Overview
The lecture explains the difference between using concentrated and diluted strong bases in organic reactions, including their effects on substitution and elimination reactions, and introduces rules for determining major and minor products.
Concentrated vs. Diluted Solutions
- Concentrated means little to no water in the solution.
- Diluted means much water is present in the solution.
- Example: undiluted vs. water-mixed cordial.
Strong vs. Weak Bases
- Strong bases (e.g., NaOH, KOH) dissociate completely in water.
- Weak bases (e.g., Ca(OH)₂) dissociate only partially.
Types of Organic Reactions
- Substitution: reactants and products both have single bonds; one atom replaces another.
- Addition: reactants have double bonds, products have single bonds; two atoms are added.
- Elimination: reactants have single bonds, products have double bonds; two atoms are removed.
Effect of Base Concentration on Reaction Type
- Diluted strong base favors substitution reactions (e.g., hydrolysis).
- Concentrated strong base favors elimination reactions (e.g., dehydrohalogenation).
- Substitution forms two products (e.g., alcohol and salt); elimination forms three products (e.g., alkene, salt, and water).
Examples with Haloalkanes
- Substitution with diluted NaOH: bromide replaced by hydroxide; forms alcohol and sodium bromide (hydrolysis).
- Elimination with concentrated NaOH: bromide and adjacent hydrogen removed; forms alkene, sodium bromide, and water (dehydrohalogenation).
Major and Minor Products
- Major and minor products arise when multiple products form from a reaction.
- Markovnikov’s Rule (addition): Hydrogen attaches to the carbon with more hydrogens; defines the major product in addition with asymmetric molecules.
- Zaitsev’s Rule (elimination): Major product is the alkene with the more highly substituted double bond.
Key Terms & Definitions
- Concentrated — Solution with little or no water.
- Diluted — Solution with much water added.
- Strong base — A base that completely dissociates in water (e.g., NaOH).
- Substitution reaction — An atom in a molecule is replaced by another atom.
- Addition reaction — Two atoms are added across a double bond.
- Elimination reaction — Two atoms are removed to form a double bond.
- Markovnikov’s Rule — In addition, hydrogen adds to the most hydrogenated carbon.
- Zaitsev’s Rule — In elimination, the most substituted alkene is favored.
Action Items / Next Steps
- Memorize the difference between concentrated and diluted bases.
- Review examples for substitution and elimination reactions.
- Learn Markovnikov’s and Zaitsev’s rules for determining major products.