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
“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
Oxidizing agent – the species that is reduced, it gains the electrons the species being
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
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
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
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
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