Why does the oxidation-reduction reaction between iron, water, and $\ce{O2}$ gas to form rusts lack an electron pushing mechanism and a specific sequence of steps? $$\ce{4Fe^0(s) + 3 O2(g) + 2n H2O(l) -> 2 Fe2O3·nH2O(s)}$$

I've noticed that most of the reactions described in undergraduate organic chemistry textbooks have detailed and widely accepted electron pushing mechanisms whereas most inorganic reactions (such as the corrosion of iron) lack electron pushing mechanisms. Why?

Also, which type of $\ce{O2}$ is involved in the corrosion of iron? The singlet excited state or the triplet ground state or doesn't matter?

$$ \underset{\text{triplet oxygen}~\ce{^3O2}}{ \ce{ ·\overset{\Large .\!\!.}{\underset{\Large .\!\!.}{O}}-\overset{\Large .\!\!.}{\underset{\Large .\!\!.}{O}}· } } \qquad \underset{\text{singlet oxygen}~\ce{^1O2}}{ \ce{ \overset{\Large .\!\!.}{\underset{\Large .\!\!.}{O}}-\overset{\Large .\!\!.}{\underset{\Large .\!\!.}{O}} } } $$

  • $\begingroup$ My tip to the hat (i.e., a compliment) for typing this nicely the two Lewis formulae with plain LaTeX' \underset instead of using a mol sketcher. $\endgroup$ – Buttonwood May 11 '19 at 13:33

Arrow pushing mechanism is just a formal way of thinking how the reaction might be occurring. Unfortunately, organic chemistry textbooks present it as if all of these reactions were occurring in real time, step by step, and we are able to watch it like a movie. The reality must be far more complex. If you were truly investigating a reaction mechanism, we have to design very clever experiments (there are books on this topic). It is experimental data that determines the true outcome of an organic reaction not the arrow pushing formalism. Usually, when chemists find out the experimental products, then they postulate arrow pushing mechanisms.

Coming to the rust question. Rust has puzzled chemists and engineers for decades because it causes millions of dollars of losses every year. Your equation is a very simplistic way of thinking that rust forms in a single step.

$$\ce{4Fe^0(s) + 3 O2(g) + 2n H2O(l) -> 2 Fe2O3·nH2O(s)}$$

It does not proceed in a single step. Our atmospheric chemistry is very complex, you have carbon dioxide, you have sulfur dioxide, nitrogen oxides, ozone, water vapors, sunlight, plenty of free radicals and plenty of minor trace components. Then there are different phases of rust too. You can have a quick look at Google Scholar and one such representative abstract [1].

In short, people still do PhD in this field and one can only imagine how complex corrosion science is.


  1. Misawa, T.; Hashimoto, K.; Shimodaira, S. On the Mechanism of Atmospheric Rusting of Iron and Protective Rust Layer on Low Alloy Steels. Corrosion Engineering 1974, 23 (1), 17–27. https://doi.org/10.3323/jcorr1974.23.1_17.
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