Overview
This lecture covers the fundamentals of acids and bases in organic chemistry, focusing on the Bronsted-Lowry theory, proton transfer mechanisms, use of curved arrow notation, and quantitative analysis using the pKa chart.
Bronsted-Lowry Acid-Base Theory
- A Bronsted-Lowry acid donates a proton (H+); a base accepts a proton.
- Proton transfer creates conjugate acid-base pairs, which differ by one hydrogen ion.
- Curved arrow notation shows electron flow in acid-base reactions, from electron-rich base to electron-deficient acid.
- Acid-base reactions typically involve a concerted (one-step) mechanism with two arrows.
- The conjugate acid has one more hydrogen than its conjugate base.
Mechanisms and Examples
- Example: In HCl + H2O, water acts as the base, accepting a proton and forming H3O+ and Clโ.
- Arrow starts at the base (electron pair) and points towards the acidic proton; the bond to H breaks and forms the conjugate base.
- Conjugate pairs: water/hydroxide, ammonia/ammonium, acetic acid/acetate, etc.
- Proton transfers always occur with the base attacking the proton and electrons shifting simultaneously.
Quantitative Perspective: pKa and Acid Strength
- Acid strength is measured by its ability to ionize: strong acids fully ionize, weak acids only partially.
- Ka is the acid dissociation constant; larger Ka means stronger acid.
- pKa = โlog(Ka); smaller pKa means stronger acid.
- Strong acids have weak conjugate bases; the relationship is inverse.
- pKa values provide a quick way to compare acid (and base) strengths.
Using the pKa Chart
- The left column lists acids, middle shows pKa values, right column lists conjugate bases.
- Compare acidity: lower pKa = stronger acid.
- Compare basicity: the conjugate base with the higher pKa of its conjugate acid is the stronger base.
- Position on the pKa chart (higher or lower) determines relative strengths for acids and bases.
- The equilibrium in acid-base reactions favors formation of the weaker acid/base (the side with the larger pKa value).
Applying pKa to Predict Equilibria
- Label conjugate pairs and identify acids/bases on both sides of the equation.
- Find pKa for both acids; equilibrium favors the side with the weaker acid (larger pKa).
- The difference in pKa values (ฮpKa) quantifies the favorability of the equilibrium direction.
Key Terms & Definitions
- Bronsted-Lowry Acid โ species that donates a proton (H+).
- Bronsted-Lowry Base โ species that accepts a proton.
- Conjugate Acid/Base โ pair differing by one proton.
- Ka โ acid dissociation constant, measures acid strength.
- pKa โ negative log of Ka; smaller value indicates stronger acid.
- Curved Arrow Notation โ shows movement of electron pairs in mechanisms.
Action Items / Next Steps
- Print and familiarize yourself with the pKa chart from your e-learning or textbook.
- Complete assigned homework problems, practicing mechanism drawing and pKa comparisons.
- Prepare for qualitative acidity analysis and further acid-base theory in the next lesson.