15.5: Chemical Properties of Carboxylic Acids- Ionization and Neutralization

Learning Objectives

• Name the typical reactions that take place with carboxylic acids.
• Describe how carboxylic acids react with basic compounds.

Water-soluble carboxylic acids ionize slightly in water to form moderately acidic solutions.

\[\mathrm{RCOOH + H_2O\rightleftharpoons RCOO^{-}+H_3O^+} \nonumber\]

Their aqueous solutions exhibit the typical properties of acids, such as changing litmus from blue to red.

The anion formed when a carboxylic acid dissociates is called the carboxylate anion (RCOO⁻).

Whether soluble in water or not, carboxylic acids react with aqueous solutions of sodium hydroxide (NaOH), sodium carbonate (Na₂CO₃), and sodium bicarbonate (NaHCO₃) to form salts:

RCOOH + NaOH(aq) → RCOO⁻Na⁺(aq) + H₂O

2RCOOH + Na₂CO₃(aq) → 2RCOO⁻Na⁺(aq) + H₂O + CO₂(g)

RCOOH + NaHCO₃(aq) → RCOO⁻Na⁺(aq) + H₂O + CO₂(g)

In these reactions, the carboxylic acids act like inorganic acids: they neutralize basic compounds. With solutions of carbonate (\(CO_3^{2–}\)) and bicarbonate (\(HCO_3^{–}\)) ions, they also form carbon dioxide gas.

Carboxylic acid salts are named in the same manner as inorganic salts: the name of the cation is followed by the name of the organic anion. The name of the anion is obtained by dropping the -ic ending of the acid name and replacing it with the suffix -ate. This rule applies whether we are using common names or International Union of Pure and Applied Chemistry (IUPAC) names:

The salts of long-chain carboxylic acids are called soaps. We discuss the chemistry of soaps elsewhere.

Physical Properties of Carboxylate Salts

1. State and Appearance:
   • Typically solid at room temperature.
   • Often appear as white crystalline powders.
2. Solubility:
   • Generally soluble in water due to their ionic nature.
   • Solubility varies depending on the cation (e.g., sodium, potassium) and the carbon chain length in the carboxylate ion.
3. Melting and Boiling Points:
   • Higher melting and boiling points compared to their parent carboxylic acids.
   • Strong ionic bonds between the carboxylate anion and the metal cation contribute to these properties.
4. Conductivity:
   • Conduct electricity in aqueous solution because they dissociate into ions.
   • This makes them effective electrolytes.
5. Hygroscopic Nature:
   • Some carboxylate salts are hygroscopic, absorbing moisture from the air.
   • Notable in salts like sodium acetate.
6. Odor:
   • Generally odorless, unlike their parent carboxylic acids, which often have strong, pungent smells.

Examples of Carboxylate Salts in Consumer Products

1. Sodium Benzoate:
   • Used as a preservative in foods like cider, jellies, pickles, and syrups¹.
2. Calcium and Sodium Propionate:
   • Added to processed cheese and bakery goods to prevent mold growth¹.
3. Sodium Sorbate and Potassium Sorbate:
   • Commonly used in fruit juices, sauerkraut, soft drinks, and wine as preservatives¹.

These properties and applications make carboxylate salts valuable in various consumer products, enhancing their shelf life and safety.

Example \(\PageIndex{1}\)

Write an equation for each reaction.

1. the ionization of propionic acid in water (H₂O)
2. the neutralization of propionic acid with aqueous sodium hydroxide (NaOH)

Solution

Propionic acid has three carbon atoms, so its formula is CH₂CH₂COOH.

1. Propionic acid ionizes in water to form a propionate ion and a hydronium (H₃O⁺) ion. CH₃CH₂COOH(aq) + H₂O(ℓ) → CH₃CH₂COO⁻(aq) + H₃O⁺(aq)
2. Propionic acid reacts with NaOH(aq) to form sodium propionate and water. CH₃CH₂COOH(aq) + NaOH(aq) → CH₃CH₂COO⁻Na⁺(aq) + H₂O(ℓ)

Exercise \(\PageIndex{1}\)

Write an equation for each reaction.

1. the ionization of formic acid in water
2. the ionization of p-chlorobenzoic acid in water

Example \(\PageIndex{2}\)

Write an equation for the reaction of decanoic acid with each compound.

1. aqueous sodium hydoxide (NaOH)
2. aqueous sodium bicarbonate (NaHCO₃)

Solution

1. Decanoic acid has 10 carbon atoms. It reacts with NaOH to form a salt and water (H₂O). CH₃(CH₂)₈COOH + NaOH(aq) → CH₃(CH₂)₈COO⁻Na⁺(aq) + H₂O(ℓ)
2. With NaHCO₃, the products are a salt, H₂O, and carbon dioxide (CO₂). CH₃(CH₂)₈COOH + NaHCO₃(aq) → CH₃(CH₂)₈COO⁻Na⁺(aq) + H₂O(ℓ) + CO₂(g)

Exercise \(\PageIndex{3}\)

Write an equation for the reaction of benzoic acid with each compound.

1. aqueous sodium hydroxide (NaOH)
2. aqueous sodium bicarbonate (NaHCO₃)

To Your Health: Organic Salts as Preservatives

Some organic salts are used as preservatives in food products. They prevent spoilage by inhibiting the growth of bacteria and fungi. Calcium and sodium propionate, for example, are added to processed cheese and bakery goods; sodium benzoate is added to cider, jellies, pickles, and syrups; and sodium sorbate and potassium sorbate are added to fruit juices, sauerkraut, soft drinks, and wine. Look for them on ingredient labels the next time you shop for groceries.

Key Takeaways

Chemical Properties of Carboxylic Acids

Acidity:

• Carboxylic acids are weak acids, meaning they partially dissociate in water to release hydrogen ions (H⁺).
• The carboxyl group (COOH) can donate a proton (H⁺), forming a carboxylate ion (R-COO⁻).
• The acidity is influenced by the presence of electron-withdrawing groups, which stabilize the carboxylate ion and increase acidity.

Reactivity with Bases:

• Carboxylic acids react with bases to form salts and water.
• For example, acetic acid reacts with sodium hydroxide to form sodium acetate and water:

  CH₃​COOH + NaOH → CH₃​COONa + H₂​O

Esterification:

• Carboxylic acids react with alcohols in the presence of an acid catalyst to form esters and water.
• This reaction is known as esterification:

  R-COOH + R’-OH → R-COOR’ + H₂​O

Reduction:

• • Carboxylic acids can be reduced to primary alcohols using reducing agents like lithium aluminium hydride (LiAlH₄).
  • The reduction process involves the conversion of the carboxyl group to a hydroxyl group.

Decarboxylation:

• Carboxylic acids can undergo decarboxylation, where they lose a carbon dioxide molecule (CO₂) to form hydrocarbons.
• This reaction typically requires heat and a catalyst.

Formation of Acid Chlorides:

• Carboxylic acids react with thionyl chloride (SOCl₂) or phosphorus trichloride (PCl₃) to form acid chlorides (R-COCl).
• Acid chlorides are highly reactive intermediates used in various organic synthesis reactions.
• Soluble carboxylic acids are weak acids in aqueous solutions.
• Carboxylic acids neutralize bases to form salts.