Orgo I - Oxidation

[skyisblue]

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THIS IS A DOUBLE POST; SDN TOOK FOREVER TO POST MY THREAD, DIDN'T THINK IT TOOK. SORRY

In Organic Chemistry oxidation is defined as a reaction that results in a loss of electron density by carbon. One way this occurs is by bond breaking between carbon and a less electronegative atom (usually hydrogen).

In this case, if the carbon is more electronegative, why wouldn't there be a greater increase in electron density for the carbon atom when the bond breaks between C and H. Hydrogen, being less electronegative wouldn't take the electrons with it when the bond breaks right??

 

[BodybldgDoc]

THIS IS A DOUBLE POST; SDN TOOK FOREVER TO POST MY THREAD, DIDN'T THINK IT TOOK. SORRY

In Organic Chemistry oxidation is defined as a reaction that results in a loss of electron density by carbon. One way this occurs is by bond breaking between carbon and a less electronegative atom (usually hydrogen).

In this case, if the carbon is more electronegative, why wouldn't there be a greater increase in electron density for the carbon atom when the bond breaks between C and H. Hydrogen, being less electronegative wouldn't take the electrons with it when the bond breaks right??

If the bond breaks heterolytically, the carbon will have lost an electron due to the bond being broken which would also mean that its oxidation state increases with a drop in electron density due to the loss of an electron because of the bond being broken. Correct me if i'm wrong anyone.

 

[poc91nc]

THIS IS A DOUBLE POST; SDN TOOK FOREVER TO POST MY THREAD, DIDN'T THINK IT TOOK. SORRY

In Organic Chemistry oxidation is defined as a reaction that results in a loss of electron density by carbon. One way this occurs is by bond breaking between carbon and a less electronegative atom (usually hydrogen).

In this case, if the carbon is more electronegative, why wouldn't there be a greater increase in electron density for the carbon atom when the bond breaks between C and H. Hydrogen, being less electronegative wouldn't take the electrons with it when the bond breaks right??

Doesn't the formation of a carbanion result in higher electron density of the carbon? For example take any diketone compound in the presence of a strong base such as LDA...you get the formation of a carbanion whereby the carbon has a negative charge with a lone electron pair. Alpha hydrogen in the presence of a strong base yields a carbanion.

 

[skyisblue]

Doesn't the formation of a carbanion result in higher electron density of the carbon? For example take any diketone compound in the presence of a strong base such as LDA...you get the formation of a carbanion whereby the carbon has a negative charge with a lone electron pair. Alpha hydrogen in the presence of a strong base yields a carbanion.

You're right. A carbanion does have greater electron density. BUT according to textbook, breaking of a C-H bond decreases electron density on the carbon thereby being an oxidation reaction. This is why I'm confused.....

 

[poc91nc]

Well here is another example of oxidation. Think of an E2 reaction involving an alkyl halide and tert-butoxide. You get an alkene...you break the carbon hydrogen bond and the two electrons that were shared between carbon and hydrogen are now shared between two carbons. The new bond formed is between carbon and an atom that is more electronegative than hydrogen (in this case carbon)....therefore by definition it is an oxidation isn't it? Just think of it as a relative scale.....

 

[poc91nc]

"why wouldn't there be a greater increase in electron density for the carbon atom when the bond breaks between C and H. Hydrogen"

Do you mean greater electron density for the final product or during the transition state or intermediate? If it is for the transition state or intermediate....wouldn't it depend on the mechanism?

 

[skyisblue]

"why wouldn't there be a greater increase in electron density for the carbon atom when the bond breaks between C and H. Hydrogen"

Do you mean greater electron density for the final product or during the transition state or intermediate? If it is for the transition state or intermediate....wouldn't it depend on the mechanism?

I think I'm just going to go with the book on this one. I don't know how it's right, but I'll just consider it as a rule.

 

[jfitzpat]

None of the books I have read explain this well at all, but here is how I think about it.

The formation of a carbanion is not a redox reaction in itself. For instance, if you take CH4 and make it CH3- and H+, you haven't changed the oxidation state of anything so the reaction is neutral.

I think the rule comes from the fact that any loss of hydrogen (which is always 1+ unless it is in a metal hydride) will result in the formation of a new molecule with a higher oxidation number for carbon. This is certainly true for almost everything that Carbon typically forms covalent bonds with (nonmetals).

I have no idea if my way of thinking is right, but I can say that is usually works.