Sometimes, it is useful to assign oxidation numbers in organic compounds; for #CH_4# we gots #stackrel(-IV)C# and #4xxstackrel(+I)H#, for ethanol, we gots #H_3stackrel(-III)C-stackrel(-I)Cstackrel(+I)H_2stackrel(-II)OH#; for acetic acid, we gots #H_3stackrel(-III)C-stackrel(+III)C(=O)OH#...as always the sum of the individual oxidation numbers adds to the charge on the ion or molecule, and here we generally gots a neutral molecule. When we (for the purposes of assigning oxidation states) break a #C-C# bond, the electrons are assumed to be shared by the bound atoms, i.e. #H_3C-CH_3rarr H_3dotC+dotCH_3; 2xxstackrel(-III)C+6xxstackrel(+I)H#.
And oxygen is more electronegative than most elements; it gets the electrons when we formally break the bond... Oxygen assumes a #-II# oxidation state in ethers, and carboxylic acids, and alcohols... Usually, oxygen only expresses a different oxidation number in the element, i.e. #stackrel(0)O_2#, or in peroxides...#H-stackrel(-I)O-stackrel(-I)O-H#.
Of course, in #stackrel(+IV)CO_2#, carbon assumes its maximum oxidation state....
