Comproportionation or synproportionation is a chemical reaction where two reactants containing the same element but with different
oxidation number
In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...
s, form a compound having an intermediate oxidation number. It is the opposite of
disproportionation
In chemistry, disproportionation, sometimes called dismutation, is a redox reaction in which one compound of intermediate oxidation state converts to two compounds, one of higher and one of lower oxidation states. More generally, the term can ...
.
[Shriver, D. F.; Atkins, P. W.; Overton, T. L.; Rourke, J. P.; Weller, M. T.; Armstrong, F. A. “Inorganic Chemistry” W. H. Freeman, New York, 2006. .]
Frost diagrams
The tendency of two species to disproportionate or comproportionate can be determined by examining the
Frost diagram
A Frost diagram or Frost–Ebsworth diagram is a type of graph used by inorganic chemists in electrochemistry to illustrate the relative stability of a number of different oxidation states of a particular substance. The graph illustrates the free ...
of the oxidation states; if a species' value of Δ''G''/''F'' is lower than the line joining the two oxidation numbers on either side of it, then it is more stable and if in a solution, these two species will undergo comproportionation.
A Frost Diagram is another way of displaying the reduction potentials for the various oxidation states of a given element, X. It shows nE against the oxidation number N: here, E is the reduction potential for the X(N)/X(0) couple, and n is the number of electrons transferred in the conversion of X(N) to X(0)
Examples
* In
lead batteries
Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cut, l ...
, the spontaneous reaction is:
:: Pb + PbO
2 + 2 H
2SO
4 → 2 PbSO
4 + 2 H
2O
*The laboratory preparation of
manganese dioxide involves comproportionation of Mn(II) and Mn(VII) reagents:
:2 + 3 + 2 → 5 + + 2
* 15 Se + SeCl
4 + 4 AlCl
3 → 2 Se
8 lCl4sub>2
* In the
Claus Process, two gaseous compounds of
sulfur comproportionate in the presence of a catalyst to give elemental sulfur:
::2 H
2S + SO
2 → 3 S + 2 H
2O
* In
halogen chemistry:
::IO
3− + 5 I
− + 6 H
+ → 3 I
2 + 3 H
2O
* In
anammox
Anammox, an abbreviation for anaerobic ammonium oxidation, is a globally important microbial process of the nitrogen cycle that takes place in many natural environments. The bacteria mediating this process were identified in 1999, and were a gre ...
chemistry:
:: NH
4+ + NO
2− → N
2 + 2H
2O
*In
organotin chemistry, the mixed alkyl tin chlorides are produced by redistribution, a reaction called the Kocheshkov comproportionation:
: 3 SnBu
4 + SnCl
4 → 4 SnBu
3Cl
References
{{reflist
Chemical reactions