Chapter 4: Acids, Bases, and pKa

Definition of Acids and Bases

  • Arrhenius Definition (Svante Arrhenius, 1884)

    • Acid: A substance that dissolves in water to produce $H^+$ ions (protons).

    • Base: A substance that dissolves in water to produce $OH^-$ ions (hydroxide ions).

Lewis Definitions

  • Lewis Acid: A species that can form a new covalent bond by accepting a pair of electrons.

  • Lewis Base: A species that can form a new covalent bond by donating a pair of electrons.

Lewis Structures and Examples

  • Example of Lewis Structures:

    • CH₃CH₂F and $BCl_3$ interaction:

    • Diethyl ether acts as a Lewis base (donates a pair of electrons).

    • BF₃ acts as a Lewis acid (accepts a pair of electrons).

  • Nomenclature: Lewis acids are typically electrophiles, and Lewis bases are nucleophiles.

Chemical Interaction:

  • In the case of CH₃CH₂F and BF₃:

    • CH₃CH₂F (Lewis base) + BF₃ (Lewis acid) forms a BF₃-ether complex.

Brønsted-Lowry Definitions (1923)

  • Brønsted Acid: A proton donor.

  • Brønsted Base: A proton acceptor.

  • Acid-base Reaction: Defined as a proton-transfer reaction.

Conjugate Acid-Base Pairs

  • Definition: A pair of molecules or ions that are related by the transfer of a proton.

    • When an acid donates a proton, it becomes its conjugate base.

    • When a base accepts a proton, it becomes its conjugate acid.

  • Example:

    • CH₃COOH (acetic acid) + NH₃ (ammonia) → CH₃COO⁻ (acetate, conjugate base) + NH₄⁺ (ammonium ion, conjugate acid).

Visual Representation of Proton Transfer

  • Curved arrows are used to show the movement of protons during acid-base reactions, illustrating electron movement in Lewis structures.

  • Example of proton transfer in H-Cl bond:

    • $HCl
      ightarrow H^+ + Cl^-$ (The H goes as a $H^+$).

Growth of Acidity and Basicity: pKa Values

  • Acid Dissociation Constant (Ka): Defines the strength of an acid in water.

    • K<em>a=rac[H</em>3O+][A−][HA]K<em>a = rac{[H</em>3O^+][A^-]}{[HA]}

  • pKa Definition: pK<em>a=−extlog(K</em>a)pK<em>a = - ext{log}(K</em>a)

    • Smaller pKa values indicate stronger acids, larger pKa values indicate weaker acids.

Correlation Between Acidity, Basicity, and pKa:

  • Strong acids have weaker conjugate bases and vice versa.

  • Traits of Strength:

    • Higher pKa → weaker acid → stronger conjugate base.

    • Lower pKa → stronger acid → weaker conjugate base.

pKa Values for Common Acids

  • A tabulated list of various acids and their pKa values:

    • Ethane: pKa = 51 (Weaker acid)

    • Ethylene: pKa = 44

    • Ammonia: pKa = 38

    • Water: pKa ≈ 15.7

    • Acetic Acid: pKa ≈ 4.76

    • Nitric Acid: pKa = -1.5 (Strong acid)

Factors Affecting Acidity:

  • Atomic Size: As the size of atom increases down the periodic table, acidity increases.

    • The longer the H-X bond, the easier it is to release $H^+$ (proton).

  • Electronegativity: For atoms within the same period, acidity increases with increasing electronegativity.

  • Conjugate Base Stability: The more stable the conjugate base, the stronger the original acid.

Electron Withdrawing vs. Electron Donating Groups

  • Electron Withdrawing (EW) Groups:

    • Stabilize negative charge through resonance/induction improving acidity.

  • Electron Donating (ED) Groups:

    • Decrease acidity by destabilizing negative charges affecting conjugate bases.

Ranking Acidity in Compounds

  • Methodology for ranking compounds based on acidity:

    • Identify the most stable conjugate base after deprotonation.

    • Recognize that the equilibrium of an acid-base reaction typically favors the weaker acid (higher pKa).

  • Example questions to predict acidity or rank compounds:

    • Ranking acidity based on inductive effects (e.g., proximity and number of EW groups).

    • Analyzing resonance effects to determine acid strength (e.g., phenol vs. alcohols).

Visual Examples of Equilibrium

  • Examine equilibrium reactions to determine the favored side:

    • If given pKa values, equilibrium favors the side with the weaker acid (higher pKa).

    • In absence of pKa values, evaluate the stability of the conjugate base.