In organic chemistry, the oxidation of a carbon atom can be recognized by:
Reduction is just the reverse. Furthermore, increasing alkylation is the equivalent of increasing numbers of hydrogens; that is, it is the same as reduction. Here is a chart to illustrate these relationships:
The important connection we want to keep in mind is that moving upward in this chart requires an oxidizing agent, whereas moving downward requires a reducing agent. If no species lies above a particular entry in the chart, it cannot be oxidized. If nothing lies below it, it is in the lowest oxidation state possible, and cannot be reduced.