Chapter 3: Acids and Bases
The Logic of Organic Chemistry
Introduction
Acid-base chemistry appears throughout both semesters of organic chemistry.
Although students often associate acids and bases with introductory chemistry, these concepts become even more important in organic chemistry because they help explain:
- stability,
- equilibrium,
- reaction mechanisms,
- and molecular reactivity.
Many reactions encountered in organic chemistry are, in one form or another, acid-base reactions.
Understanding why molecules donate or accept protons provides a framework for understanding much of the subject.
Why Acids and Bases Matter
Acidity and basicity influence:
Equilibrium
Which side of a reaction is favored.
Stability
Stable conjugate bases generally correspond to stronger acids.
Reactivity
Acids and bases frequently initiate reaction mechanisms.
Mechanisms
Proton transfers are among the most common steps encountered in organic chemistry.
Stability Determines Acidity
One of the most important ideas in organic chemistry is:
The stability of the conjugate base determines the strength of the acid.
This principle appears repeatedly throughout both semesters.
Factors Influencing Stability
Several factors contribute to stability:
Resonance
Delocalized charge increases stability.
Atom (Electronegativity and Size)
More electronegative atoms stabilize negative charge more effectively. Atomic size matters too, and dominates within a group: acidity increases down a column (HI > HBr > HCl > HF) even though electronegativity decreases, because a larger atom spreads the negative charge over more volume.
Inductive Effects
Electron-withdrawing groups influence charge distribution.
Hybridization
Atoms with greater s-character generally stabilize charge more effectively.
These ideas eventually combine into systematic frameworks for ranking acidity.
Thinking About Acids and Bases
Helpful questions include:
- Which atom bears the charge?
- Can resonance stabilize the charge?
- Is the charge localized or delocalized?
- Which conjugate base is more stable?
Gentle Exercises
Determine:
- strongest acid,
- strongest base,
- conjugate acid,
- conjugate base.
Explain:
Why are carboxylic acids stronger acids than alcohols?
Common Mistakes
Memorizing pKa Values
Better approach:
Understand stability.
Ignoring Resonance
Better approach:
Ask whether charge can be delocalized.
Self-Assessment
I can:
☐ Identify acids and bases.
☐ Determine conjugate acids and bases.
☐ Understand the meaning of pKa.
☐ Explain acidity in terms of stability.
☐ Appreciate the importance of resonance.
Further Study
Reading
LibreTexts Organic Chemistry — Ch. 2, Acids and Bases — Acids and bases; pKa; stability.
Videos
Khan Academy — Resonance and Acid-Base Chemistry — Acids and bases; pKa.
Organic Chemistry Tutor — Acids and bases; pKa.
Supplementary
Master Organic Chemistry — Acidity and basicity; ARIO; pKa table.
Looking Ahead
Molecules are not merely collections of atoms.
They possess shape.
The next chapter explores stereochemistry and the importance of three-dimensional structure.