Acids and Bases in Organic Chemistry

Arrhenius Acids

The Arrhenius definition of acids is foundational in understanding acid-base chemistry. According to this concept, an Arrhenius acid is a substance that dissociates in water to produce hydronium ions (H3O+).

• Definition: An Arrhenius acid increases the concentration of H3O+ ions in aqueous solution.

• Example: Sulfuric acid dissociates as follows:

• Acetic acid dissociates as:

• Strength: Stronger acids dissociate to a greater extent than weaker acids.

Arrhenius Bases

Arrhenius bases are substances that dissociate in water to yield hydroxide ions (OH-).

• Definition: An Arrhenius base increases the concentration of OH- ions in aqueous solution.

• Examples:

• Strength: Stronger bases (e.g., NaOH) dissociate more completely than weaker bases (e.g., Mg(OH)2).

Brønsted–Lowry Acids and Bases

The Brønsted–Lowry theory broadens the definition of acids and bases beyond aqueous solutions.

• Brønsted–Lowry acid: Any species that donates a proton (H+).

• Brønsted–Lowry base: Any species that accepts a proton.

• Examples:

  • HCl donates a proton (acid), NaOH accepts a proton (base).

  • H2SO4 donates a proton (acid), NH3 accepts a proton (base).

Conjugate Acids and Bases

Every acid-base reaction involves the transfer of a proton, resulting in the formation of conjugate acid-base pairs.

• Conjugate acid: The species formed when a base gains a proton.

• Conjugate base: The species formed when an acid loses a proton.

• Example:

  • HSO4- is the conjugate base of H2SO4; H3O+ is the conjugate acid of H2O.

  • CH3COO- is the conjugate base of acetic acid; CH3OH is the conjugate acid of methoxide.

Acid Strength

The strength of an acid is determined by its tendency to donate a proton, which is quantified by the acid dissociation constant (Ka).

• Acid dissociation reaction:

• Acid dissociation constant:

• pKa:

• Interpretation: A larger value of Ka (or a smaller pKa) indicates a stronger acid.

Additional info: These slides introduce the fundamental acid-base theories and the concept of conjugate acid-base pairs, which are essential for understanding reactivity and mechanisms in organic chemistry.