Oxidation and Reduction in Organic Chemistry

by James

in Alcohols, Aldehydes, Carboxylic acids, Esters, Nomenclature

Iron smelting! 
Photo credit: http://sclowcountryoutdoors.blogspot.com/2011/02/iron-smelting-at-acba.html

In the beginning, the term actually made sense. When the alchemists and medieval metallurgists started doing experiments to quantify exactly how much iron, say was formed from the smelting of a given amount of iron ore, they found that the weight of the solid iron was always less than that of the ore.

Given the decrease in mass, a sensible name for the process was made: reduction.

That was more than 500 years ago.

Later on, Scheele,  Lavoisier and Priestley independently discovered that the loss of mass was due to the expulsion of an element named (by Lavoisier) oxygen, and subsequent burning the metal in air led to its recombination.  Hence, “oxidation”.

That was about 200 years ago.

Then came a general understanding of how atoms are composed of a positively charged nucleus and negatively charged electrons, and the introduction of the formalism known as the “oxidation state“, which is the hypothetical charge that an atom would have if all bonds to atoms of different elements of different elements were 100% ionic. It also works for ions, of course. So here’s the same reaction. See how oxidation leads to removal of electrons, and reduction leads to a gain of electrons.

 That was about 70 years ago.

This also happens to be the first definition of oxidation and reduction I first learned in high school. When this was introduced in class, my first question – which is still asked by many students today, was:

“In what world does it make sense to call a process where electrons are gained “reduction” ? “

The answer from my high school chemistry teacher was , “well, you’re reducing the oxidation state – making it more negative”. Which was a very clever answer, completely jettisoning the inconvenient historical definition in favor of a simplistic mathematical one. Fortunately for him, I wasn’t fast or clever enough to counter with “then why isn’t “oxidation” called “addition”? (Feel free to use this yourself, however).

Result: I just memorized that “reduction” meant “adding electrons” and “oxidation” meant “removing electrons”.  Which came in handy in general chemistry, with its seemingly endless balancing of complex redox reactions.

Just when this seemed settled in my mind, along came organic chemistry, with what was seemingly yet another way of defining oxidation and reduction.  Agh!!!

At first glance, this seems a long way away from the Gen chem definition of oxidation being loss of electrons and reduction a gain of electrons.

But if you go back to the concept of the oxidation state, it might make some more sense. If you just pay attention to what’s happening to the oxidation state of the carbons, you can follow along to see if it’s an oxidation or reduction. If the oxidation state is becoming more negative, it’s a reduction (gaining electrons). If the oxidation state is becoming more positive, it’s an oxidation (losing electrons).

Let’s look at those examples again (putting in an extra example for fun), paying attention to the change in oxidation state.

So is there a quick way to figure out if a carbon is being oxidized or reduced? Why yes there is.

A reduction will result in a net increase in the number of C-H bonds, or a net decrease in the number of C-O bonds (or equivalent, such as C-Cl, C-Br, etc).

An oxidation will result in a net decrease in the number of C-H bonds, or a net increase in the number of C-O bonds (or equivalent). 

All of these events affect the oxidation state of the carbon, and this ties back to the concept of oxidation that I originally learned in high school: keeping track of the gaining (and losing) of electrons.

When I finally understood this I was happy to note that the term “oxidation” finally made sense again.

“Reduction” still didn’t, but I learned to live with it and moved on.  You will too.


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{ 11 comments… read them below or add one }

azmanam August 1, 2011 at 1:10 pm

In gen chem (at least in the US), we sometimes learn the mnemonic OILRIG for redox reactions: Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons).

In organic chemistry, we rarely calculate out the actual oxidation state of carbon. We more often use the mnemonic you have above: Oxidation is loss of C-H bond or gain of C-X bonds, and Reduction is the gain of C-H bonds or loss of C-X bonds.

More generally, in organic chemistry, we talk about “oxidation levels” rather than “oxidation states.” It takes an oxidation of an alkane to make an alkyl halide, or an alcohol, or an alkene, so we typically lump all of these together and call an alcohol, alkene, and alkyl halide at ‘the same oxidation level.’

The next oxidation level up would be alkynes, aldehydes, or ketones, and organic chemists would typically consider these at ‘the same oxidation level.’ Next is esters, amides, acids, acid chlorides. Finally, carbonates, carbamates, and carbon dioxide are all at the ‘most oxidized oxidation level.

If I oxidize a primary alcohol with PCC, the molecule is oxidized one oxidation level to an aldehyde. If I oxidize a secondary alcohol with PCC, the molecule is oxidized one oxidation level to a ketone, thus ketones and aldehydes are ‘at the same oxidation level,’ right?

This chart somewhat illustrates my point. Everything within a vertical column an organic chemist would consider ‘at the same oxidation level.’ To move to a different column, one needs to perform an oxidation or a reduction reaction. To move within a column (from an aldehyde to an acetal), one only needs to use acid/base chemistry (thus, chemists also learn that acid/base reactions are not considered redox reactions).

Yet, when you actually go through and calculate the oxidation state of carbon in each case… an aldehdye and a ketone are definitely not at the same carbon oxidation state… and can never be!

I think this speaks to the ‘formalism’ aspect of oxidation states. It’s a bookkeeping tool (and a useful one), but depending on how you use it, it can be confusing. I’ve discussed this in two other places, and I never seem to find the right words. Basically, one must be careful how one defines ‘oxidation state,’ because organic chemistry and organic chemistry define it differently.

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James August 1, 2011 at 2:14 pm

Thanks for such a detailed response. I find it hard to find the right words too. As an organic chemist, I consider aldehydes and ketones to be at the same “oxidation state” but like you said the oxidation number at carbon is different. It’s a mess, but I’m not sure what can be done about it.

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John Spevacek August 2, 2011 at 9:14 am

Thanks for exposing me to the historic perspective that I was unaware of. I love it, but always worry if that approach isn’t too academic. For (too) many people, history began the day they were born.

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James August 3, 2011 at 12:42 pm

True… but I think it’s something you grow out of though :-)

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MJ February 19, 2013 at 5:52 am

This explanation (and whole site really) is awesome! Wish I’d stumbled across this site months ago.
I remember learning that “reduction” meant “reducing the overall charge on the atom”. Meaning when you gain an electron, the atom is going to be more negative and its charge is going to be reduced! That helped me make sense of the word reduction.

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Piya June 1, 2013 at 1:51 am

Thanks for a great explanation! But what does “CONTROLLED OXIDATION” mean then?

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Oyin June 7, 2013 at 9:08 pm

How are you counting the oxidation state on carbon, like C: -1, C: -3. I thought on the first example that there are 2 C-H bonds, So its not C:-2

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james June 8, 2013 at 7:18 pm

don’t forget the Cl –> this makes the oxidation state higher

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Maryzze November 27, 2013 at 11:46 am

The fun part in studying Chemistry is that the branches are undoubtedly inter and correlated and yet you’ll find each in a very different perspective/approach as compared to the others. I never thought that redox is very much used even in organic Chem. I so love it! Thanks for the interesting “history” and explanation :))

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caters June 6, 2014 at 12:31 pm

I have heard of oxidation as loss of electrons, gain of oxygen, and as loss of hydrogen.

I have also heard of reduction as gain of electrons, loss of oxygen, and as gain of hydrogen.

In other words Fe + H2O = H2 + FeOx(with the x standing for unknown amount of O) where the water is reduced to hydrogen and the iron is oxidized to one or more of lots of different iron oxides.

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Dietmar November 14, 2014 at 3:24 am

Hi everybody,
In the meantime I understood what oxidation is all about, and I understood what acidity is all about. What I didn’t understand yet is how the two interact. I know that there are RedOx reactions that are favored by acidity, but can you say that “oxidation is favored by acidity” (probably not?!)? Is it true that iron oxidizes faster under acidic conditions? Does it therefore oxidize more slowly under basic conditions?
Regards, Dietmar

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