Overview
This lecture introduces the reactions of alkenes, focusing on electrophilic addition mechanisms and the factors that determine product outcomes, including carbocation stability and the Hammond postulate.
Electrophilic Addition Reactions of Alkenes
- Alkenes undergo electrophilic addition reactions due to the electron-rich pi bond acting as a nucleophile.
- The pi bond attacks an electrophile (e.g., H in HBr), forming a carbocation intermediate.
- A nucleophile (e.g., Br-) then adds to the carbocation, resulting in an alkyl halide.
- The reaction is exergonic, with products having lower energy than reactants.
- This general mechanism applies regardless of the specific electrophile or nucleophile.
Mechanistic Steps and Product Formation
- The first step is addition of the electrophile to one sp2 carbon, forming a carbocation.
- The nucleophile adds to the other sp2 carbon.
- The mechanism predicts that varying the nucleophile/electrophile allows synthesis of alcohols, ethers, epoxides, ketones, and more from alkenes.
- With unsymmetrical alkenes, the position where the electrophile adds affects which carbocation intermediate forms, influencing the final product.
Carbocation Stability and Product Prediction
- Carbocation stability increases with more alkyl groups: tertiary > secondary > primary > methyl.
- Alkyl groups stabilize carbocations via hyperconjugation, dispersing positive charge.
- The more stable carbocation intermediate forms faster and leads to the major product.
The Hammond Postulate
- The transition state resembles the species (reactant or product) to which it is closest in energy.
- In exergonic steps, the transition state resembles the reactant; in endergonic steps, it resembles the product/intermediate.
- Factors stabilizing a carbocation also stabilize the transition state leading to it.
- Lower activation energy for more stable carbocations results in their preferential formation.
Product Distribution
- If activation energies for forming different carbocations are similar, multiple isomeric products may form.
- When one carbocation is much more stable, only one major product is formed.
Key Terms & Definitions
- Electrophilic Addition Reaction — A reaction where an electrophile and a nucleophile add across a double bond.
- Pi Bond — A type of covalent bond formed from sideways overlap of p orbitals, present in alkenes.
- Carbocation — An intermediate with a positively charged carbon atom.
- Hyperconjugation — Stabilization of a carbocation by delocalization of electrons from adjacent sigma bonds.
- Hammond Postulate — The transition state of a reaction resembles the structure (reactants or products) closest in energy.
Action Items / Next Steps
- Complete assigned homework problems on electrophilic addition reactions and carbocation stability.
- Review the concepts of carbocation formation, stability, and the Hammond postulate.
- Ask questions if any concepts from this lecture are unclear.