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Offline Logan

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Oxidation numbers
« on: July 27, 2010, 08:53:06 PM »
I have to calculate the oxidation number on each carbon atom for

a) Ascorbic acid and

b) Dehydro-ascorbic acid.

I worked out that ascorbic acid has a mol formula of C6H8O6, so the oxidation number on C=4 (+8 and -12 for H8 and O6 respectively)
and Dehydro-ascorbic has a mol formula of C6H6O6, so C= 6.

Is the above correct or have I made an error somewhere?

Offline Schrödinger

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Re: Oxidation numbers
« Reply #1 on: July 27, 2010, 10:09:12 PM »
There is more than one carbon atom in each molecule. So, you need to know the structure in order to calculate the oxidation number in each of these carbons atoms, because the arrangement of atoms is what matters, not the mere number and type of atoms.

To give you more insight into what I'm talking about, the different oxidation numbers of carbons present in ascorbic acid : +3,+1,0,-1 (no particular order)
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Offline opti384

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Re: Oxidation numbers
« Reply #2 on: July 27, 2010, 11:07:23 PM »
Oxidation numbers in organic compounds are not simple as those of other compounds.

Offline Logan

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Re: Oxidation numbers
« Reply #3 on: July 27, 2010, 11:43:45 PM »
There is more than one carbon atom in each molecule. So, you need to know the structure in order to calculate the oxidation number in each of these carbons atoms, because the arrangement of atoms is what matters, not the mere number and type of atoms.

To give you more insight into what I'm talking about, the different oxidation numbers of carbons present in ascorbic acid : +3,+1,0,-1 (no particular order)

Oh I see. So with respect to the structure I can see how the +1 (C-OH), -1 (C-H) oxidation numbers arise but I don't see how +3 or 0 arises. To my knowledge Ascorbic acid has 4 OH groups, a C=O bond and a C-H bond, if you could explain how the 0 or +3 arises I would be very grateful. Thanks

Offline Schrödinger

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Re: Oxidation numbers
« Reply #4 on: July 28, 2010, 02:13:05 AM »
The Carbon in the ring that is attached to an oxygen via a double bond is also bonded to another oxygen(the heteroatom) via a single bond. That carbon's oxidation state is +3

There are 2 carbon atoms with 0 oxidation state. One of the them is the atom that is attached directly to the ring and other one is the atom at the point of attachment of the ring and the chain.

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Offline AWK

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Re: Oxidation numbers
« Reply #5 on: July 28, 2010, 03:36:34 AM »
Quote
I have to calculate the oxidation number on each carbon atom for

a) Ascorbic acid and

b) Dehydro-ascorbic acid.
Write down structural formula for each compound
http://en.wikipedia.org/wiki/Ascorbic_acid
http://en.wikipedia.org/wiki/Dehydroascorbic_acid
Omit all CC bonds (single, double)
In each case you will get 5 part. Treat each part as simple compound and calculate oxidation number for each carbon atom:
eg CHCl Cl -1, H +1 hence C 0
Hope this helps you.
AWK

Offline Logan

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Re: Oxidation numbers
« Reply #6 on: July 29, 2010, 02:08:23 AM »
The Carbon in the ring that is attached to an oxygen via a double bond is also bonded to another oxygen(the heteroatom) via a single bond. That carbon's oxidation state is +3

There are 2 carbon atoms with 0 oxidation state. One of the them is the atom that is attached directly to the ring and other one is the atom at the point of attachment of the ring and the chain.

I don't know if my description has been vivid; if it isn't please don't hesitate to ask
Quote
I have to calculate the oxidation number on each carbon atom for

a) Ascorbic acid and

b) Dehydro-ascorbic acid.
Write down structural formula for each compound
http://en.wikipedia.org/wiki/Ascorbic_acid
http://en.wikipedia.org/wiki/Dehydroascorbic_acid
Omit all CC bonds (single, double)
In each case you will get 5 part. Treat each part as simple compound and calculate oxidation number for each carbon atom:
eg CHCl Cl -1, H +1 hence C 0
Hope this helps you.

Thanks guys for the help, I got it now. It was the C-O single bond that was confusing me. I thought the oxidation number for that O atom was -2.

Offline AWK

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Re: Oxidation numbers
« Reply #7 on: July 29, 2010, 04:59:46 AM »
Oxygen nearly always -2 (exceptions: F2O, KO2, KO3,  H2O2 and its derivatives, organic ozonides)
AWK

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