Oxidation and Reduction
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An important part of the task of electron book-keeping.
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Oxidation, [O], and reduction, [R], are opposites and both must occur simultaneously,
hence redox reactions.
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Organic chemists will normally describe a reaction as either oxidation
or reduction depending on the fate of the major organic component.
Oxidation:
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more C-O bonds (or other atoms more electronegative than C)
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less C-H bonds
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loss of electrons
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increased oxidation state, e.g. +1 to +3 (see below)
Reduction:
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more C-H bonds
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less C-O bonds (or other atoms more electronegative than C)
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gain of electrons
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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.
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for attached C atoms, i.e. C-C bonds electrons shared, \
count 0
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for attached X atoms, i.e. C-X bonds (X more electronegative), \
count -1 (per bond)
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for attached H atoms, i.e. C-H bonds (H is less electronegative
than C), \ count +1
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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, but it is better to count each bond to the 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.