Oxidation notes from knote

WEIRD REACTIONS THAT ARE WONDERFUL: OXIDATION AND REDUCTION OF ORGANIC MOLECULES

Overview of Oxidation and Reduction

• Oxidation:
    - Definition: Results in an increase in the number of C−Z bonds (usually C−O bonds) or a decrease in the number of C−H bonds.

• Reduction:
    - Definition: Results in a decrease in the number of C−Z bonds (usually C−O bonds) or an increase in the number of C−H bonds.

HYDRIDE REDUCTION

Adding Hydrogen

• Method: To add hydrogen ($H_2$), a hydride ($H^-$) and a proton ($H^+$) are utilized.

• Common Hydride Reducing Agents: Typically contain a hydrogen atom bonded to a metal, often boron or aluminum. Examples include:
    - Sodium Borohydride (NaBH₄)
    - Lithium Aluminum Hydride (LiAlH₄)
    - Mechanism of Action: Both reagents transfer $H^-$ to the substrate, followed by the addition of a proton ($H^+$) from sources such as water ($H_2O$) or alcohol.

Reducing Agent Properties

• LiAlH₄ and NaBH₄ Capabilities:
    - Both can reduce aldehydes and ketones to alcohols.
    - Only LiAlH₄ can reduce carboxylic acids and esters to alcohols.

• Comparison of Reactivity:
    - NaBH₄: Less reactive nucleophilic hydride, considered more useful.
    - LiAlH₄: Highly reactive Bronstead base.
    - Hydrides Comparison:
      - Very reactive electrons not in a covalent bond
      - Conduct longer, weaker Al-H bonds versus shorter, stronger B-H bonds.

INTRODUCTION TO CARBONYL CHEMISTRY

Overview of Carbonyl Groups

• Carbonyl Group Characteristics:
    - Presence of electronegative oxygen creates a polarized bond.
    - This polarization renders the carbonyl carbon electron deficient.

• Resonance Structures:
    - The carbonyl group is effectively represented by two resonance structures, highlighting the nature of the double bond.

RACEMIC PRODUCTS

Stereochemistry in Carbonyl Reduction

• Planarity of Carbonyls:
    - The flat structure of carbonyls allows hydride to approach the double bond with equal probability from either side, potentially leading to racemic mixtures in products.

METAL HYDRIDE REDUCING AGENTS

Usage of LiAlH₄

• Strength: LiAlH₄ is recognized as a vigorous reducing agent, reacting with all carboxylic acid derivatives.

OTHER METAL HYDRIDE REDUCING AGENTS

DIBAL-H

• Compound Structure: Diisobutylaluminum hydride [(CH₃)₂CHCH₂]₂AlH

• Reactivity: Characterized by two bulky isobutyl groups which reduce reactivity compared to LiAlH₄ but ideal for reducing carboxylic acid derivatives to aldehydes.

OXIDATION OF ORGANIC MOLECULES

General Oxidizing Agents

• Types of Oxidizing Agents:
    1. Reagents with oxygen-oxygen bonds (e.g., peroxides, peroxyacids).
    2. Reagents containing metal-oxygen bonds (e.g., potassium dichromate).

• Common Metal-Oxygen Bond Oxidants:
    - Chromium (+6 oxidation state).
      - Common chromium reagents:
          - $CrO_3$
          - Sodium or potassium dichromate ($Na_2Cr_2O_7$, $K_2Cr_2O_7$).
      - Selective Oxidant: Pyridinium chlorochromate (PCC).
    - Manganese (+7 oxidation state).
      - Common reagent: Potassium permanganate ($KMnO_4$).
    - Additional Oxidizing Agents:
      - Osmium tetroxide ($OsO_4$)
      - Silver(I) oxide ($Ag_2O$)

OXIDATION REACTIONS OF ALCOHOLS

Oxidation of Alcohols

• General Observation: Alcohols are oxidized to various carbonyl compounds depending on alcohol type.
    - 1° Alcohols: Can oxidize to either aldehydes or carboxylic acids by replacing one or two C−H bonds with C−O bonds.
      - Reaction Mechanism:
        - One or two C-H bonds replaced by C-O bonds yielding:
          - $R^{1}OH <br>ightarrow$ aldehyde or carboxylic acid.
    - 2° Alcohols: Oxidized to ketones by replacing one C−H bond with a C−O bond.
      - Reaction Mechanism:
        - One C-H bond replaced by C-O bond yielding:
          - $R^{2}OH <br>ightarrow$ ketone.
    - 3° Alcohols: Lack hydrogen atoms on the carbon with the $OH$ group, thus are not readily oxidized.
      - Reaction Mechanism:
        - No reaction occurs for 3° alcohols:
          - $R^{3}OH <br>ightarrow$ No reaction.

CHROMIUM OXIDIZING REAGENTS

Mechanism of Alcohol Oxidation

• Cr6+ oxidants:
    - Typically used for the oxidation of alcohols to carbonyl compounds. These oxidants are reduced to produce Cr3+ products.
      - Examples:
        - $CrO_3$, $Na_2Cr_2O_7$, $K_2Cr_2O_7$.
    - Conditions for Use:
      - Strong oxidants are usually utilized in aqueous acid (e.g., $H_2SO_4 + H_2O$).
    - Selective Oxidation:
      - PCC can be employed in non-strong acid conditions (soluble in $CH_2Cl_2$), considered a milder oxidant.

OXIDATION OF 2° ALCOHOLS

Reactants and Reactions

• Oxidation Efficiency:
    - Any of the Cr6+ oxidizing agents effectively oxidize 2° alcohols to produce ketones.

MECHANISM OF OXIDATION

Alcohols :Visual Mechanisms

• CrO₃ Oxidation Mechanism: Shown for illustrative purposes only; typically not required knowledge for exams.

• 1° Alcohols Oxidation Mechanism: Similarly illustrative and not exam-relevant.

SUMMARY OF ALCOHOL OXIDATION REACTIONS

Carbonyl Reactions Overview

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• Carbonyl Groups as Functional Groups:
    - Important in chemistry.

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• Reagent Selection: Control of product outcomes is achievable via selective reagent choice.

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• Table of Results:
  
  

  
  

  Alcohol Type	Na2Cr2O7 / H2SO4	PCC / CH2Cl2
  1° Alcohol	Carboxylic acid	Aldehyde
  2° Alcohol	Ketone	Ketone
  3° Alcohol	No reaction	No reaction
  SUMMARY OF CARBONYL REDUCTION REACTIONS

  Overview

  
  

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  • Functional Importance: Carbonyl groups are critical functional groups in organic chemistry.

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  • Reagent Control: Product formation can be accurately manipulated through reagent selection.

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  • Table of Results:
    
    

    
    

    Carbonyl Type	NaBH4	LiAlH4	DIBAL-H
    Aldehyde	1° Alcohol	1° Alcohol	1° Alcohol
    Ketone	2° Alcohol	2° Alcohol	2° Alcohol
    Carboxylic Acids	No Reaction	1° Alcohol	Aldehyde (ONLY with 1 equivalent)
    UNCOVERING THE ISSUES WITH ALKYNES

    Challenges Presented by Alkynes

    • General Summary: Discusses how alkynes present difficulties that require attention and solutions.