Ask A Scientist© Chemistry Archive

name         Kevin O.
status       educator
age          30s

Question -   What are the oxidation numbers of the elements in the
acetate  polyatomic ion?  If the following rules for determining
oxidation numbers are applied, the carbon would have an oxidation number
of zero.  Please help!
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Rules
Oxygen will have an oxidation number of -2 except in peroxides.
Hydrogen will have an oxidation number of -1 except in metallic hydrides.
The algebraic sum of the oxidation numbers in a compound will equal zero.

Where did you get those rules?  Hydrogen should be +1 except in the saline
hydrides, where it is -1.

Trying to determine the oxidation state of carbon can give some funny
answers.  In carbon dioxide or carbon tetrachloride, it would be +4;  in
methane -4.  In formaldehyde, it would be zero.  The problem is that
calculating oxidation numbers basically assumes that all compounds are
purely ionic, when in fact many have covalent bonds.  Especially where
carbon is concerned.

Even so, the oxidation numbers are useful.  Different carbon-containing
functional groups can be interconverted by oxidation/reduction (if the
oxidation number is to change) or by other methods (if the oxidation number
is not to change).  Knowing whether a transformation requires
oxidation/reduction or not is actually useful.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
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The concept of "oxidation numbers" loses its utility when applied to
organic compounds because carbon can form multiple bonds with itself and
with other elements, e.g. oxygen adn nitrogen. A better model to use for
organic chemicals is the concept of hybridization, which you will find in
the early chapters of most organic text books.


Vince Calder
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