Chem 3/7

Overview of Addition Reactions

• Focus on addition reactions as a reminder, moving from substitution and elimination reactions.

• Introduction of new types of addition reactions and carbocation considerations.

Electrophilic Addition Reactions

• Basic Concept: Involves adding a nucleophile (like HX or H2O) to an alkene.

• Acid-Catalyzed Addition of Water/Alcohol:

  • Catalyzed by enzymes which show stereospecificity, producing only the S stereoisomer of S-malic acid.

  • Fumarate does not react due to geometric isomerism: the cis double bond does not fit the enzyme's active site.

Stereochemical Features

• Enzyme Specificity:

  • Enzymes exhibit high specificity in binding and reacting with substrates.

• Dehydration Reaction:

  • Reverse of acid-catalyzed addition, typically involving the formation of water.

  • Commonly shown in laboratories with various yield conditions tailored for reaction efficiency.

Carbocation Stability and Rearrangement

• Secondary vs. Tertiary Carbocations:

  • Considerations on proton addition leading to secondary or tertiary carbocations.

  • Resonance playing a key role in determining the stability of carbocations.

• Hydride Shifts:

  • Common rearrangement mechanism where a hydrogen atom moves, affecting carbocation position and potential product outcomes.

  • Stability often governs the likelihood of these shifts occurring in reactions.

Dimerization and Polymerization

• Dimerization Process:

  • Dimerization occurs with specific conditions, distinct from polymerization where continuous addition occurs to form long chain compounds like polyisobutylene.

• Cationic Polymerization:

  • Reactions involving carbocation intermediates that lead to polymers.

Importance of Carbocation Arrangements

• Mechanisms Involving Rearrangement:

  • Secondary alcohols converting to more stable tertiary cations when using phosphorus trichloride instead of hydrogen halides.

  • Biological processes also utilize carbocation rearrangements, albeit less frequently.

• Control Issues in Synthetic Reactions:

  • Difficulty in controlling the rearrangements leads to multiple products, challenging synthetic methods.

Complex Rearrangement Mechanism Example

• Karilenol to Isoclobine:

  • Reaction details, including the removal of water to generate a tertiary carbocation.

  • Ring Expansion:

    • Involves migrating bonds leading to different ring structures, showcasing the complexity of the rearrangement mechanisms.

  • Geometry and stability considerations affect product formation in significant ways.

Conclusion

• Summation of recent topics on addition reactions and carbocation mechanisms.

• Importance of understanding various reaction pathways and their implications for organic synthesis.