Disproportionation Reaction

In a disproportionation reaction, the same species is simultaneously oxidized and reduced, producing two different substances. This type of reaction typically occurs in compounds where the element has an intermediate oxidation state that can either be increased or decreased.

1.0What is a Disproportionation Reaction?

A disproportionation reaction, also known as a dismutation reaction, is a specific type of redox reaction(oxidation-reduction) in which a single substance undergoes simultaneous oxidation and reduction, resulting in the formation of two distinct products. This process is characterized by the substance both gaining and losing electrons.

2.0Redox and Disproportionation Reaction

Disproportionation reactions are a subset of redox reactions. In a typical redox reaction, two different species undergo oxidation and reduction. In disproportionation, the same species undergoes both.

Oxidation States: In a disproportionation reaction, the element starts in an intermediate oxidation state and ends up in two different oxidation states—one higher (oxidized) and one lower (reduced).

3.0General Characteristics of Disproportionation Reaction

1. Single Reactant with Intermediate Oxidation State: The element in the reactant is initially in an intermediate oxidation state and gets both oxidized and reduced.
2. Formation of Two Products: The products formed have the same element in two different oxidation states—one higher and one lower than the original state.
3. Element with Multiple Oxidation States: The reacting element must be capable of existing in at least three different oxidation states.

4.0Examples of Disproportionation Reaction

1. Decomposition of Hydrogen Peroxide (H₂O₂):

2H₂O₂  →  2H₂O  +  O₂

Oxidation States:

• Hydrogen peroxide (H₂O₂) has oxygen in the oxidation state of -1.
• In water (H₂O), oxygen is in the oxidation state of -2.
• In oxygen gas (O₂), oxygen is in the oxidation state of 0.
• Thus, hydrogen peroxide both reduces to water and oxidizes to oxygen gas.

2. Disproportionation of Chlorine in Basic Solution:

Cl₂ + 2NaOH  →  NaCl + NaClO +H₂O

Oxidation States:

• Chlorine (Cl₂) has an oxidation state of 0.
• In sodium chloride (NaCl), chlorine is in the oxidation state of -1.
• In sodium hypochlorite (NaClO), chlorine is in the oxidation state of +1.

5.0Steps to Identify a Disproportionation Reaction

1. Check the Oxidation States: 

Identify the oxidation states of the reactant before and after the reaction.

2. Identify Oxidation and Reduction: 

Ensure that the same element is both oxidized (increasing its oxidation state) and reduced (decreasing its oxidation state).

3. Verify Products: 

Confirm that the reaction results in the formation of two different products, each with the element in different oxidation states.

6.0Comproportionation Reactions

Comproportionation, also known as synproportionation, is the reverse process of disproportionation. In a comproportionation reaction, two species containing the same element in different oxidation states react to form a single product in an intermediate oxidation state. These reactions are essentially the reverse of disproportionation reactions.

Basically, disproportionation reactions can be reversed by changing the medium (e.g., from acidic to basic or vice versa). This backward reaction exemplifies comproportionation.

7.0General Characteristics of Comproportionation Reactions

1. Reactants in Different Oxidation States: 

The reactants contain the same element but in different oxidation states.

2. Formation of Single Product: 

The product formed has the element in an intermediate oxidation state.

3. Reverse of Disproportionation: 

Many comproportionation reactions can be considered as the reverse of corresponding disproportionation reactions.

8.0Examples of Comproportionation Reaction

Ex. Reaction Between Iodide and Iodate in Acidic Medium:

I⁻+ IO₃⁻ + 6H⁺ → 3 I₂ + 3H₂O

Oxidation States:

• In I⁻, iodine is in the -1 oxidation state.
• In IO₃⁻​, iodine is in the +5 oxidation state.
• In I₂​, iodine is in the 0 oxidation state.

Explanation:

• Iodide (I⁻) and iodate (IO₃⁻​) ions react in the presence of an acid to form iodine (I₂​) and water (H₂O).
• This reaction can be viewed as the reverse of the disproportionation of iodine in a basic medium.

Ex. Hydrogen Peroxide in Acidic Medium:

Disproportionation of hydrogen peroxide in basic medium: 

2H₂​O₂​ → 2H_2​O + O₂​

Reverse reaction (comproportionation) in an acidic medium: 

O₂ + 2H₂O + 2e⁻ → 2H₂O₂

In an acidic medium, oxygen can react with water to form hydrogen peroxide, representing a comproportionation reaction.

Conclusion:

Comproportionation reactions are the reverse of disproportionation reactions, involving the reaction of two different oxidation states of the same element to form a single product with an intermediate oxidation state. 

9.0Difference Between Disproportion and Comproportionation Reaction

Here is a table summarizing the differences between disproportionation and comproportionation reactions.