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CHMB42H3 (16)
Chapter 20

CHAPTER 20 - Textbook Notes.docx

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Lana Mikhaylichenko

CHAPTER 20: MORE ABOUT OXIDATION–REDUCTION REACTIONS  In an oxidation-reduction reaction (redox reaction), one species loses electrons and one gains electrons. The species that loses electrons is oxidized, and the one that gains electrons is reduced.  “LEO the lion says GER”: Loss of Electrons is Oxidation; Gain of Electrons is Reduction.  Oxidation is always coupled with reduction.  Reducing agent – the species that is oxidized, it loses the electrons that the species being reduced gains.  Oxidizing agent – the species that is reduced, it gains the electrons the species being oxidized loses. 20.1 Oxidation–Reduction Reactions of Organic Compounds: An Overview  If the reaction increases the number of C–H bonds or decreases the number of C–O, C–N, or C–X bonds (where X denotes a halogen), the compound has been reduced.  If the reaction decreases the number of C–H bonds or increases the number of C–O, C–N, or C–X bonds, the compound has been oxidized.  Reduction at the carbon increases the number of C–H bonds or decreases the number of C–O, C–N, or C–X bonds.  Oxidation at the carbon decreases the number of C–H bonds or increases the number of C–O, C–N, or C–X bonds.  Hydrogen, hydrazine, and sodium borohydride are reducing agents.  Bromine and chromic acid (H C2O ) 4re oxidizing agents. 20.2 Reduction Reactions Reduction by Addition of Two Hydrogen Atoms  Hydrogen can be added to carbon–carbon double and triple bonds in the presence of a metal catalyst. These reactions, called catalytic hydrogenations, are reducing reactions because there are more C–H bonds in the products than in the reactants. Alkenes and alkynes are both reduced to alkanes.  Rosenmund reduction – reduction of acyl chloride to an aldehyde by using a deactivated palladium catalyst. Reduction by Addition of an Electron, a Proton, an Electron, and a Proton  Dissolving-metal reduction – a reduction brought about by the use of sodium or lithium metal dissolved in liquid ammonia. Reduction by Addition of a Hydride Ion and a Proton  Carbonyl groups are easily reduced by metal hydrides such as sodium borohydride (NaBH ) 4 or lithium aluminum hydride (LiAlH ). The actual reducing agent in metal-hydride - 4 reductions is hydride ion (H: ).  Hydride ion adds to the carbonyl carbon, forming an alkoxide ion that is subsequently protonated. In other words, the carbonyl group is reduced by the addition of an H followed by the addition of an H .  Aldehydes, ketones, and acyl halides can be reduced to alcohols by sodium borohydride.  The numbers in front of the reagents above or below a reaction arrow indicate that the second reagent is not added until reaction with the first reagent is completed.  LiAlH i4 generally used to reduce only compounds – such as carboxylic acids, esters, and amides – that cannot be reduced by the milder reagent.  The carbonyl group of an amide is reduced to a methylene group (CH ) b2 lithium aluminum hydride.  The multiply bonded carbon atoms of alkenes and alkynes do not possess a partial positive charge and therefore will not react with reagents that reduce compounds by donating a hydride ion. 20.3 Chemoselective Reactions  A chemoselective reaction is a reaction in which a reagent reacts with one functionally group in preference to another. For example, NaBH in4isopropyl alcohol reduces aldehydes faster than it reduces ketones.  Because sodium borohydride cannot reduce an ester, an amide, or a carboxylic acid, it can be used
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