IRELAND 13

Learning Outcomes

  • Recall general structures of carboxylic acids, fatty acids, and prostaglandins.
  • Define Ka and pKa.
  • Explain why carboxylic acids are more acidic than alcohols.
  • Explain substituent effects on acidity.

Overview of Carboxylic Acids

  • Occurrence
  • Functional group: Contains the -COOH (carboxyl) group.
  • Properties:
    • Solubility in water due to hydrogen bonding.
    • Exists predominantly as dimers due to hydrogen bonds.
    • Soluble in water if R ≤ 5 carbons.
  • Acidity: Explores ionization behavior.
  • Synthesis: How to create carboxylic acids.
  • Reactions: Basic chemical interactions involving carboxylic acids.

Properties of Carboxylic Acids

  • Solubility is enhanced due to the ability to form hydrogen bonds.
  • Dimers can form between carboxylic acid molecules (2 hydrogen bonds per dimer).
  • Hydrophobic region varies with the carbon chain length.

Acidity of Carboxylic Acids

  • Carboxylic acids dissociate in water, producing carboxylate (anion) and H2O.
    • Example reaction:
      RCOOH + NaOH
      ightarrow RCOO^- Na^+ + H2O
  • Extent of dissociation is given by acidity constant Ka.
  • Stronger acids have larger Ka values.
  • Weak acids: Carboxylic acids are categorized as weak acids.
    • Example Ka values:
    • (ClCH2COOH: K_a hickapprox 1.4 imes 10^{-3})
    • (CH3COOH: K_a hickapprox 1.8 imes 10^{-5})
    • Conclusion: ClCH2COOH is the stronger acid.

pKa Values

  • Definition: pKa is the negative logarithm of Ka:
    pKa = - ext{log} (Ka)
  • Relationship between Ka and pKa
    • Strong acids (large Ka) have low pKa.
    • Weak acids (low Ka) have high pKa.
  • Example Values:
    • Hydrochloric acid: pKa = -7
    • Ethanol: pKa = 16
    • Ethanoic acid: pKa = 4.7
    • Comparison: Ethanoic acid is 100 billion times more acidic than ethanol.

Why are Carboxylic Acids More Acidic than Alcohols?

  • Comparison of conjugate bases:
    • Carboxylate (conjugate base of carboxylic acid) is stabilized by resonance, unlike alkoxide (conjugate base of alcohol).
  • Result: Carboxylic acids dissociate more easily than alcohols, establishing stronger acidic properties.

Stabilization of Carboxylate Anions

  • Resonance Delocalization:
    • Charge is dispersed over multiple atoms, enhancing stability.
    • Alkoxide does not benefit from resonance stabilization, resulting in localized charge and reduced stability.

Effects of Substituents on Acidity

  • Substituent R influences the acidity of the -COOH group:
    • Electron-withdrawing group (EWG) increases acidity by stabilizing the conjugate base.
    • Electron-donating group (EDG) decreases acidity by destabilizing the conjugate base.

Examples of pKa Values

  • Ethanoic acid: pKa = 4.7
  • Various other acids compared:
    • pKa < 4.2: weaker than Benzoic acid
  • Notable values demonstrate the range of acidity influenced by different substituents.

Synthesis of Carboxylic Acids

  • Method: Oxidation of primary alcohols or aldehydes using strong oxidizing agents like:
    • Potassium permanganate (KMnO4)
    • Potassium dichromate (K2Cr2O7)

Reactions of Carboxylic Acids

  • Interaction with strong bases (e.g., NaOH) leads to the formation of water-soluble carboxylate salts.
  • Characteristics: Carboxylate salts are water-soluble regardless of the R chain length.
  • Interaction with ammonia and amines creates amides and supports synthetic applications (e.g., nylon production).

Introduction to Lipids

  • Definition: Organic molecules categorizing lipids based on solubility rather than structure.
  • Classification:
    • Lipids containing ester linkages (hydrolyzable): e.g., fats, phospholipids.
    • Lipids without ester linkages (non-hydrolyzable): e.g., fatty acids, eicosanoids.
  • Functions in the body:
    • Energy source, cell membrane formation, skin health, organ protection, hormonal production, and vitamin transport.

Fatty Acids & Eicosanoids

  • Fatty Acids: Long-chain carboxylic acids characterized by even-numbered carbon chains, typically synthesized from two-carbon units.
  • Eicosanoids: Derived from fatty acids (e.g., arachidonic acid).

Saturated vs. Unsaturated Fatty Acids

  • Saturated Fatty Acids: Chain saturated with hydrogens (no double bonds) e.g., stearic acid.
  • Unsaturated Fatty Acids: One or more double bonds, typically in cis configuration (e.g., oleic acid).
  • Polyunsaturated Fatty Acids: Multiple double bonds, with specific arrangements
    (e.g. linolenic acid).

Eicosanoids & Their Importance

  • Biosynthesis: Derived from phospholipids in cell membranes, specifically arachidonic acid.
  • Classes of Eicosanoids: Includes prostaglandins, thromboxanes, and leukotrienes, which act locally (paracrine hormones).