Alcohols, Epoxides and Ethers

By James Ashenhurst

Oxidation Ladders

Last updated: November 12th, 2022 |

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.

oxidation ladder example of going from less oxidized carbon alkane to more oxidized carbon primary alcohol aldehyde carboxylic acid

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.

oxidation ladder showing oxidation states of alkane to secondary alcohol ketone ester going from less oxidized to more oxidized

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.

oxidation ladder of alkane to alkene to alkyne showing increase of oxidation state at carbon due to loss of hydrogen

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.

reactions that are neither oxidations nor reductions since oxidation and reduction happens on adjacent carbon eg hydration of alkene

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.


Comment section

21 thoughts on “Oxidation Ladders

  1. I have this query. Before the beginning of second ladder, the note says “similarly, if you start with an alkane with secondary carbon…….”
    My question is:
    An alkane with a secondary carbon also has primary carbons. How a primary or a secondary carbon be oxidized selectively as per our requirement, using the same oxidizing agents(PCC, H2CrO4)

  2. Is it just me or does it seem like there is oxidation in the final examples? Take the bottom example 1 alcohol -> alkene-> 2 alcohol. The oxidation number of the carbon attached to OH in the 1 alcohol is -1 oxidation state, then in the 2 alcohol it seems to be 0? Is that not an oxidation?

    1. Good eye.

      I would say that in order to call something an oxidation, you’d have to look at the *net* change across two carbons if it’s occuring across a double bond.

      For conversion of the primary alcohol to the alkene, the end carbon goes from -1 to -2 (reduction). The internal carbon goes from -2 to -1 (oxidation). So on net this is neither oxidation nor reduction.

      For hydration of the alkene (alkene to secondary alcohol) the end carbon goes from -2 to -3 (reduction). The internal carbon goes from -1 to 0 (oxidation). So on net this is again neither oxidation nor reduction.

  3. I’m a little sketched out by the aldehyde alkyne ketone horizontal “rung”. Looking at the carbonyl carbon, its oxidation state is +1 in the aldehyde and +2 in the ketone. The alkyne’s two functional carbons are 0 and -1. Even adding the oxidation states of the alpha carbons to the ketone and aldehyde, it doesn’t seem like these three compounds are all the same oxidation state. I agree they are in a sense on the same oxidation “level”, but the numbers don’t seem to add up. The bond breaking/forming argument is a strong one, but it may be complicated by the fact that the aldehyde is missing a carbon relative to the two other compounds.

  4. Maybe this is a silly question but how come in the horizontal reactions diagram all the left sided products have one carbon atom less than the alkene/alkyne/ynol they are formed from yet all the products on the right maintain the same number of carbon atoms?

    1. It’s not a silly question at all – it’s impossible to make a drawings such as this one and have everything be self-consistent.

      1. Sorry, this is still a little unclear to me. What is meant by “self-consistent”? Does it mean that to illustrate the idea clearly you have to assume that these “extra carbons” came from another reagent that isn’t shown?

  5. I am just wondering if there is such an oxidizing agent that only converts aldehyde to carboxylic acid but does not react with alcohol. I was thinking of a weak oxidizing agent but I’m not sure. Could anyone help? Many thanks.

    1. The weak oxidizing agent that will only react with aldehydes and convert them to Carboxylic acids is the Tollens Reagent, which is Ag2O and NH4OH. It ignores alcohols

  6. James, This page is perfect. Thanks for building and sharing it. I came upon it while puzzling over how to number the carbons in the molecules of the TCA cycle. I’ve read that the numbering begins at the end closest to the most oxidized carbon, but I’m still puzzled about how to number the carbons in a molecule like citrate.

  7. Hello James, I tried to find the mechanism for the oxidation of alkane to alcohol, but to no avail on the net.

    is it that you have to dehydrogenate alkane to alkene then through hydration to obtain alcohol?


  8. Come across this website and find it quite useful… hope it can ease my hatred of chemistry lol

  9. Ah snakes… great post – and congratulations for beating me to it. I have had a post on my page called “Oxidation states are pointless. Oxidation levels is the shit” lingering as a draft for months. I suppose I can safely delete my version in silence now :)

    1. Why not post it anyway? It’s rare that two people come at the same topic from the exact same angle, anyway. If you write it, I’ll definitely point people to it.

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