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 ($H_3{O}^{+}$), establishing an equilibrium.

$R-COOH+H_{2}O\rightleftharpoons R-CO{O}^{-}+H_3{O}^{+}$

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.

$C{H}_{3}COOH(\text{Acetic acid})+NaOH\to C{H}_{3}COONa(\text{Sodium acetate})+H_{2}O$

b) Reaction with Carbonates and Bicarbonates

Carboxylic acids are strong enough to react with carbonates ($N{a}_{2}C{O}_3$) and bicarbonates ($NaHC{O}_3$), liberating carbon dioxide gas with a characteristic effervescence. This reaction is often used as a test for the carboxyl group.

$2C{H}_{3}COOH+N{a}_{2}C{O}_3\to 2C{H}_{3}COONa+H_{2}O+C{O}_2\uparrow$ $C{H}_{3}COOH+NaHC{O}_3\to C{H}_{3}COONa+H_{2}O+C{O}_2\uparrow$

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.

$2C{H}_{3}COOH+2Na\to 2C{H}_{3}COONa+H_2\uparrow$

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 $H_{2}S{O}_4$), 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: $R-COOH(\text{Carboxylic Acid})+R^{\prime }-OH(\text{Alcohol})\stackrel{H^{+}}{\to }R-COO{R}^{\prime }(\text{Ester})+H_{2}O$

Example: Acetic acid reacts with ethanol to form ethyl acetate. $C{H}_{3}COOH(\text{Acetic Acid})+C_2{H}_{5}OH(\text{Ethanol})\stackrel{H_{2}S{O}_4}{\to }C{H}_{3}COO{C}_2{H}_5(\text{Ethyl Acetate})+H_{2}O$

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 ($LiAl{H}_4$) is typically used.
• Carboxylic acids are generally less reactive to hydride reduction than aldehydes, ketones, or esters.

Example: Reduction of acetic acid to ethanol. $C{H}_{3}COOH+4[H]\stackrel{LiAl{H}_4}{\to }C{H}_{3}C{H}_{2}OH+H_{2}O$

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 $SOC{l}_2$ to form acid chlorides. This is preferred over $PC{l}_5$ because the by-products ($S{O}_2$ and $HCl$) are gases and escape easily.