Overview
This lecture introduces organic acids and bases, focusing on their properties, pKa values, how to predict acid-base behavior, and the relationship between pKa and equilibrium in reactions.
Common Organic Acids and Bases
- Carboxylic acids are the most common acids in organic chemistry, with acetic and formic acid as key examples.
- Acetic acid (found in vinegar) has a pKa of 4.76; formic acid (from ant stings) has a pKa of 3.75.
- Alcohols (like methyl and ethyl alcohol) are less acidic, with pKa values around 15.
- Amines (e.g., methylamine, ammonia) have high pKa values (35-40), making them weak acids but good bases.
Protonated Species and Acidity
- Protonated compounds have gained a proton (H+), lowering their pKa and increasing their acidity.
- Protonated alcohols and carboxylic acids have pKa < 0, qualifying as strong acids.
- Protonated amines have pKa around 10–11, making them more acidic than regular amines but still not strong acids.
- Having an extra proton does not automatically make a substance a strong acid.
Acid-Base Reactions of Functional Groups
- Alcohols can act as both acids (lose H+) and bases (accept H+) depending on reaction partners.
- Carboxylic acids also behave as both acids and bases.
- Amines can act as acids (lose H+) or, more commonly, as bases (accept H+), with reactions often favoring the base pathway due to high pKa.
Using Curved Arrow Notation
- Curved arrows show electron movement: arrows start at electron pairs (negative sites) and point toward electron-deficient (positive) sites.
- Helps track bond breaking and forming during acid-base reactions.
Understanding pKa and Predicting Acid-Base Roles
- pKa scale increases in increments of 5: protonated alcohols/carboxylic acids (<0), carboxylic acids (~5), protonated amines (~10), alcohols/water (~15), amines (~40).
- Lower pKa = stronger acid; the reactant with the lower pKa acts as the acid in a reaction.
- Use pKa values to predict which compound donates a proton.
Equilibrium and pKa
- Equilibrium favors formation of the weaker acid (higher pKa) in an acid-base reaction.
- To predict equilibrium, compare the pKa of acids on both sides: the side with the higher pKa acid is favored.
- Calculating equilibrium constant: subtract product acid pKa from reactant acid pKa, then calculate 10^(difference).
Key Terms & Definitions
- Carboxylic Acid — Organic acid with the group -COOH, generally strong among organic acids.
- Alcohol — Organic compound with an -OH group.
- Amine — Organic compound with a nitrogen atom.
- Protonated — Describes a molecule that has gained a hydrogen ion (H+).
- pKa — Numeric value indicating acid strength; lower pKa means stronger acid.
- Conjugate Acid/Base — Product formed when an acid gives up or a base gains a proton.
- Curved Arrow Notation — Diagrammatic method to show electron flow in reactions.
Action Items / Next Steps
- Memorize approximate pKa values for common functional groups.
- Review acid-base reaction mechanisms using curved arrow notation.
- Practice predicting acids, bases, and reaction equilibrium using pKa.
- Complete assigned practice problems on acid-base equilibria.