Oxidation and Reduction 

OXIDATION

REDUCTION

Combination with oxygen

Removal of oxygen

C + O2 à CO2
carbon is oxidized

2 HgO à 2Hg + O2
mercury is reduced

Loss of electrons

Gain of electrons

Fe à Fe3+ + 3 e-
iron is oxidized

Zn2+ + 2e- à Zn
zinc ion is reduced

Algebraic increase in oxidation number
 [becomes more positive]

Algebraic decrease in oxidation number
 [becomes more negative]

2KClO3  à 2 KCl + 3O2
the oxidation number of O goes from -2 to 0
oxygen is oxidized

PbO2 + Mg à PbO + MgO
the oxidation number of Pb goes from +4 to +2
lead is reduced

2KClO3  à 2 KCl + 3O2
the oxidation number of Cl goes from +5 to -1
chlorine is reduced

Removal of hydrogen atoms

Addition of hydrogen atoms

Oxidation and reduction always occur together!

The redox reaction:  2“H2” + O2 à 2H2O
is the major source of energy in aerobic organisms!

  

“Hydrogen” Comes in Many Guises 

Representation

Description

H2

H:H, H-H molecular hydrogen, a diatomic
molecule consisting of two hydrogen nuclei
[protons] joined by a s1s-1s single covalent bond
containing two electrons

“H2”

molecular hydrogen or any of its equivalents listed below

2H+ + 2 e-

two protons and two electrons

2H•

two hydrogen atoms

H• + H+ + e-

hydrogen atom and a proton and an electron

H:- + H+

hydride ion and a proton

SubH2

substrate with two removable hydrogens

Organic Reducing Agents
a preview of good things to come...
This list is not complete by any means!

name formula comments
hydrogen + catalyst H2/Ni, Ru, Rh, Pd, Pt etc. hydrogen gas
at pressures ranging from 1 to 200 atm;
metal catalyst in powder form to maximize surface area
lithium aluminum hydride LiAlH4 especially useful for reducing polar double and triple bonds like C=O, C=N, C≡N, etc.
sodium metal
in liquid ammonia		 Na in NH3(l) used to reduce alkynes to trans-alkenes and in the Birch reduction of aromatics
sodium borohydride NaBH4 milder, more selective for bond type than LiAlH4
iron or tin metal
with hydrochloric acid		 Fe + HCl
Sn + HCl		 used to reduce aromatic nitro groups -NO2 to amino groups -NH2
zinc amalgam
with hydrochloric acid		 Zn(Hg) + HCl used in the Clemmensen reduction of ketones to alkanes
reduced nicotinamide-adenine dinucleotide
and
reduced nicotinamide-adenine dinucleotide phosphate		 NADH (+ H+)

NADPH (+ H+)

 used in enzyme-catalyzed biological reductions (NADPH) and as the main source of electrons for mitochondrial oxidative phosphorylation (NADH); both of these coenzymes function as hydride ion donors
reduced flavin-adenine dinucleotide FADH2 also donates electrons for electron transport
     

Organic Oxidizing Agents
a preview of even more good things to come...
This list is not complete by any means!

name formula comments
oxygen O2 used in combustion analysis and bomb calorimetry
ozone O3 cleaves C=C to C=O + O=C
hydrogen peroxide H2O2 used in "hydroboration-oxidation"
and other procedures
silver oxide Ag2O oxidizes phenolic groups to quinones;
also used in Tollens test for aldehydes
osmium tetroxide OsO4 used to convert alkenes to vicinal diols
potassium permanganate KMnO4 used to convert alkenes to vicinal diols;
oxidizes most alkyl side chains attached to aromatic rings, leaving only a carboxyl group
periodic acid HIO4 oxidative cleavage of vicinal diols; used in structural analysis of carbohydrates
chromic acid H2CrO4
usually generated from Na2Cr2O7 + H2SO4(aq)		 oxidizes primary and secondary alcohols
pyridine chlorochromate (PCC)

pyridine dichromate (PDC)

 C5H5NH+ClCrO3¯

(C5H5NH+)2Cr2O72¯

 selectively oxidize primary alcohols to aldehydes
nicotinamide-adenine dinucleotide
and
nicotinamide-adenine dinucleotide phosphate		 NAD +

NADP+

 coenzymes used in biological oxidation of alcohol and aldehyde groups and numerous other reactions
flavin-adenine dinucleotide FAD coenzyme used in converting alkyl groups to alkenyl groups and numerous other biological reactions
     
     

© Ronald W. Rinehart, 2002