3306 Ch12_Lecture_HG

Chapter 12: Oxidation and Reduction of Organic Molecules

Oxidation vs. Reduction

  • Oxidation involves increasing the number of C-O bonds and decreasing C-H bonds.

  • Reduction involves increasing the number of C-H bonds and decreasing C-O bonds.

Forms of Carbon

  • Most Reduced Form: High number of C-H bonds (e.g., alkanes).

  • Most Oxidized Form: High number of C-O bonds (e.g., carbon dioxide, ketones).

Reductions of Organic Molecules

Types of Reductions

  1. H2 as Reducing Agent: Typically involves a metal catalyst.

  2. Dissolving Metal Reductions: Involves adding protons and electrons directly to a substrate.

  3. Hydride Reduction: Involves adding hydride (H−) and a proton (H+).

Catalytic Hydrogenation

  • Definition: The addition of H2 in the presence of a metal catalyst (commonly Pd, Pt, or Ni).

  • Mechanism: The process occurs with H2 adding in a syn fashion.

  • Heat of Hydrogenation: Used to evaluate the relative stability of alkenes.

Catalytic Hydrogenation Mechanism

  1. H2 adsorbs onto the catalyst, leading to cleavage of the H-H bond.

  2. Alkenes bond with the metal.

  3. Two H atoms are transferred to the alkene in sequence, forming an alkane.

  4. The alkane is released from the catalyst.

  • Reactivity: Increases with the amount of alkyl substitution (most reactive = least substituted).

Reduction of Alkynes

Hydrogen Addition Methods

  • H2 can be added to the triple bond using:

    • Catalytic Hydrogenation to form a cis alkene.

    • Dissolving Metal Reduction to form a trans alkene.

Dissolving Metal Reduction of Alkynes

Mechanism for Formation of Trans Alkene

  1. An electron is added to the alkyne, forming a radical anion.

  2. Protonation results in a radical.

  3. A second electron addition produces a carbanion.

  4. The carbanion is protonated to yield a trans alkene.

Oxidation Reactions

Types of Oxidation Reactions

  1. Epoxidation: Formation of epoxides from alkenes.

  2. Dihydroxylation: Addition of OH groups from alkenes to produce 1,2-diols.

  3. Oxidative Cleavage: Breaking down alkenes and alkynes into smaller carbonyl compounds, such as aldehydes and acids.

Mechanism of Dihydroxylation

  • Syn Dihydroxylation: Achieved using KMnO4 or OsO4, adds two hydroxy groups to form a glycol.

  • Anti Dihydroxylation: Involves epoxidation followed by ring-opening reactions (either with −OH or H3O+).

Ozonolysis: Oxidative Cleavage of Alkenes

  • Process: Involves reaction with ozone (O3) followed by treatment with zinc and water to yield carbonyl compounds.

Oxidative Cleavage of Alkynes

  • Summary: Similar to alkenes, alkynes undergo oxidative cleavage, resulting in either carboxylic acids or CO2 depending on the substitution of alkynes.

Oxidation of Alcohols

  • Primary Alcohols: Oxidized to aldehydes or carboxylic acids.

  • Secondary Alcohols: Convert to ketones upon oxidation.

  • Tertiary Alcohols: Do not undergo oxidation due to lack of hydrogen atoms on the carbon bearing the hydroxyl group.

Mechanism of Alcohol Oxidation

  • Primary Alcohols to Carboxylic Acids: Involves strong oxidizing agents; e.g., K2Cr2O7 in acidic conditions.

  • Secondary Alcohols to Ketones: Similar oxidants can be applied but with different outcomes.

The Sharpless Epoxidation

  • Enantioselective Reaction: Uses the Sharpless reagent to selectively form one enantiomer preferentially, creating a stereogenic center.

  • Applications: Important for synthesizing chiral compounds, such as insect pheromones.