Oxidation States

[cfive22]

What is the oxidation state of the carbonyl carbon in acetyl coa? I thought that it was +2...but TBR says that it is +3. Apparently the sulfur somehow affects the oxidation state. I've included the diagram from the passage as well. Any help would be appreciated!

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[cfive22]

I think I've found the answer to my own question. In TBR Bio, says that sulfur is more electronegative than carbon, probably showing that sulfur is simply stealing the electron from the C-S bond and increasing carbon's oxidation state.

 

[chiddler]

Sulfur is 2.58, carbon is 2.55 in electronegativity! virtually equivalent.

 

[cfive22]

Sulfur is 2.58, carbon is 2.55 in electronegativity! virtually equivalent.

Ah that's interesting...I did think that sulfur usually made covalent bonds. Hm I don't know then. Sulfur's electronegativity seems to be TBR's only reasoning for an oxidation number of +3 for the carbonyl carbon...

 

[BerkReviewTeach]

Sulfur is 2.58, carbon is 2.55 in electronegativity! virtually equivalent.

While they are close, S is still more electronegative than C, so the rules for determining oxidation states assign the S the electrons and C the lack thereof. And when you get right down to it, oxidation states are nothing more than book keeping.

And although 2.58 and 2.55 seem too close to make much of a difference, H2CO3 has a pKa1 that is about 3 higher than H2SO3. So that small difference can still manifest itself on the reactivity of compounds.

 

[MedPR]

I thought it was +4 🙂 Oh well.

 

[pfaction]

F O N CL B R I S C H

This is TPR's Electronegativity. Therefore, both S and O are more electroneg, so C = +3.

 

[BerkReviewTeach]

F O N CL B R I S C H

This is TPR's Electronegativity. Therefore, both S and O are more electroneg, so C = +3.

I think that's pretty much everyone's mnemonic for the relative electronegativity of common elements. High school teachers often have some of the very best mnemonics. There was an ongoing thread of mnemonics for a while. It's a good one to go through if you can find it.

 

[gettheleadout]

F O N CL B R I S C H

This is TPR's Electronegativity. Therefore, both S and O are more electroneg, so C = +3.

Didn't know there was an R element. 😉 😛

Reminds me of the Br I N Cl H O F mnemonic for the diatomics.

 

[MedPR]

Can someone explain how you know the oxidation state of carbon is +3? I get that it is connected to two oxygens, which total (-4), but where does the other +1 come from?

 

[BerkReviewTeach]

Can someone explain how you know the oxidation state of carbon is +3? I get that it is connected to two oxygens, which total (-4), but where does the other +1 come from?

When it comes to organic molecules, it's usually easier to look at the bonds an atom makes. In acetyl coA, the carbonyl carbon has one bond to C, one bond to S, and two bonds to O. C is equally electronegative as C, so that bond earns a 0 for the C. S is more electronegative than C, so that bond earns a +1 for the C. O is more electronegative than C, so both of those bonds earn a +1 each for the C. From the four bonds, C gets a 0, +1, +1, and +1. The total is +3, so it gets a +3 oxidation state assigned to it.

Just to make sure it works, consider an aldehyde. The carbonyl C has a bond to C, a bond to H, and two bonds to O. It gets a 0 for the bond to C, a -1 for the bond to H, and +1 each for the two bonds to O. Overall it gets a 0 -1 +1 +1 = +1, so its oxidation state is +1.

Double checking with one more, let's consider carbon 2 of 2-butanol. Carbon 2 has two bonds to C, one bond to H, and one bond to O. It gets a 0 for each bond to C, -1 for the bond to H, and a +1 for the bond to O. Overall it gets a 0 for its oxidation state.

If there is a formal charge on an atom, that gets considered too.

 

[MedPR]

When it comes to organic molecules, it's usually easier to look at the bonds an atom makes. In acetyl coA, the carbonyl carbon has one bond to C, one bond to S, and two bonds to O. C is equally electronegative as C, so that bond earns a 0 for the C. S is more electronegative than C, so that bond earns a +1 for the C. O is more electronegative than C, so both of those bonds earn a +1 each for the C. From the four bonds, C gets a 0, +1, +1, and +1. The total is +3, so it gets a +3 oxidation state assigned to it.

Just to make sure it works, consider an aldehyde. The carbonyl C has a bond to C, a bond to H, and two bonds to O. It gets a 0 for the bond to C, a -1 for the bond to H, and +1 each for the two bonds to O. Overall it gets a 0 -1 +1 +1 = +1, so its oxidation state is +1.

Double checking with one more, let's consider carbon 2 of 2-butanol. Carbon 2 has two bonds to C, one bond to H, and one bond to O. It gets a 0 for each bond to C, -1 for the bond to H, and a +1 for the bond to O. Overall it gets a 0 for its oxidation state.

If there is a formal charge on an atom, that gets considered too.

Wow I didn't know you could calculate oxidation states like that. Thanks!

Obviously this depends on knowing FONClBrISCH though, correct?