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17.2: Acidity of Carboxylic Acids

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    86295
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    Learning Objectives
    • Define \(pK_a\) and use it to determine acidity of different carboxylic acids.
    • Describe the reactions between carboxylic acids and strong bases.

    Ionization of Carboxylic Acids

    Carboxylic acids are named such because they tend to be more acidic than other functional groups in organic chemistry. In dilute aqueous solutions, they act as weak acids that partially dissociate to produce the corresponding carboxylate anion and hydronium cation (H3O+). Carboxylate anions are named by replacing the -ic acid ending from the carboxylic acid with -ate, see examples below.

    clipboard_e5d5231ee29b0e09302d9be278e14ba78.png

    clipboard_ee96e4bdb092d57d93f99a790d3dab8dd.png

    The extent of dissociation of these weak acids in water is described by \(K_a\) values. Remember that a compound with a smaller \(K_a\) value will be a weaker acid.

    \({RCOOH+H_2O\rightleftharpoons RCOO^-+H_3O^+}\) \(K_a = \dfrac{[\ce{RCOO^{-}}][\ce{H3O^{+}}]}{[\ce{RCOOH}]}\)

    When comparing the acidity of organic and biomolecules, it is useful (and more preferable) to use \(pK_a\) values instead of \(K_a\) values, which are calculated by taking the negative log of \(K_a\): \(pK_a =\ –\log K_a\). When using the \(pK_a\) scale, it is important to know that weaker acids have larger and more positive \(pK_a\) values, this is opposite of \(K_a\) values. The \(pK_a\) values of some typical carboxylic acids are listed in Table \(\PageIndex{1}\). (Remember that \(pK_a\) is a log expression, which means that every 1 \(pK_a\) unit represents a 10-fold change in acidity.)

    Table \(\PageIndex{1}\): Comparisons of Carboxylic Acid \(K_a\) and \(pK_a\) Values
    Name Compound \(K_a\) \(pK_a\)
    formic acid HCOOH 1.8 X 10–4 3.74
    acetic acid CH3COOH 1.8 X 10–5 4.74
    propanoic acid CH3CH2COOH 1.3 X 10–5 4.89
    butanoic acid CH3CH2CH2COOH 1.5 X 10–5 4.82
    chloroacetic acid ClCH2COOH 1.4 X 10–3 2.85
    trichloroacetic acid Cl3CCOOH 2.3 X 10–1 0.64
    hexanoic acid CH3(CH2)4COOH 1.3 X 10–5 4.89
    benzoic acid C6H5COOH 6.5 X 10–5 4.19
    oxalic acid HOOCCOOH 5.4 X 10–2 1.27
      OOCCOOH 5.2 X 10–5 4.28
    glutaric acid HOOC(CH2)3COOH 4.5 X 10–5 4.35
      OOC(CH2)3COOH 3.8 X 10–6 5.42

    Neutralization of Carboxylic Acids

    Carboxylic acids will react with bases such as sodium hydroxide (NaOH), sodium carbonate (Na2CO3), and sodium bicarbonate (NaHCO3) to form water and a carboxylic acid salt:

    RCOOH + NaOH(aq) → RCOONa+(aq) + H2O

    2RCOOH + Na2CO3(aq) → 2RCOONa+(aq) + H2O + CO2(g)

    RCOOH + NaHCO3(aq) → RCOONa+(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:

    carboxylic acid salts.jpg
    Note

    The salts of long-chain carboxylic acids are called soaps.

    soaps.jpg

    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) → CH3CH2COONa+(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)8COONa+(aq) + H2O(ℓ)
    2. With NaHCO3, the products are a salt, H2O, and carbon dioxide (CO2). CH3(CH2)8COOH + NaHCO3(aq) → CH3(CH2)8COONa+(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)
    Note 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.

    organic salts.jpg


    17.2: Acidity of Carboxylic Acids is shared under a CC BY-NC-SA 3.0 license and was authored, remixed, and/or curated by LibreTexts.