Ch 12- Oxidation-Reduction (1)

Chapter 12: Oxidation/Reduction

Thermite Reaction

  • Reaction: Fe2O3 + 2Al → 2Fe + Al2O3

    • Iron is reduced: It gains electrons (e-).

    • Aluminum is oxidized: It loses electrons (e-).

  • Important concept for understanding oxidation and reduction in inorganic reactions.

Overview of Oxidation and Reduction in Organic Reactions

  1. Recognizing oxidation and reduction in organic reactions.

  2. Specific reactions:

    • a. Alkene/alkyne reduction

    • b. Alkene oxidation

    • c. Alcohol oxidation

  3. Role of oxidation/reduction in synthesis.

  4. Biochemical oxidation/reduction processes.

Oxidation/Reduction in Organic Compounds

  • Assessing changes in C-H and C-Z bonds (Z represents electronegative atoms, usually oxygen).

  • Oxidation:

    • Increase in C-Z bonds and/or decrease in C-H bonds.

  • Reduction:

    • Decrease in C-Z bonds and/or increase in C-H bonds.

  • Expected questions about the comparison of C-I and C-O bonds.

Identifying Oxidation and Reduction

  • Evaluate whether the reaction is oxidation, reduction, or neither:

    • Breaking C-H bonds = oxidation.

    • Making C-H bonds = reduction.

    • Example: CH3 CH3 (neither, as one change corresponds to oxidation and another to reduction).

Addition Reactions in Alkenes

  • Addition of reagents can also be seen as oxidation/reduction processes.

Types of Reducing Agents

  • Most reducing agents result in net addition of two H atoms.

  • Common reduction mechanisms include:

    1. Addition of molecular hydrogen (H2).

    2. Addition of 2e- and 2H+ in separate steps.

    3. Addition of H- followed by H+.

Reduction of Alkenes

  • General reaction format:

    • Alkene + H2 (using Pd-C as catalyst) → Alkane.

    • Addition reactions are syn (same side) and do not dissolve in the reaction medium.

  • Example: H2, Pd-C reduces CH3-CH=CH2 to CH3-CH2-CH3.

  • Observe stereochemistry; trans products not typically formed in certain contexts.

Reduction of Alkynes

  1. H2 addition with Pd-C catalyst produces alkanes.

  2. Reduction with Na in NH3 yields cis alkenes.

  3. H2 addition with Lindlar catalyst produces cis alkenes as well.

Reduction of Carbonyl Compounds

  • Treatment with H- sources (e.g., LiAlH4 or NaBH4) converts carbonyls to alcohols.

  • Involves H- transfer to carbon and H+ transfer to oxygen.

Oxidation Agents

  • Common oxidizing agents mention M-O or O-O bonds:

    • Ozone (O3), mCPBA, hydrogen peroxide, potassium chlorochromate (PCC).

  • Oxidation typically involves forming C-O bonds and breaking C-H bonds.

Epoxidation of Alkenes

  • Alkenes can be oxidized to epoxides using mCPBA.

  • Mechanism is concerted, maintaining stereochemistry in the starting materials.

Dihydroxylation of Alkenes

  • Two hydroxy groups can be added either syn (same side) or anti (opposite sides).

    • Syn addition often involves OsO4/NMO; anti addition through epoxide openings.

Oxidative Cleavage of Alkenes

  • Reaction with ozone followed by a reducing agent leads to the formation of carbonyls.

  • Example: Treatment of CH3-CH=CH2 with O3 followed by DMS.

Oxidation of Alcohols

  • Primary alcohols oxidize to form carbonyl compounds (aldehydes or carboxylic acids).

    • Use PCC to stop at the aldehyde stage.

  • Secondary alcohols convert to ketones, while tertiary alcohols are resistant to oxidation.