Organic acids and bases are crucial players in chemical reactions. They're defined by specific structural features that determine their behavior. Acids have polarized O-H bonds, while bases have lone electron pairs on nitrogen or oxygen atoms.

Understanding how these compounds interact is key to grasping organic chemistry. Their strength is measured by pKa values, and they form conjugate pairs when reacting. Biological compounds like amino acids showcase both acidic and basic properties, highlighting their importance in living systems.

Organic Acids and Bases

Structural features of organic acids/bases

• Organic acids contain polarized O-H bond
  • Hydrogen partially positive ($\delta+$) oxygen partially negative ($\delta-$) due to electronegativity difference
  • Common functional groups include carboxylic acids (-COOH), phenols (Ar-OH), and sulfonic acids (-SO3H)
• Organic bases contain lone pair of electrons on nitrogen or oxygen
  • Allows acceptance of proton (H+) to form conjugate acid
  • Common functional groups include amines (primary R-NH2, secondary R2NH, tertiary R3N), aromatic amines (Ar-NH2), and imines (R2C=NR)

Charge stabilization in conjugate bases

• Conjugate bases of organic acids form when acid donates proton (H+)
  • Resulting negative charge stabilized by electronegativity of atom bearing charge (oxygen better stabilizes than carbon)
  • Resonance delocalization of negative charge spreads it over multiple atoms more stable with more resonance structures
• Carboxylate anion (RCOO-) stabilized by oxygen electronegativity and resonance between two oxygens
• Phenoxide anion (ArO-) stabilized by oxygen electronegativity and resonance with aromatic ring
• Sulfonate anion (RSO3-) stabilized by oxygen electronegativity and resonance among three oxygens

Acid-base theories and quantitative measures

• Brønsted-Lowry theory defines acids as proton donors and bases as proton acceptors
• Lewis theory expands definition to include electron pair acceptors (acids) and donors (bases)
• pKa measures acid strength, with lower values indicating stronger acids
• Henderson-Hasselbalch equation relates pH, pKa, and concentrations of acid-base conjugate pairs
• Buffer solutions resist pH changes and can be prepared using weak acids/bases and their conjugate partners

Acid-base behavior of biological compounds

• Nitrogen-containing compounds
  1. Amines act as bases lone pair on nitrogen accepts proton (H+) forms ammonium ions (R-NH3+)
  2. Pyridine and imidazole aromatic nitrogen heterocycles act as bases due to lone pair on nitrogen
• Oxygen-containing compounds
  • Alcohols and phenols weakly acidic due to polarized O-H bond can donate proton (H+) to form alkoxide or phenoxide anions
  • Carboxylic acids moderately strong acids donate proton (H+) to form carboxylate anions
• Amino acids contain both acidic (carboxyl -COOH) and basic (amino -NH2) groups
  • Zwitterionic form proton transfer from carboxyl to amino group net neutral charge but localized positive (NH3+) and negative (COO-) charges
  • pH-dependent behavior
    1. Low pH protonated amino group neutral carboxyl group
    2. Isoelectric point (pI) zwitterionic form predominates
    3. High pH deprotonated carboxyl group neutral amino group