Useful Concepts

Oxidation and Reduction

• An important part of the task of electron book-keeping.
• Oxidation, [O], and reduction, [R], are opposites and both must occur simultaneously, hence redox reactions.
• Organic chemists will normally describe a reaction as either oxidation or reduction depending on the fate of the major organic component.
Oxidation:
• more C-O bonds (or other atoms more electronegative than C)
• less C-H bonds
• loss of electrons
• increased oxidation state, e.g. +1 to +3 (see below)
Reduction:
• more C-H bonds
• less C-O bonds (or other atoms more electronegative than C)
• gain of electrons
• decreased oxidation state, e.g. +1 to -1 (see below)
Calculating Oxidation Number or State (there are several methods for doing this, pick the one that works for you !)
This allows for a more formal, quantitative decription of the oxidation state for the C atoms and is based on looking at what atoms are attached to the C atom in question. The algebraic sum of the oxidation states must equal the charge of the molecule.
• for attached C atoms, i.e. C-C bonds electrons shared, \count 0
• for attached X atoms, i.e. C-X bonds (X more electronegative), \count -1 (per bond)
• for attached H atoms, i.e. C-H bonds (H is less electronegative than C), \ count +1
• Add the total for atoms attached to the C in question, then switch the sign.
Here are a few examples, and a couple of schemes that show important relationships:

Tips:  In General Chemistry or High School you may have learnt that O is always -2, however it is better to count each bond to an O as -1 since there is a difference between C=O and C-O as seen by comparing the aldehyde and the alcohol in the diagram above. After all , the -2 statement is wrong.... for example in oxygen gas, the oxidation state is zero

 © Dr. Ian Hunt, Department of Chemistry, University of Calgary