Comproportionation

Comproportionation is a type of chemical reaction where two substances containing the same element in different oxidation states react to form a single product in which the element has an intermediate oxidation state. It is also known as a redox reaction, specifically a type of disproportionation reaction running in reverse. In a comproportionation reaction, the oxidation states of the reacting elements move closer together. For example, when iodine ($I_2$) reacts with iodide ions ($I^-$), both containing iodine, the reaction forms triiodide ions ($I_3^-$). In this case, iodine in its elemental form ($I_2$) has an oxidation state of 0, and iodine in the iodide ion ($I^-$) has an oxidation state of -1. The product, triiodide ($I_3^-$), contains iodine with an average oxidation state of -1/3.

This type of reaction is driven by the tendency of elements to achieve a more stable oxidation state. Often, comproportionation reactions involve the formation of a product with an oxidation state that is thermodynamically favorable. The reaction can be represented by a general equation where an element in a higher oxidation state ($A^{n+}$) reacts with the same element in a lower oxidation state ($A^{m+}$) to form a product with an intermediate oxidation state ($A^{p+}$), where $m < p < n$. An example is the reaction between permanganate ion ($MnO_4^-$, manganese in oxidation state +7) and manganese(II) ion ($Mn^{2+}$, manganese in oxidation state +2) in acidic solution to form manganese dioxide ($MnO_2$, manganese in oxidation state +4). Understanding comproportionation is important in various fields, including inorganic chemistry, analytical chemistry, and materials science.