Once you get a handle on oxidations and reductions, you might start to notice that with some molecules these reactions can proceed in sequences.
For example, if you start with an alkane with a CH3 group,
- the alkane can be oxidized to a primary alcohol.
- The alcohol can be oxidized to an aldehyde
- The aldehyde can be oxidized to a carboxylic acid.
(The reverse reactions would all be reductions, of course)
Each of these reactions involves the gradual increase in oxidation state at carbon. If you arrange these reactions with increasing oxidation state on the y axis, you get patterns which are often called oxidation ladders, and they are extremely useful way of organizing reactions. (We could do the reverse reactions and call it a “reduction ladder” – for some reason the name “oxidation ladder” has stuck).
That’s why we often say that we oxidize the alcohol “up” to an aldehyde, and reduce an aldehyde “down” to an alcohol.
Similarly, if you start with an alkane with a secondary carbon:
- the secondary carbon can be oxidized to a secondary alcohol
- the secondary alcohol can be oxidized to a ketone
- The ketone can even be oxidized to an ester
Here’s the “oxidation ladder” for that sequence.
Finally, you can also think about oxidation ladders involving double bonds.
- Alkanes can be oxidized to alkenes.
- Alkenes can be oxidized to alkynes
- Some alkynes can even be oxidized further into ynols, an interesting but somewhat exotic species I won’t get into.
It’s also a useful concept for organizing reactions that don’t involve climbing or descending the oxidation ladder. For instance, alkenes can be converted into either primary or secondary alcohols, depending on the choice of reagent – and either of these can be converted back into alkenes. Similarly, alkynes can be converted into either aldehydes or ketones, depending on the choice of reagent, and neither of these transformations are considered to be oxidations nor reductions.
In the big picture, you can think of two types of reactions: “vertical” reactions, in which the oxidation state of a molecule is changed, and “horizontal” reactions, in which functional groups are interconverted.
One final note. In general, oxidation is a thermodynamically more favorable process than reduction (due to the higher bond strength of C-O vs. C-H). At this very second, sugars (alcohols) in your body are ascending the oxidation ladder to become carbon dioxide, releasing energy in the process. Conversely, photosynthetic bacteria and plants are employing sunlight (an external source of energy) to assist in the conversion of carbon dioxide “down” the oxidation ladder to become aldehydes, alcohols, and alkanes.
So as long as there is life on earth, carbon atoms will be going up and down the oxidation ladder.