Organic Chemistry I: Polar Covalent Bonds; Acids & Bases - In-Depth Notes

Topic 2: Polar Covalent Bonds; Acids & Bases

Learning Objectives

• Bronsted-Lowry Acids/Bases:
  • Acid: Proton donor.
  • Base: Proton acceptor.
• Mechanism Arrows: Show proton transfer in acid-base reactions.
• pKa and Acid Strength: Understand how pKa relates to the strength of acids. Memorize pKa values and how they influence protonation states in buffer solutions.
• Acidity/Basicity Ranking: Rank compounds based on electronegativity, resonance, size, and inductive effects.
• Equilibrium Predictions: Predict which side of the equilibrium is favored based on relative acidity/basicity.

Brønsted-Lowry Definitions

• Brønsted-Lowry Acid Example:
  • Reaction: $\text{HCl + H}<em>2\text{O} \rightarrow \text{H}</em>3\text{O}^+ + \text{Cl}^-$
  • Here, HCl donates a proton (H+).
• Brønsted-Lowry Base Example:
  • Reaction: $\text{Na}^+ + \text{OH}^- + \text{H}_2\text{O} \rightarrow \text{Na}^+ + \text{HOH} + \text{O}^-$
  • Hydroxide (dOH) accepts a proton.

Conjugate Acid/Base Relationship

• Definition: In a Brønsted-Lowry acid-base reaction, $\text{HA} + \text{B} \rightleftharpoons \text{HB}^+ + \text{A}^-$
  • Conjugate Acid: Formed when a base accepts a proton.
  • Conjugate Base: Formed when an acid donates a proton.
• Equilibrium: Acid-base reactions can shift between forms depending on proton acceptance or donation.

Curved Arrows for Mechanism

• Arrow Usage: Curved arrows indicate electron transfer in reactions.
  • Arrows point from electron-rich areas (negative) to electron-poor areas (positive).
  • Mechanism Insight: Two arrows indicate simultaneous electron pair movement as the base attacks the acid.

Lewis Acid/Base Definition

• Lewis Acid: Accepts electron pairs.
• Lewis Base: Donates electron pairs.
• Curly arrows: Start from the lone pair of the Lewis base to an electron-deficient Lewis acid atom.

Equilibrium Principles

• Equilibrium Constant (Keq): Indicates favored side in reactions.
  • $K_{eq} = \frac{[products]}{[reactants]}$
• Acid-Base Strength: Strong acids have weak conjugate bases and vice versa. Stability of conjugate base impacts acid strength.
  • Inequalities: If $K<em>{eq} > 1$, products favored; if K{eq} < 1, reactants favored.

Factors Affecting Acid/Base Strength

1. Size: Larger atoms stabilize negative charge, leading to weaker bases and stronger acids.
2. Electronegativity: More electronegative atoms hold electrons tighter, resulting in stable anions and thus weaker bases/stronger acids.
3. Resonance: Delocalization of negative charges through resonance increases stability and acidity.
4. Induction: Electronegative groups stabilize anions through inductive effects, impacting acidity.
5. Hybridization: More s-character in hybridized orbitals leads to greater stability of electrons; more stable anions = weaker bases.

pKa and Acid Strength

• Definitions of pKa:
  • High pKa indicates weak acids (less dissociation) and strong conjugate bases.
  • Low pKa indicates strong acids (more dissociation) and weak conjugate bases.
• Examples of Acidic Hydrogens:
  • $C<em>{2}H</em>{6}$ (ethane) has a pKa of approx. 50 (very weak acid).
  • $CH_{3}COOH$ (acetic acid) has a pKa of approx. 5 (moderate acid).

Summary of Structural Effects on Acidity

• Table of Key Influences:
  • Electronegativity: More electronegative atoms stabilize conjugate bases (e.g., $HCl$ vs. $H_2S$).
  • Size: Larger atoms can stabilize charge better (e.g., $HI$ vs. $HF$).
  • Resonance: More resonance structures indicate more stable anions (e.g., acetate).
  • Inductive Effects: Presence of electronegative atoms can increase acidity.
  • Hybridization: Higher s-character enhances stability of conjugate bases.

Application Examples

• Determine Acid Strength: Compare pairs based on pKa values and structural factors.
• Protonation States: Assess protonation based on pH vs. pKa, determining the states at different pH levels.
• Levothyroxine Study: Examine reactivity concerning pH and interaction with other compounds such as calcium carbonate which can affect absorption.

Conclusion

• Understand these key concepts as they intertwine to form the foundation of Acid-Base chemistry which is crucial in organic reactions and mechanisms.