Determining an Organic Carbon's Oxidation Level
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Determining an organic carbon's oxidation level means comparing how many of its bonds go to oxygen versus to hydrogen: replacing a carbon-hydrogen bond with a carbon-oxygen bond oxidizes that carbon, and the reverse replacement reduces it, without needing a full oxidation-number calculation for large molecules.
Methane's carbon, bonded only to hydrogen, is fully reduced; sequentially replacing each carbon-hydrogen bond with a carbon-oxygen bond carries that carbon through methanol, to formaldehyde, to formic acid, to fully oxidized carbon dioxide, each step trading one more C-H bond for one more C-O bond.
Applying this shortcut to a carbon atom bonded to other carbons, rather than only to the bonds it makes to hydrogen and oxygen specifically, misreads which bonds actually count toward the comparison.