CHMB42 Chapter 19.doc

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17 Apr 2012
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Chapter 19: More About Oxidation-Reduction Reactions
Fundamentals
Redox Reactions: organic reactions that involve the transfer of electrons from
one molecule to another.
LEO: loss of electrons is oxidation
GER: gain of electrons is reduction
Oxidation is always coupled with reduction; a species cannot gain electrons
unless another species in the reaction simultaneously loses electrons.
Reducing Agent: the species that is oxidized because it loses the electrons that
are used to reduce the other species.
Oxidizing Agent: the species that is reduced because it gains the electrons
given up by the other species when it is oxidized.
Reduction at carbon increases the number of C-H bonds or decreases the
number of C-O, C-N or C-X bonds.
Oxidation at carbon decreases the number of C-H bonds or increases the
number of C-O, C-N or C-X bonds.
Reduction Reactions
Catalytic Hydrogenations: reduction reactions because there are more C-H
bonds in the products than in the reactants; hydrogen can be added to carbon-
carbon double and triple bonds in presence of a metal catalyst (H2 and
Pt/Pd/Ni).
oAn alkyne can be stopped at a cis alkene if a partially deactivated
catalyst is used (Lindlar catalyst).
oIt can also be used to reduce carbon-nitrogen double and triple bonds to
produce amines.
oThe carbonyl group of ketones and aldehydes can be reduced by
catalytic hydrogenation with Raney nickel as the metal catalyst (H2 and
Raney nickel).
oRosenmund Reaction: reduction of an acyl chloride can be stopped at an
aldehyde if a partially deactivated catalyst is used (H2 and partially
deactivated Pd).
oCarboxylic acids, esters and amides are less reactive than aldehydes
and ketones so they cannot be reduced by catalytic hydrogenation.
Dissolving-Metal Reaction: when a compound is reduced using sodium in liquid
ammonia, sodium donates an electron to the compound and ammonia donates
a proton (overall reaction adds two electrons and two protons to the
compound).
oAn alkyne can be stopped at a trans alkene using Na or Li with NH3 (liq).
oThis cannot reduce a carbon-carbon double bond, only triple bonds.
Metal-Hydride Reductions: carbonyl groups are easily reduced by metal
hydrides (NaBH4 or LiAlH4).
oAldehydes, ketones and acyl halides can be reduced to alcohols by
sodium borohydride (NaBH4).
oLithium aluminum hydride is a stronger reducing agent and also reduces
carboxylic acids, esters and amides (making two compounds).
oIf DIBALH is used as a hydride donor at low temperature (-780C), the
reduction of the ester can be stopped after the addition of one
equivalent of hydride ion.
oThe carbonyl group of an amide is reduced to a methylene group (CH2)
by lithium aluminum hydride.
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