CHEM 3603 Ch. 2 Acids and bases with class notes

Acids and Bases in Organic Chemistry

  • Course Title: Organic Chemistry I (CHEM 3603)

  • Instructor: Delmy Díaz González

  • Textbook: Org. Chem. J.G.Smith, 6th Ed.

  • Chapter Overview:

    • 2.1 Brønsted-Lowry Acids and Bases

    • 2.2 Reactions of Brønsted-Lowry Acids and Bases

    • 2.3 Acid Strength and pKa

    • 2.4 Predicting the Outcome of Acid-Base Reactions

    • 2.5 Factors That Determine Acid Strength

    • 2.6 Common Acids and Bases

    • 2.7 Reading Assignment: Aspirin

    • 2.8 Lewis Acids and Bases

    • Suggested Problems: Ch. 2: 2.38, 2.39, 2.68, 2.40 - 2.49, 2.30 - 34, 2.38, 2.41 - 42, 2.49, 2.62, 2.69 – 70, 2.63 - 2.66

Brønsted-Lowry Acids and Bases

  • Definition of Brønsted-Lowry Acids and Bases:

    • Acid: Can donate a hydrogen ion (H+)

    • Base: Can accept a hydrogen ion (H+)

  • Proton (H+): A hydrogen atom without its electron.

  • Compounds: Compounds containing both hydrogen atoms and lone pairs can act as either acids or bases, depending on the reaction.

    • Examples include H2O, CH3OH, morphine.

Reactions of Brønsted-Lowry Acids and Bases

  • Result in the transfer of a proton from an acid to a base.

  • Formation of conjugate acid of the base and conjugate base of the acid.

  • A double reaction arrow indicates equilibrium, showing that the reaction can proceed in both directions.

Acid Strength and pKa

  • Acid Strength: The tendency of an acid to donate a proton.

  • Equilibrium Constant (Keq) and Ka: Useful for measuring acid strength.

  • Using pKa values is often more convenient than using Ka values.

  • Relationship between Acid Strength and Conjugate Bases:

    • Stronger acids have weaker conjugate bases.

    • Stronger bases have weaker conjugate acids.

Factors That Determine Acid Strength

  • Factors that stabilize the conjugate base increase the acidity of the acid.

  • Key factors affecting acidity include:

    • Element Effects

    • Inductive Effects

    • Resonance Effects

    • Hybridization Effects

  • Always analyze the stability of conjugate bases when comparing acid strengths.

Element Effects

  • Acid strength increases as the size of the element bonded to hydrogen increases.

  • Stability of charges is spread over a larger volume.

  • Example: H—C < H—N < H—O < H—F (acidity increases across a period).

Inductive Effects

  • Defined as the influence of electronegativity difference pulling electron density through sigma bonds.

  • More electronegative atoms stabilize negative charges by withdrawing electron density.

  • Acidity of H-A increases with the presence of electron-withdrawing groups.

Resonance Effects

  • Charge delocalization through resonance can enhance acidity.

  • Example: Conjugate base of acetic acid is resonance stabilized, while that of ethanol has a localized charge.

Summary of Factors Determining Acid Strength

  • Step 1: Identify atoms bonded to hydrogen and assess using periodic trends.

  • Step 2: If the same element is involved, analyze the conjugate bases for additional differences such as stabilization by electron-withdrawing groups or resonance.

Lewis Acids and Bases

  • Lewis Base: Electron pair donor.

  • Lewis Acid: Electron pair acceptor.

  • Any electron-deficient species capable of accepting an electron pair qualifies as a Lewis acid.

  • All Brønsted-Lowry acids are Lewis acids, but not vice versa.

Conclusion on Lewis Acid-Base Reactions

  • Interaction involves a Lewis base donating an electron pair to a Lewis acid, resulting in the formation of new bonds, often leading to carbocations.