[HusamEddin]

Hi there

which of the following is more oxidized .. and why ?
methanol CH3OH
or
dimethyl ether CH3OCH3

I searched for it , and found that they are the same !
what is the reason ?

I know that the more C-O bonds , the more oxidized
but at the same time , the more C-H bonds , the less oxidized
there is more C-O in the second compound , but the same time there is also more C-H !!!

So how to determine in such a situation ?

thanks

[Borek]

What is the quantitative definition of "oxidized" in this context? I have never heard about one.

[Babcock_Hall]

The sixth edition of McMurry's Organic Chemistry textbook has a section on oxidation (pp. 332-334).  Based on his treatment, one could define an oxidation number for a carbon atom as equal to the number of bonds to O, N, or X minus the number of bonds to H.  I find this useful in biochemistry also, except that sulfur is a bit of a question mark.

[Borek]

one could define an oxidation number for a carbon atom as equal to the number of bonds to O, N, or X minus the number of bonds to H

OK, but oxidation number is a property of an atom, while OP posted two molecules.

[Schrödinger]

@OP : More often than not in organic molecules, you need to specify which carbon you want to calculate the Oxidation number for, in case the molecule has more than one C atom.

[Babcock_Hall]

Borek,

Sorry, I was writing quickly this morning, and I left out some important stuff.  One can then sum the oxidation numbers of the individual carbon atoms to calculate an oxidation number for all of the carbons of a molecule.  Suppose we consider the hydration of ethene (ethylene) to ethanol.  The oxidation numbers of each of the two carbons are -2 in ethylene, summing to -4.  The oxidation numbers of the carbons of ethanol are -1 and -3, which also sum to -4.  Overall, the process is neither an oxidation nor a reduction, but each carbon atom changes its oxidation number.  Fermentation processes are also interesting subjects for treatment by oxidation numbers:  "Assigning and Using Oxidation Numbers in Biochemistry Lecture Courses"  Halkides, CJ, J. Chem. Educ., 2000, 77 (11), p 1428 

[HusamEddin]

Borek,

Sorry, I was writing quickly this morning, and I left out some important stuff.  One can then sum the oxidation numbers of the individual carbon atoms to calculate an oxidation number for all of the carbons of a molecule.  Suppose we consider the hydration of ethene (ethylene) to ethanol.  The oxidation numbers of each of the two carbons are -2 in ethylene, summing to -4.  The oxidation numbers of the carbons of ethanol are -1 and -3, which also sum to -4.  Overall, the process is neither an oxidation nor a reduction, but each carbon atom changes its oxidation number.  Fermentation processes are also interesting subjects for treatment by oxidation numbers:  "Assigning and Using Oxidation Numbers in Biochemistry Lecture Courses"  Halkides, CJ, J. Chem. Educ., 2000, 77 (11), p 1428

depending on this,,
CH3OH will have an oxidation number of 1-3=-2
but CH3OCH3 is (1-3=-2)+(1-3=-2)=-4
-2 & -4 , they are not the same
but I figured out the correct answer to be (the same) -without mention numbers or reasons- !!!

still the reason is unclear

[Schrödinger]

@ Babcock_Hall : But what's the point of summing up the ON's of all Carbons? Let's say we have 1-pentanol and I'm oxidizing it to pentanal. The only C where there is an ON change is the C-1. All the others' ON's remain unchanged. So, isn't it more useful if you specify them one by one? i.e., mention ON atom by atom.

@ OP : Without summing them up, each carbon in dimethylether has the same oxidation number of -2. And so does the carbon in methanol. It so happens the molecule they've given you (the ether) has both the Carbons in the same oxidation number. Hence the reasonable comparison with methanol.

[Babcock_Hall]

Husam Eddin, A complication here is that you are comparing two carbons against one carbon.  For the question you asked, it might be reasonable to take the average of the two oxidation numbers of dimethyl ether and compare it with methanol.  Or to think in terms of having two molecules of methanol for every molecule of dimethyl ether.  One could reframe your question as: For the equation 2(Methanol)  dimethyl ether + water, would one need to invoke an oxidizing or reducing agent?  (No)  The bottom line for your problem is that each of the three carbon atoms in question has an oxidation number of -2.

Schrodinger, When only one carbon changes oxidation number and when the stoichiometric coefficients on the reactant side and product side are equal, I agree with you.  I am not sure how one would treat the hydration of ethene, however.

The utility of summing the oxidation numbers might become more apparent by considering the following problem from the paper I cited:  Can three moles of glucose be fermented (catabolized anaerobically) to 2 moles of butanediol, 2 moles of glycerol, and four moles of carbon dioxide?  3(0) = 2(-6) + 2(-2) + 4(+4) = 0.  The numbers in parenthesis are the sums of the oxidation numbers of the carbons from glucose, butanediol, glycerol, and carbon dioxide, respectively.  These are multiplied by the stoichiometric coefficients, and the reactant side compared with the product side.  If the reactants and products had not provided the same number, the difference is the number of electrons not taken into account.  For example one mole of glucose cannot be fermented into two moles of ethylene glycol and two moles of carbon dioxide, because four electrons have gone missing.