TBR: Calculating Oxidation State of Carbon in Glucose

[justadream]

I was under the impression that when you calculate the oxidation state of an atom, you only look at things directly connected to it.

But TBR does not follow this.

For example, in calculating the oxidation state of Carbon #2, TBR does the calculation as follows:
Hydrogen directly bonded is +1
Oxygen directly bonded is -2
Hydrogen bound to the oxygen is +1 [WHAT!!! Why do you do this]

Because the overall charge must = 0, Carbon #2 must have an oxidation state of 0.

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[sillyjoe]

Can you ask your question again? Is the red carbon at an oxidation state of +1? Where in TBR is this?

 

[justadream]

@sillyjoe

Yes, TBR says the red carbon (carbon #1) is +1

I agree with that because the red carbon is bound to oxygen (-2) and hydrogen (+1).

This is TBR Book II page 245 #54

I just don't understand why you look at atoms not bound to the carbon in question...

 

[sillyjoe]

@sillyjoe

Yes, TBR says the red carbon (carbon #1) is +1

I agree with that because the red carbon is bound to oxygen (-2) and hydrogen (+1).

This is TBR Book II page 245 #54

I just don't understand why you look at atoms not bound to the carbon in question...

It's just a matter of perspective in organic chemistry. The reactions center around carbon and it's oxidation state i.e. -CH3 is more reduced than, -CH2OH, -CHO , -CO2.

That's the main reason why you look at carbon.

 

[justadream]

@sillyjoe

So how do you decide which atoms not directly bound to the carbon should still be "considered" when doing oxidation state?

 

[sillyjoe]

Every atom bound to the carbon of interest is taken into consideration.

 

[justadream]

@sillyjoe

Do you mean to say "every atom bound to the carbon of interest and anything else bound to it"?
If so, why don't you consider every single atom in the molecule (since every atom is technically bound in some way to the carbon of interest)?

If not, then your rule wouldn't apply to Carbon #2 (since the Hydroxyl's Hydrogen -which is not bound to the carbon of interest directly - is still accounted for).

 

[sillyjoe]

@sillyjoe

Do you mean to say "every atom bound to the carbon of interest and anything else bound to it"?
If so, why don't you consider every single atom in the molecule (since every atom is technically bound in some way to the carbon of interest)?

If not, then your rule wouldn't apply to Carbon #2 (since the Hydroxyl's Hydrogen -which is not bound to the carbon of interest directly - is still accounted for).

What is the oxidation state of carbon #2?

 

[justadream]

@sillyjoe

TBR says it is 0 (for the reasons I mentioned in the OP).

But when I follow TBR's rule to determine oxidation state (look at what is bonded to the carbon) I find that:

Hydrogen directly bonded is +1
Oxygen directly bonded is -2

Thus, Carbon #2 is +1.

Of course, when I look at the entire molecule, I realize that wouldn't work (the overall charge is 0).

But it seems somewhat "arbitrary" to include the Hydroxyl's Hydrogen in the calculation for carbon 2. As in, how do you know when to "stop" considering atoms not directly bound to carbon #2?

 

[sillyjoe]

@sillyjoe

TBR says it is 0 (for the reasons I mentioned in the OP).

But when I follow TBR's rule to determine oxidation state (look at what is bonded to the carbon) I find that:

Hydrogen directly bonded is +1
Oxygen directly bonded is -2

Thus, Carbon #2 is +1.

Of course, when I look at the entire molecule, I realize that wouldn't work (the overall charge is 0).

But it seems somewhat "arbitrary" to include the Hydroxyl's Hydrogen in the calculation for carbon 2. As in, how do you know when to "stop" considering atoms not directly bound to carbon #2?

Ah, I see. You're mistake is counting the oxygen as -2. It is not -2, it is -1. The only reason it was -2 on the carbonyl carbon is because it has a double bond to oxygen.

 

[justadream]

@sillyjoe

Oh you're right! What a genius!

I'm so used to thinking of oxygen as -2 automatically!

 

[The Brown Knight]

the oxidation state for every carbon can be seen as follows:
H = -1
single-bonded O = +1
double bonded O = +2
another C = 0
(the numbers indicate oxidation states of carbon)
the single/double bond thing works because a double bonded oxygen makes a more electrophilic carbon than a single-bonded carbon; i.e. you see electrophilic additions much more frequently on carbonyls than alcohols.