Lab 7 Carboxylic acid and Ester

Experiment 7: Carboxylic Acids and Esters

Objectives

  • Identify acidic properties of carboxylic acids.
  • Write equations for the ionization of carboxylic acids.
  • Prepare salts of carboxylic acids and compare their solubility with that of the acids.
  • Write equations for the neutralization of carboxylic acids.
  • Prepare esters and assess their odors.
  • Write equations for esterification.

Background

  • Carboxylic Acids:
    • Organic compounds containing the carboxyl group (-COOH).
    • Exhibit acidic properties in aqueous solutions.
    • Common examples include:
      • Formic acid
      • Acetic acid
      • Citric acid
      • Benzoic acid
    • Table 1 provides a list of common carboxylic acids and their natural sources:
      • Formic Acid: Found in ants
      • Acetic Acid: Found in vinegar
      • Citric Acid: Found in lemons and limes
      • Benzoic Acid: Found in cranberries and blueberries
  • IUPAC Naming of Carboxylic Acids:
    • Steps:
      1. Identify the longest carbon chain containing the carboxyl group as the parent chain.
      2. Replace the "-e" from the corresponding alkane name with "-oic acid".
        • Ex: Ethane becomes ethanoic acid, Propane becomes propanoic acid.
      3. Number the carbon chain from the end nearest to the carboxyl group, assigning it position 1.
      4. Name and number substituents (halogens, alkyl groups) as required.
    • Common Names:
      • Formic Acid (Methanoic Acid)
      • Acetic Acid (Ethanoic Acid)
      • Propionic Acid (Propanoic Acid)
      • Butyric Acid (Butanoic Acid)
    • Greek Letter Designation for Carbons:
      • Alpha (α): Carbon adjacent to the carboxyl group
      • Beta (β): Next carbon

A. Solubility and Ionization of Carboxylic Acids in Water

  • Solubility Factors:
    1. Small-Chain Carboxylic Acids:
      • Highly soluble in water (1-4 carbon atoms, e.g., formic acid, acetic acid) due to hydrogen bonding with water.
      • Figure illustrating hydrogen bonding between the carboxyl group in ethanoic acid and water molecules.
    2. Long-Chain Carboxylic Acids:
      • Decreased solubility due to hydrophobic effects from the longer alkyl chains (e.g., benzoic acid, palmitic acid, stearic acid).
    3. Temperature Effect:
      • Increased temperature enhances solubility as it disrupts intermolecular interactions, promoting dispersion of acid molecules in water.
  • Ionization of Carboxylic Acids:
    • Carboxylic acids are classified as weak acids, undergoing partial ionization in water.
    • Ionization Equation:extCH<em>3ext−COOH(aq)+H</em>2extO(l)ightleftharpoonsextH<em>3extO+(aq)+extCH</em>3ext−COO−(aq)ext{CH}<em>3 ext{-COOH (aq) + H}</em>2 ext{O (l)} ightleftharpoons ext{H}<em>3 ext{O}^+ (aq) + ext{CH}</em>3 ext{-COO}^- (aq)
      • Ethanoic acid (acetic acid) ionizes to form acetate ion.
    • Breakdown of the Ionization Process:
      1. Proton transfer occurs as the carboxyl group donates H⁺ to water, acting as a Brønsted-Lowry acid.
      2. Water, acting as a Brønsted-Lowry base, accepts the proton to form a hydronium ion (H₃O⁺).
      3. The acetate ion (CH₃-COO⁻) remains after the proton is released, indicating an equilibrium exists between the reactants and products, with undisputed acid remaining.

B. Solubility and Odor of Carboxylic Acid Salts

  • Neutralization Reaction with Base (e.g., NaOH):
    • Reaction process yields salt and water:
      extCH<em>3ext−COOH+NaOHightarrowextCH</em>3ext−COO−extNa++extH2extOext{CH}<em>3 ext{-COOH + NaOH} ightarrow ext{CH}</em>3 ext{-COO}^- ext{Na}^+ + ext{H}_2 ext{O}
    • Properties of the Produced Sodium Salts:
      • Highly soluble in water due to robust interactions with water molecules.
      • Resulting salt solutions are odorless.
    • Naming Convention for Salts:
      • Written by naming the cation followed by the name of the acid with the suffix "–ic acid" replaced by "–ate".
  • Reactions with Sodium Bicarbonate or Carbonate:
    • When treated with these bases, carbon dioxide gas (CO₂) is released, observable as fizzing/bubbling:
      • 2extRCOOH+Na<em>2extCO</em>3<br/>ightarrow2extRCOO−extNa++extH<em>2extO+extCO</em>2ext↑2 ext{RCOOH + Na}<em>2 ext{CO}</em>3 <br /> ightarrow 2 ext{RCOO}^- ext{Na}^+ + ext{H}<em>2 ext{O} + ext{CO}</em>2 ext{ ↑}
      • extRCOOH+NaHCO<em>3ightarrowextRCOO−extNa++extH</em>2extO+extCO2ext↑ext{RCOOH + NaHCO}<em>3 ightarrow ext{RCOO}^- ext{Na}^+ + ext{H}</em>2 ext{O} + ext{CO}_2 ext{ ↑}

C. Esters

  • Definition and Structure of Esters:
    • Functional group characterized by the formula RCOOR', where R and R' are alkyl or aryl groups and CO represents the carbonyl group (C=O).
    • Illustrated esters have moieties derived from carboxylic acid (R) and alcohol (R').
    • Fragrance in Fruits:
      • Esters contribute to the characteristic aromas of fruits (e.g., ethyl hexanoate denotes apple aroma, benzyl acetate produces floral jasmine scent).
    • Examples of Esters and Their Odors in Table 3:
      • IUPAC Names and Characteristic Flavors:
        • Propyl ethanoate: Pear
        • Pentyl ethanoate: Banana
        • Octyl ethanoate: Orange
        • Methyl butanoate: Apple
        • Ethyl butanoate: Pineapple
  • IUPAC Naming of Esters:
    • Steps to Name:
      1. Identify the alkyl group from alcohol, replacing the "-ol" with "-yl" (i.e., methanol to methyl, ethanol to ethyl).
      2. Identify the acid part from carboxylic acid, replacing "-ic acid" with "-ate" (i.e., ethanoic acid to ethanoate).
      3. Combine the two parts correctly (alkyl group followed by acid part).
    • Table 4 showcases IUPAC and common names of esters:
      • Methyl acetate: Derived from methanol and acetic acid
      • Propyl ethanoate: Derived from propanol and ethanoic acid
      • Ethyl propanoate: Derived from ethanol and propanoic acid
  • Esterification Reaction:
    • Direct chemical reaction between a carboxylic acid and alcohol resulting in ester formation and water:
      extCH<em>3ext−COOH+HO−CH</em>3<br/>ightarrowextCH<em>3ext−COO−CH</em>3+extH2extOext{CH}<em>3 ext{-COOH + HO-CH}</em>3 <br /> ightarrow ext{CH}<em>3 ext{-COO-CH}</em>3 + ext{H}_2 ext{O}
    • Components of Esterification:
      • Acetic acid provides the carbonyl group (-CO-) and alcohol provides the alkoxy group