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Understanding Organic Redox Reactions: Mechanisms and Key Reagents

This chapter explores the fundamentals of redox reactions in organic chemistry, detailing the processes of oxidation and reduction. It elucidates how oxidation involves the loss of hydrogen or gain of oxygen, while reduction entails the opposite. Key reagents such as LiAlH4, NaBH4, and various chromic acid forms are discussed for synthesizing alcohols, carboxylic acids, and aldehydes. The chapter also highlights mechanisms involving hydride attacks and the impact of organometallic reagents like Grignard and organolithium compounds, emphasizing their roles as strong bases and nucleophiles.

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Understanding Organic Redox Reactions: Mechanisms and Key Reagents

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  1. Organic Chemistry ReviewsChapter 12 Cindy Boulton February 22, 2009

  2. Redox • Reduced: Gain electrons • Oxidation: Loose electrons • Organic Redox: • Oxidation [O]: loss of Hydrogen and/or gain of Oxygen • Reduction [H]: loss of Oxygen and/or gain of Hydrogen

  3. Reduction by Hydride • Making Alcohols • Hard to reduce: Carboxcylic Acids and Esters • Use LiAlH4/H3O+ • Easy to reduce: Ketone and Aldehyde • Use NaBH4/H2O • Use LiAlH4/H3O+ • Mechanism: • Hydride attacks the partially positive Carbon and then partial negative Oxygen gains a Hydrogen

  4. Oxidation • Cr6+ Oxidizing reagents • Cr6+ -> Cr3+ • Primary or Secondary Alcohols are oxidized to a Ketone or Aldehyde • Aldehyde is oxidized to Carboxycylic acid • Forms a aldehyde hydrate, Gem-diol • Only in the presence of Aqueous Chromic Acid (water) • Aqueous Chromic Acid • HCrO3/H2O • NaCr2O7/H2SO4/H2O • CrO3/H2O/acetone • Organic Chromic Acid • CrClO3- • PCC/CH2Cl2

  5. Oxidation • KMnO4 Oxidation • Organic oxidaion • Mn+7 -> Mn+4 reduction • Primary Alcohol reacts with KMnO4/H2O/OH- to form carboxcylic acid • No aldehyde intermediate

  6. Organometallic Reagents • Carbon attached to a metal • Carbanion sources: Carbon is partial negative • Organolithium • C-Br + 2Li -> C-Li + LiBr • Carbon is reduced • Lithium is oxidized • Organomagnesium Bromide • Grignard Reagents • C-Br + Mg -> C-MgBr • Carbon is reduced • Magnesium is oxidized

  7. Organometallic Reagents • Carbanion acts as a base • Electron Pair Donor/Gains a Hydrogen • The more substituted the stronger the base • Alkyl groups act as electron density donors • Carbanion acts as a nucleophile • If the base is too strong than an elimination reaction is more likely • Carbon-Carbon bond is formed

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