarboxylation, where the carboxyl group is removed as carbon dioxide ($CO_2$). * **Reaction with Soda-lime:** Heating a sodium salt of a carboxylic acid with soda-lime ($NaOH + CaO$) produces an alkane. $$ R-\mathrm{COONa} + \mathrm{NaOH} \xrightarrow{CaO, \Delta} R-\mathrm{H} + \mathrm{Na_2CO_3} $$ ## Worked Example **Problem:** Predict the product of the reaction between propanoic acid ($\mathrm{CH_3CH_2COOH}$) and ethanol ($\mathrm{CH_3CH_2OH}$) in the presence of a sulfuric acid catalyst. **Solution:** This reaction is a Fischer Esterification, a type of nucleophilic acyl substitution. 1. **Identify Reactants and Reaction Type:** * Carboxylic Acid: Propanoic acid ($\mathrm{CH_3CH_2COOH}$) * Nucleophile: Ethanol ($\mathrm{CH_3CH_2OH}$) * Catalyst: $\mathrm{H_2SO_4}$ (provides $H^+$) * Reaction: Esterification 2. **Write the General Equation:** The general equation for esterification is: $$ R-\mathrm{COOH} + R'-\mathrm{OH} \stackrel{H^+}{\rightleftharpoons} R-\mathrm{COOR'} + \mathrm{H_2O} $$ 3. **Substitute the Specific Molecules and Solve:** * $R = \mathrm{CH_3CH_2-}$ (from propanoic acid) * $R' = \mathrm{CH_3CH_2-}$ (from ethanol) * The product ($R-\mathrm{COOR'}$) will be **Ethyl Propanoate**. The balanced chemical equation is: $$ \mathrm{CH_3CH_2COOH} + \mathrm{CH_3CH_2OH} \stackrel{H_2SO_4}{\rightleftharpoons} \mathrm{CH_3CH_2COOCH_2CH_3} + \mathrm{H_2O} $$