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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.

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.

Comments

18 thoughts on “Oxidation Ladders”

azmanam says:
August 8, 2011 at 10:27 am
well said :)
Reply
DrFreddy says:
August 12, 2011 at 8:34 am
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 :)
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1. James says:
  August 12, 2011 at 2:12 pm
  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.
  Reply
Wolfgang Chem Hater says:
March 4, 2013 at 8:23 am
Come across this website and find it quite useful… hope it can ease my hatred of chemistry lol
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Ps says:
April 7, 2013 at 12:20 am
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?
PS.
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1. james says:
  April 10, 2013 at 9:56 pm
  Not an easy reaction, oxidation of alkanes to alcohols. Why might that be?
  Reply
Rob says:
July 30, 2014 at 2:05 pm
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.
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Xuan says:
September 18, 2014 at 4:28 pm
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.
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1. Patrick Keller says:
  June 12, 2016 at 5:56 pm
  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
  Reply
Roman says:
March 13, 2015 at 10:18 pm
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?
Reply
1. James says:
  March 15, 2015 at 3:27 pm
  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.
  Reply
  1. Neil says:
    December 9, 2016 at 9:58 pm
    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?
    Reply
    1. James Ashenhurst says:
      September 18, 2019 at 11:13 am
      Yes, it’s not “self-consistent” because the number of carbons in each molecule differs – exactly as you said.
      Reply
2. Jeremy says:
  November 15, 2016 at 9:08 am
  Also noted a single bond is identified as a sigma bond and a double bond is termed a pi bond.
  Source
  http://Www.masterorganicchemistry.com
  Reply
Heather says:
January 1, 2016 at 6:28 pm
Thank you so much for this!! People like you are a godsend.
Reply
Rich Aversa says:
June 28, 2016 at 10:46 am
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.
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Sneha Basu says:
June 29, 2021 at 12:40 pm
What are the reagents that help conversion of alkane to alkene and alkyne?
Reply
1. James Ashenhurst says:
  June 29, 2021 at 3:22 pm
  Halogenation of alkanes, followed by elimination with strong base.
  Reply