Analyzing Organic Reactions

Objectives

Acids and Bases
- Define Arrhenius, Brønsted–Lowry, and Lewis definitions of acids/bases.
- Discuss acid-base strength and common functional groups.
Nucleophiles, Electrophiles, and Leaving Groups
- Detailed discussion of nucleophilic substitution reactions.
Oxidation-Reduction Reactions
- Define oxidation, reduction, oxidizing agents, and reducing agents.
- Discuss key reactions for the MCAT.
Chemoselectivity
Problem-Solving Organic Chemistry Reactions
- Step-by-step approach to problem solving.

Arrhenius Definition (most restrictive)
- Acid: Dissociates to form H⁺ ions in solution
- Base: Dissociates to form OH⁻ ions in solution
Brønsted–Lowry Definition
- Acid: Donates H⁺ ions
- Base: Accepts H⁺ ions
- More inclusive than Arrhenius
- E.g., Hydroxide, ammonia are Brønsted–Lowry bases but not Arrhenius bases
Lewis Definition
- Acid: Electron pair acceptor
- Base: Electron pair donor
- Most inclusive definition
- Focuses on electrons forming coordinate covalent bonds
- Lewis acids often used as catalysts in organic reactions
Hierarchy
- Every Arrhenius acid/base is a Brønsted–Lowry acid/base
- Every Brønsted–Lowry acid/base is a Lewis acid/base
- Converse not necessarily true

Acid Dissociation Constant (Ka) and pKa
- Ka = \([H⁺][A⁻])/[HA]
- pKa = -log(Ka)
- Strong acids: Low or negative pKa (< -2)
- Weak organic acids: pKa between -2 and 20
Common Functional Groups and pKa Values
- Alkane (C-H): pKa ~50 (very weak acid)
- Alkene (C=C): pKa ~44
- Amine (NH₃): pKa ~38
- Alkyne (C≡C): pKa ~25
- Alcohol (OH): pKa ~16-18
- Water (H₂O): pKa ~15.7
- Phenol (ArOH): pKa ~10
- Carboxylic Acid (COOH): pKa ~4-5
- Hydronium Ion (H₃O⁺): pKa ~-1.7
Trends
- Bond strength decreases down the periodic table → Acidity increases
- Higher electronegativity → Higher acidity
- Low bond strength takes precedence when trends oppose

We will cover Nucleophiles, Electrophiles, and Leaving Groups in the next lecture.