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Alcohols, Epoxides and Ethers

By James Ashenhurst

Calculating the oxidation state of a carbon

Last updated: November 12th, 2022 |

The Oxidation State Formalism

By the end of Gen Chem, calculating oxidation states of different metals should be pretty familiar.

Here’s what you do for a typical compound like FeCl3 for instance.

  • Treat every bond between the metal and a different atom as if it were an ionic bond.
  • That means the more electronegative elements (like chlorine, say, or oxygen) bear negative charges, and the less electronegative element (such as the metal) bears the positive charge.
  • If the compound is neutral, the sum of the oxidation states also has to be neutral. (If the compound has a charge, you adjust the oxidation states accordingly so that their sum equals the charge).

1. Calculating Oxidation States In Inorganic Compounds

oxidation state formalism oxidation is hypothetical charge atom would have if all bonds to atoms of different elements were treated as ionic

Now here’s a fun exercise. Try applying the same rules to carbon.

It’s going to feel a little bit weird. Why? Because there are two key differences.

  • First, carbon is often more electronegative (2.5) than some of the atoms it’s bound to (such as H, 2.2). So what do you do in this case?
  •  Secondly, unlike metal-metal bonds, carbon-carbon bonds are ubiquitous. So how do you deal with them?

Two answers.

  1. In a C-H bond, the H is treated as if it has an oxidation state of +1. This means that every C-H bond will decrease the oxidation state of carbon by 1.
  2. Any two bonds between the same atom do not affect the oxidation state (recall that the oxidation state of Cl in Cl-Cl (and that of H in H-H) is zero. So a carbon attached to 4 carbons has an oxidation state of zero.

So unlike metals, which are almost always in a positive oxidation state, the oxidation state of carbon can vary widely, from -4 (in CH4) to +4 (such as in CO2). Here are some examples.

2. Calculating The Oxidation State Of Carbon

examples-of-oxidation-state-calculation-involving-carbon

(Don’t forget that this is called a “formalism” for a reason. The charge on the carbon isn’t really +4 or –4. But the oxidation state formalism helps us keep track of where the electrons are going, which will come in handy very soon).

With an understanding of how to calculate oxidation states on carbon, we’re ready for the next step: understanding changes in the oxidation state at carbon, through reactions known as oxidations (where the oxidation state is increased), and reductions (where the oxidation state is reduced). More on that next time.

00 General Chemistry Review
01 Bonding, Structure, and Resonance
02 Acid Base Reactions
03 Alkanes and Nomenclature
04 Conformations and Cycloalkanes
05 A Primer On Organic Reactions
06 Free Radical Reactions
07 Stereochemistry and Chirality
08 Substitution Reactions
09 Elimination Reactions
10 Rearrangements
11 SN1/SN2/E1/E2 Decision
12 Alkene Reactions
13 Alkyne Reactions
14 Alcohols, Epoxides and Ethers
15 Organometallics
16 Spectroscopy
17 Dienes and MO Theory
18 Aromaticity
19 Reactions of Aromatic Molecules
20 Aldehydes and Ketones
21 Carboxylic Acid Derivatives
22 Enols and Enolates
23 Amines
24 Carbohydrates
25 Fun and Miscellaneous
26 Organic Chemistry Tips and Tricks
27 Case Studies of Successful O-Chem Students

Comments

Comment section

21 thoughts on “Calculating the oxidation state of a carbon”

  1. what if the carbonyl group is protonated? what will be the oxidation number of the carbon carrying carbonyl group?

    Reply

  2. 35 yrs of scientific studies before realizing that carbon could have 9 oxidation state! Thx. But it has always 4 ‘legs’ (unsaturated). I corrected a 5 legs carbon in a PhD draft appendices once!

    Reply

  3. The + and – charges depicted on the C-OH bond of the primary alcohol should be reversed. The + should be near the C and the – should be near the O. Otherwise an excellent explanation?

    Reply

  5. If the carbon has a higher oxidation state in ketones than in aldehydes, then why are aldehydes easier to reduce? Is it only due to sterics? I assume sterics is why aldehydes are more reactive to nucleophilic addition.

    Reply
  6. Pingback: Oxidation State of Organic Compound | aboutstephen

  7. Some people say that the carbon atoms in acetic acid have oxidation numbers of +2 & -2 by referring to their states of hybridisation (sp2 & sp3) and applying the concept of electronegativity .
    Is it correct?
    If it is correct, then why don’t we apply it to all other compounds?

    Reply

  8. Wow, I never understood the concept of oxidation states until just now. I always just memorized how to do them.

    Reply

  9. Yaay! I love this!! Helped me so much in understanding biochemical reactions :)

    Thanks James.. wish I could like this on facebook or something :)

    Reply

  12. “With an understanding of how to calculate oxidation states on carbon, we’re ready for the next step: understanding changes in the oxidation state at carbon, through reactions known as oxidations”

    Were you planning on making a post specifically on this topic?

    Reply

    1. What do you mean by “large version”? The png is 1175px x 2120px. It’s just over 2 landscape chemdraw pages tall. You should be able to zoom in on the picture to get a larger version.

      I redid it slightly just now to make it more general and less specific (now listing the oxidation state of any methylene carbon, not just the oxidation state of propane). The new version’s on the chem reddit and here:

      http://i.imgur.com/wyZaa.png

      Reply

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