General Trends in Acid Strength

Periodic Table Trends

Acid strength varies systematically across the periodic table. Understanding these trends is essential for predicting reactivity and stability in organic chemistry.

• Down a Column/Group: Acid strength increases as you go down a group (column) in the periodic table.

• Across a Row/Period: Acid strength increases as you move to the right across a period.

• Example: (acid strength increases down the halogen group)

Reason for Stability

• Ionic Volume: Larger ionic volume (radius) allows charge to spread out over a larger volume, stabilizing the ion.

• Charge Density: Smaller ions have higher charge density, making them less stable.

• Example: is more stable than due to its larger size.

Trends Across a Period

• Acidity increases as you move to the right in a period.

• Example:

pKa Values

• pKa: A quantitative measure of acid strength. Lower pKa means stronger acid.

• Examples:

  • : pKa ≈ 50

  • : pKa ≈ 35

  • : pKa ≈ 15.7

  • : pKa ≈ 3.2

Lewis Acidity

Lewis acids are species willing to accept an electron pair from a Lewis base.

• Lewis Acid: Electron pair acceptor

• Lewis Base: Electron pair donor

Curved Arrow Notation in Organic Chemistry

Arrow-Pushing Formalism

Curved arrows are used to show the movement of electron pairs during chemical reactions. This notation is fundamental for illustrating reaction mechanisms.

• Movement: Arrows indicate movement of an electron pair from a region of high electron density to low electron density.

• Can Represent:

  • A reaction step

  • Resonance forms

Resonance Forms

• Resonance involves delocalization of electrons, often shown with curved arrows.

• Example: →

Charge Changes

• If an electron pair moves from an anionic center, it becomes neutral.

• If an electron pair moves to a neutral center, it becomes anionic.

• If an electron pair moves from a neutral species, it becomes cationic.

Single Electron Movement

• Single electron movement is shown with an arrow that has half a head.

Kinetic vs. Thermodynamic Control of a Reaction

Definitions

Reactions can be controlled by either kinetic or thermodynamic factors, affecting the product distribution.

• Kinetic Product: Forms fastest, favored under low temperature and short reaction times.

• Thermodynamic Product: More stable, favored under higher temperature and longer reaction times.

Energy Diagram

• Kinetic product has a lower activation energy ().

• Thermodynamic product has a lower final energy (more stable).

= Activation energy Kinetic control: Lower $E_a$, faster formation Thermodynamic control: Lower product energy, more stable

Electrophiles and Nucleophiles

Definitions

• Electrophile: Species that wants to accept an electron pair.

• Nucleophile: Species that wants to donate an electron pair.

Comparison to Lewis Acids and Bases

• Electrophiles are analogous to Lewis acids (electron pair acceptors).

• Nucleophiles are analogous to Lewis bases (electron pair donors).

Summary Table: Acid Strength Trends

Key Equations

• pKa relationship:

• Acid dissociation:

Additional info: Academic context and examples have been expanded for clarity and completeness.