[Kalibasa]

I know the rules for oxidation numbers and this is an old topic for me, so I have no idea why I'm getting this problem wrong.

Arrange the following compounds according to the oxidation number of their carbon atom, from the lowest to the highest:
A) CH₃Cl
B) (HO)₂C=O
C)H₂C=O
D)CH₄

I got -2 for A, 0 for B, and -4 for both C and D.  The answer, though, is D<A<C<B.  How can C be greater than A, and why aren't C and D equal?

I thought that:
1) Double bonds don't play a role
2) All the oxygens are -2 here because none of them are bonded to each other

Isn't this right? Then I don't see what's wrong...

Thanks!

[renge ishyo]

Although there are formal rules for calculating oxidation number, there is a trick for comparing the level of oxidation for most organic compounds. Simply put, carbons with more hydrogens attached are more reduced than other carbons which have less hydrogens attached (and are more oxidized). Using this simple rule it is easy to see that D<A<C<B because if you compare the number of hydrogens on the carbons they decrease in that order.

[Kalibasa]

Oh that makes it easier! But to be picky, how can these methods be contradicting each other? Even if the standard rules for oxidation numbers are rarely used, I'd still expect to get the same result using them as with the shortcut. Are they both "right" because assigning oxidation numbers is just an approximation?

[renge ishyo]

Both methods should give the same answer in most cases. The oxidation number calculation will give you the correct answer every time whereas the hydrogen counting shortcut is wrong in some cases such as for acetylene with its triply bonded carbon. The value of the shortcut is that it is correct in general and is much easier to understand and retain. You can see sample oxidation states for carbon on the wiki (scroll down to the carbon section mid page):

http://en.wikipedia.org/wiki/Oxidation_state

You will see that the hydrogen counting rule generally holds, but for a few exceptions in the listing.

[Kalibasa]

Thank you!