ORGLIST: Re: Oxidation state of cellulose

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From: Jacob Zabicky (jzabicky$##$etseq.urv.es)
Date: Fri Aug 07 1998 - 05:43:02 EDT


>Dear Prof. Zabicky:
> (I am not sure wether I should write in Spanish or not+ACE-)
> Your comments below really confused me. I answered this question
>directly to the person who asked it, so my message didn't show. I thought
>the question was so basic+ACE- You mention a convention in organic chemistry
>about C-H and C-C being truely covalent and this leaves the idea that the
>C-O bond is not covalent. Is this what you mean? Also, in charcoal, for
>example, or in diamond, the oxidation state for the carbon atom should be
>zero. Going from here to CH4 would require a reduction, so carbon should
>be -4+ACE-, etc. This means I agree with Dr. Fassbender. Please, correct
>me if
>I am wrong. And thanks a lot for your time.

>Gerardo Alberto Mora
>Director
>Centro de Investigaciones en Productos Naturales
>(CIPRONA)
>Universidad de Costa Rica
>2060 San Jose, Costa Rica
>Tel. (506) 2074474+ADs- fax (506) 2259866
>E-mail: gamora+AEA-cariari.ucr.ac.cr

Dear Professor Mora,

>From a certain point of view you are totally right. We are discussing a
system where, as opposed to purely ionic compounds such as sodium chloride,
things are not sharply defined. Sodium hydroxide may serve as an inorganic
sample; sodium and hydroxide are undoubtedly ions, but the O-H bond, how
should we take it?, as totally covalent?, as totally ionic, formed of O=
and H+? Pauling pointed in his "Nature of the Chemical Bond" to the
direction where to find a reply to such querries, offering intermediate
stages for the coordination between atoms.

The question that appeared in ORGLIST was of another kind. When inquiring
about the average degree of oxidation of C in glucose or cellulose, one
does not seek those intermediate values of Pauling, but how many
"oxidation" steps were carried out starting from a certain point. Oxidation
was place in quotation marks because it refers to the introduction of atoms
of oxygen, halogen or whatever electronegative element, yielding covalent
bonds. In fact, it is totally irrelevant where one places the 0 of the
scale; in the example I gave, from methane to carbonic acid there are four
well-defined steps. In my answer to the original question I took the
corresponding alkane (hexane) as reference point, as it seemed to me the
clearest one. Perhaps I was wrong whith that.

Where is the justification of calling such steps oxidation stages? On the
one hand, a deep analysis of the structure will indicate that the electron
share of the oxygen atom is larger than that of C or H. However, one can
perform electrochemical experiments in aqueous solution with such
compounds, where one carries out reductions on proceding along the series
from bottom to top and oxidation in the opposite direction. In such
processes a different potential is required to abstract H from a C-H bond
than from a O-H bond, thus, it may be justified to ascribe a 0 oxidation
state to H in methane and +1 in water or an alcohol.

All the best

Jacob Zabicky


*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*
Prof. Jacob Zabicky Fax. xx-34-977-559694
c/o Prof. Alberto Coronas Tel. xx-34-977-???
Department of Mechanical Engineering Private No.
Universitat Rovira i Virgili xx-34-930-119552
Cra. de Salou s/n
43006 Tarragona
Spain
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