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11.9: Chemical Properties: Carboxylic Acids

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    Skills to Develop

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

    Organic acids have been known for ages. Prehistoric people likely made acetic acid when their fermentation reactions went awry and produced vinegar instead of wine. The Sumerians (2900–1800 BCE) used vinegar as a condiment, a preservative, an antibiotic, and a detergent. Citric acid was discovered by an Islamic alchemist, Jabir Ibn Hayyan (also known as Geber), in the 8th century, and crystalline citric acid was first isolated from lemon juice in 1784 by the Swedish chemist Carl Wilhelm Scheele. Medieval scholars in Europe were aware that the crisp, tart flavor of citrus fruits is caused by citric acid. Naturalists of the 17th century knew that the sting of a red ant’s bite was due to an organic acid that the ant injected into the wound. The acetic acid of vinegar, the formic acid of red ants, and the citric acid of fruits all belong to the same family of compounds—carboxylic acids. Soaps are salts of long-chain carboxylic acids.

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

    \(\mathrm{RCOOH+H_2O\rightleftharpoons RCOO^-+H_3O^+}\)

    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 (CO3) and bicarbonate (HCO3) 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.

    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 CH3CH2COOH.

    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)

    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)

    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.

    Concept Review Exercises

    1. How does the neutralization of a carboxylic acid differ from that of an inorganic acid? How are they similar?
    2. What products are formed when a carboxylic acid is neutralized with a strong base? What additional product is formed when a carboxylic acid is neutralized with a carbonate or a bicarbonate?

    Answers

    1. Insoluble carboxylic acids often form soluble carboxylate salts. Both form a salt and water.
    2. a carboxylate salt and water; carbon dioxide

    Key Takeaways

    • Soluble carboxylic acids are weak acids in aqueous solutions.
    • Carboxylic acids neutralize bases to form salts.

    Exercises

    1. Write the equation for the ionization of CH3CH2CH2COOH in water.

    2. Write the equation for the reaction of CH3COOH with sodium carbonate [Na2CO3(aq)].

    3. Write the equation for the ionization of propionic acid, CH3CH2COOH, in water.

    4. Write an equation for the reaction of butyric acid, CH3CH2CH2COOH, with each compound.

      1. aqueous NaOH
      2. aqueous NaHCO3

    Answers

    1. CH3CH2CH2COOH(aq) + H2O(ℓ) → CH3CH2CH2COO−(aq) + H3O+(aq)
    2. 2CH3COOH + Na2CO3(aq) → 2CH3COO−Na+(aq) + H2O(ℓ) + CO2(g)
    3. CH3CH2COOH(aq) + H2O(ℓ) → CH3CH2COO−(aq) + H3O+(aq)
    4.  
    1. CH3CH2CH2COOH(aq) + NaOH(aq) → CH3CH2CH2COO−Na+(aq) + H2O(ℓ)
    2. CH3(CH2)2COOH + NaHCO3(aq) → CH3(CH2)COO−Na+(aq) + H2O(ℓ) + CO2(g)

     

    11.9: Chemical Properties: Carboxylic Acids is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.

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