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Topic: Redox Reactions: " ... And I've Gone Cross-eyed."  (Read 5449 times)

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Offline azmanam

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Redox Reactions: " ... And I've Gone Cross-eyed."
« on: July 06, 2011, 02:17:23 PM »
Austin Powers voice aside, I've been looking at organic redox reactions this morning... Then I confused myself... Then I spent the rest of the morning calculating it out and blowing my mind.  Help me rationalize all this.

I've always drawn organic redox charts like shown below.  Acid/base reactions are not redox reactions, but increasing/decreasing C-O or C-H bonds is a redox reaction.  Thus moving horizontally is a redox process, moving vertically is an acid/base process. (click image for better view)



Then I started counting electrons and figuring out the exact oxidation state of carbon for several series of molecules and found a startling trend.  I know we've hashed out oxidation state of carbon on these boards before. 



What do you think?
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Offline enahs

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Re: Redox Reactions: " ... And I've Gone Cross-eyed."
« Reply #1 on: July 07, 2011, 11:25:33 AM »
I am not sure I get your first chart/figure.

On the Y axis you have it labeled acid/base chemistry

But on the highest point of the Y axis you have things that have pKa that range from ~ 25-50
Right below that, you have things that have pKa's as low as ~4.75
But then at the lowest part of the Y axis you have an amine with pKa 8-11 we will say.


And in your second chart, what is the exact basis for your "Y axis"? If you can arbitrarily chose how to group things on one axis, then you can make anything you want out of it.

Offline azmanam

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Re: Redox Reactions: " ... And I've Gone Cross-eyed."
« Reply #2 on: July 07, 2011, 01:03:27 PM »
Good point.  I should have explained better (it all made sense in my head!)

In the first chart, molecules are grouped vertically by oxidation level, not by pKa.  All molecules in the same column are "at the same oxidation level."  Alkenes are at "the same oxidation level" as alcohols.  Acids, esters, amides are all "at the same oxidation level" but are not as oxidized as carbonates or CO2.  You can (theoretically) interconvert between any two molecules in the same column by using acid/base chemistry (thus vertical movements are not oxidations which is why these molecules are "at the same oxidation level.")  Add HCl to an alkene (acid/base chemistry) to get an alkyl halide.  All carboxylic acid derivative interconversions are acid/base chemistry (add H2SO4 and methanol to an acid to get an ester).  But to convert an ester to an aldehyde, I have to reduce with a hydride source.

In the top chart, to move horizontally, you have to use a redox reaction (you don't have to stay in the same row, but it needs to be a redox reaction).  To move vertically within a column, you have to use an acid/base reaction.

In the second chart, there is no y-axis anymore.  Vertical groups of molecules all have the same numeral oxidation state of carbon, but an organic chemist would not say that any vertical group of molecules are "at the same oxidation level." 

That's the dichotomy I'm pointing out.  I guess my point was, the top chart was a chart my prof gave us in my advanced organic class in grad school.  It's a handy chart to determine what 'classification' of reaction I need to use.  If I want to move vertically within a column, I only need acid/base reactions.  If I want to jump to another column, I need redox chemistry.  And this is true.  If I have an alkene and I want an epoxide, I need to oxidize.  If I have a ketone and I want an acetal, I need alcohol and acid.  When you start actually calculating out the actual oxidation number of carbon in these functional groups, I realized it depended on more than the heteroatom.  It depended on the rest of the hydrocarbon framework.  Suddenly, things that were 'at the same oxidation level' before now had completely different actual oxidation states of carbon.   This confused me at first and I didn't like it.  So I drew out two big charts, pondered, sobbed for a while, had lunch, taught class, came back here and posted the charts to see what others thought :)

(In both charts, horizontal groupings are arbitrary based on molecules that "look alike" (hydrocarbons, halides, nitrogen-containing).)
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