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Most redox reactions have two players, an oxidising agent and a reducing agent. A disproportionation reaction has one: the same element in the same species goes up and down at once. Exam questions ask you to spot these reactions, say which species cannot disproportionate, balance them and sometimes find an n-factor. This post covers all four, plus the reverse process, comproportionation.
What does NCERT say?
The NCERT Class 11 chapter Redox Reactions defines a disproportionation reaction as one in which an element in one oxidation state is simultaneously oxidised and reduced. It adds two conditions. The reacting substance must contain an element that can exist in at least three oxidation states, and that element must start in an intermediate state, so that both a higher and a lower state can form.
NCERT's examples:
2H₂O₂ → 2H₂O + O₂ (oxygen: −1 to −2 and 0)
P₄ + 3OH⁻ + 3H₂O → PH₃ + 3H₂PO₂⁻ (phosphorus: 0 to −3 and +1)
S₈ + 12OH⁻ → 4S²⁻ + 2S₂O₃²⁻ + 6H₂O (sulphur: 0 to −2 and +2)
Cl₂ + 2OH⁻ → ClO⁻ + Cl⁻ + H₂O (chlorine: 0 to +1 and −1), the reaction that makes household bleach
Redox reactions are in both the JEE (Main) 2026 syllabus and the NEET (UG) 2026 syllabus, under Redox Reactions and Electrochemistry. Check the syllabus for your exam year.
Why do students miss these reactions?
Garnett and Treagust (1992) interviewed 32 students in their final year of high school chemistry. Many had trouble identifying which equations were redox reactions, and several misconceptions came from applying definitions of oxidation and reduction where they did not fit. Disproportionation is a natural trap, because there is no separate oxidising agent to point to. The safe habit is to write the oxidation number of every atom on both sides.
Which species cannot disproportionate?
Apply NCERT's two conditions.
Highest state: NCERT's Problem 7.5 explains that ClO₄⁻ cannot disproportionate because chlorine is already at +7, its highest state. ClO⁻ (+1), ClO₂⁻ (+3) and ClO₃⁻ (+5) all can.
Fluorine: NCERT notes that fluorine, the most electronegative element, cannot show a positive oxidation state, so it has no higher state to go to. With alkali it gives F⁻ and OF₂ instead of following chlorine.
Lowest state: Cl⁻ at −1 cannot go lower.
Worked example: balancing by half reactions
NCERT's Problem 7.6 classifies 2NO₂ + 2OH⁻ → NO₂⁻ + NO₃⁻ + H₂O as disproportionation: nitrogen goes from +4 to +3 and +5. Here is how to balance it.
Reduction: NO₂ + e⁻ → NO₂⁻. Atoms and charge balance.
Oxidation: NO₂ → NO₃⁻ + e⁻. Add oxygen in basic solution with OH⁻: NO₂ + 2OH⁻ → NO₃⁻ + H₂O + e⁻.
Add them. The electrons cancel: 2NO₂ + 2OH⁻ → NO₂⁻ + NO₃⁻ + H₂O.
Check charge: −2 on the left, −1 −1 = −2 on the right.
For sulphur, check the electrons: four S atoms go from 0 to −2 and gain 8 electrons. The other four end up in two S₂O₃²⁻ ions at an average of +2, losing 8. They match.
Where does disproportionation show up in the d-block?
The NCERT Class 12 chapter on the d- and f-block elements gives two examples:
Many copper(I) compounds are unstable in water: 2Cu⁺ → Cu²⁺ + Cu. NCERT explains this through the much more negative hydration enthalpy of Cu²⁺.
Manganate(VI) disproportionates in acid: 3MnO₄²⁻ + 4H⁺ → 2MnO₄⁻ + MnO₂ + 2H₂O. This step is used in making KMnO₄.
What is comproportionation?
It is the reverse: two species with the same element in different oxidation states react to give an intermediate state. NCERT does not use the word, but the same d-block chapter has an example. Permanganate oxidises manganese(II) to MnO₂:
2MnO₄⁻ + 3Mn²⁺ + 2H₂O → 5MnO₂ + 4H⁺
Check the electrons: two Mn go from +7 to +4 and gain 6; three Mn go from +2 to +4 and lose 6. Another example you can balance yourself is iodate with iodide in acid: IO₃⁻ + 5I⁻ + 6H⁺ → 3I₂ + 3H₂O, where iodine goes from +5 and −1 to 0.
How does this connect to n-factors?
In Cl₂ + 2OH⁻ → Cl⁻ + ClO⁻ + H₂O, one Cl atom gains an electron and the other loses one. Per Cl₂, one electron is transferred, so its n-factor is 1. Our post on n-factor and the equivalent concept explains how to use it. For more balancing practice, see balancing redox reactions.
Common mistakes
Calling a reaction disproportionation when two different elements change.
Forgetting that the element must start in an intermediate state.
Reading a fractional average, like +2 in S₂O₃²⁻, as the state of every atom. NCERT notes fractional values are averages.
Balancing atoms but not charge in basic solution.
If the idea of oxidation states itself feels shaky, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing.
A practice routine
For each of NCERT's four examples, write every oxidation number and mark which atoms go up and which go down.
Write the disproportionation of ClO₃⁻ to Cl⁻ and ClO₄⁻ from NCERT's Problem 7.5 and balance it by half reactions: 4ClO₃⁻ → Cl⁻ + 3ClO₄⁻.
Balance the iodate-iodide reaction and check its charge.
List three species that cannot disproportionate and give the reason for each.
Next time a redox reaction has only one reactant changing, check straight away whether it is disproportionation.
