Chem 3/17

Addition Reactions Overview

• Addition reactions involve the addition of atoms across a double bond in alkenes.

• The product formed can lead to carbocations which might rearrange to give more stable configurations.

Mechanistic Steps in Addition Reactions

• Protonation Step:

  • Proton adds to the alkene forming a carbocation (can use hydronium ion as proxy for acid).

  • Typically, the proton adds to the less substituted carbon to form a more stable carbocation on the more substituted carbon.

• Formation of Oxonium Ion:

  • Water can add to the carbocation resulting in an oxonium ion.

  • The excess proton is subsequently lost, leading to the final alcohol product.

• Carbocation Rearrangement:

  • Rearrangement occurs if a more stable carbocation can be formed (e.g., hydride shift).

  • Carbocation stability order: tertiary > secondary > primary.

Example of Acid-Catalyzed Addition

• Hydration of Alkenes:

  • Example: Acid-catalyzed addition of water to methylcyclohexene.

  • Major product forms on the more substituted carbon due to Markovnikov’s rule, while minor product forms on the less substituted carbon.

Anti-Markovnikov Addition (Hydroboration-Oxidation)

• Hydroboration Step:

  • Use borane (BH₃) for the initial addition.

  • Produces organoborane through a one-step reaction with syn-addition of boron and hydrogen.

  • Key result: OH ends up on the less substituted carbon.

• Oxidation Step:

  • Reaction with hydrogen peroxide (H₂O₂) and base converts boron into a hydroxyl group.

  • Maintains stereochemistry of the original alkene due to the syn addition.

Stereochemistry of Addition Reactions

• Syn Addition vs. Anti Addition:

  • Syn: Addition occurs on the same side of the double bond.

  • Anti: Addition occurs on opposite sides.

  • Stereochemical outcomes can affect the compound's properties significantly.

Three-Membered Ring Intermediates

• Addition reactions may lead to the formation of three-membered rings (e.g., bromonium ions in bromination).

• Nucleophilic Attack:

  • The nucleophile attacks from the side opposite to Y in the three-membered ring, thus leading to anti addition regardless of the nature of the initial ring structure.

Mechanism of Bromination

• Free Bromine Addition to Alkenes:

  • Alkene acts as a nucleophile, leading to bromonium ion formation.

  • Bromonium ion demonstrates anti addition of bromine.

  • Resulting products depend on both stereochemical and regiochemical considerations.

Considerations in Synthesis and Stability

• Synthesis should ideally favor stable carbocation intermediates to minimize by-products.

• Understanding carbocation stability and the regioselectivity of reactions enhances predictive capabilities in synthetic chemistry.

Chapter Overview

• Recap of key concepts discussed:

  • Acid-catalyzed hydration and appropriate mechanisms.

  • Hydroboration-oxidation sequence and its relevance in preparing less substituted alcohols.

  • Introduction to three-membered ring chemistry and stereochemistry implications in organic reactions.