Detailed Notes on Carboxylic Acids

Introduction to Chemistry

  • Course Details: CHEM 1008 / CHEM 1017
    Lecture: 13
    Instructor: Dr. Leo Clarke
    Institution: Technological University Dublin
    Location: Dublin

Overview of Carboxylic Acids

  • Definition:

    • Carboxylic acids are organic compounds containing a carbonyl group (C=O) adjacent to a hydroxyl group (–OH).

  • General Formula:

    • The formula exhibits the presence of both functional groups on the same carbon atom.

  • Importance:

    • Due to their natural occurrence, carboxylic acids were among the first studied organic compounds.
    • Common names are still widely used today, despite systematic naming conventions.

Nomenclature Rules

  • Basic Naming Convention for Carboxylic Acids:

    • Replace the ‘-e’ of the parent alkane with ‘-oic acid’.
    • The -CO₂H group is always assigned the number 1 in the carbon chain as it has the highest priority in functional group (FG) ranking.

  • Substituents and Rings:

    • Substituents are labeled as α, β, γ, δ… but should not be used in IUPAC naming.
    • For cyclic structures, use the suffix ‘-carboxylic acid’ and assign the -CO₂H group the number 1 location.

  • Dicarboxylic Acids:

    • Location numbers for dicarboxylic acids are often omitted. Both -CO₂H groups expected to be at the chain ends.

  • Salts of Carboxylic Acids:

    • Named by naming the cation and replacing ‘-ic acid’ with ‘-ate’.

Physical Properties

  • Geometry:

    • The carboxyl group exhibits a trigonal planar shape.
    • Lone pair electrons from the hydroxyl oxygen conjugate with the carbonyl’s π system, leading to planarity.
    • Carboxylic acids can donate an H⁺ ion, forming a carboxylate ion, exhibiting significant acidity.

  • Acidity:

    - Classified as the most acidic organic functional group.

    Comparison:

    • Example of pKₐ:
    • Acetic acid (~5) is far more acidic than ethanol (~17).

  • Boiling and Melting Points:

    • Boiling points increase with molecular size due to stronger intermolecular forces in larger carboxylic acids.

    - Melting points do not follow a consistent trend; acids with even C numbers have higher melting points due to differences in crystal packing.

    Cis- and Trans- isomers:

    • Significant differences noticed in melting points for cis isomers due to spatial arrangement affecting intermolecular forces.

  • Solubility:

    • High boiling points attributed to dimerization (two acidic molecules forming hydrogen bonds).
    • Solubility decreases in larger acids (>C4) due to the increased importance of dispersion forces over hydrogen bonding.
    • Ionization increases solubility by creating stronger ionic interactions.

Preparation of Carboxylic Acids

  • Oxidation of Alcohols and Aldehydes:

    • Carboxylic acids can be prepared by oxidizing alcohols (primary to carboxylic acids).
    • Example of Chemoselectivity: Tollen's reagent selectively oxidizes aldehydes without affecting ketones.
    • Common Reagents: Ag₂O, Chromic acid.

  • Tollen’s Test:

    • Utilizes silver complex in ammonia solution to confirm the presence of aldehydes: the reduction of silver leads to metallic deposition, producing a mirror effect in positive tests.

  • Reagents Preparation: AgNO₃ to Ag₂O conversions occur by reactions that utilize hydroxide ions and ammonia.