I can not understand difference between the intramolecular redox reaction and the Disproportionation redox reaction. Both involve the same molecule as substrate, but what is difference?
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Disproportionation is a redox reaction where one element works both ways, with part of it being oxidized and another part reduced. It does not have to be intramolecular in the strict sense, since the oxidizing and reducing agents do not necessarily come from the same molecule (though they come from similar molecules, of course). Think of the well-known reaction: $$\ce{3KClO -> KClO3 + 2KCl}$$ It is certainly a disproportionation, but one may hardly call it intramolecular; there is not much going on within one molecule (or rather within one ion, for all of these are ionic compounds). Chlorine oxidizes chlorine; now, one ion contains one atom of chlorine, so to oxidize anything it needs to find another ion in the first place.
Intramolecular redox reaction occurs when we have the oxidizing and reducing agents within one molecule. They may or may not be of the same element. In the former case it is going to be a disproportionation, but not in the latter. Think of TNT: $$\ce{2 C6H2(NO2)3CH3 → 3 N2 + 5 H2O + 7 CO + 7 C}$$ See what's going on? Basically, N oxidizes C. They are indeed from the same molecule, so this may be intramolecular, but they are not the same element, hence this is not a disproportionation.
answered Sep 18, 2015 at 14:57
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$\begingroup$ You're sure TNT decomposition is intramolecular? $\endgroup$– MithoronCommented Sep 18, 2015 at 19:40
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1$\begingroup$ Not really. It's a pretty violent thing, with a lot of molecules breaking up and reacting all together. I should have said that this is but an example of a reaction that can happen intramolecularly. Come to think of it, redox reactions with proven and definitive intramolecular character are relatively rare. The Wolff–Kishner reduction may be a valid example. $\endgroup$ Commented Sep 18, 2015 at 20:01
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$\begingroup$ Slight disagreement: any elimination reaction with a cyclic transition state (e.g. selenoxide pyrolysis, ester pyrolysis) is an intramolecular redox reaction although the only redox active elements are typically carbon. There are probably quite a few more of that type. $\endgroup$– JanCommented Sep 13, 2017 at 6:44
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$\begingroup$ That's right, so they are not that rare, after all. $\endgroup$ Commented Sep 13, 2017 at 6:59
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DISPROPORTIONATION DEFINITION:
Disproportionation is a chemical reaction, typically a redox reaction, where a molecule is transformed into two or more dissimilar products. In a redox reaction, the species is simultaneously oxidized and reduced to form at least two different products.
Disproportionation reactions follow the form:
$$\ce{2A -> A' + A"}$$
where $\ce{A}$, $\ce{A'}$, and $\ce{A"}$ are all different chemical species.
The reverse reaction of disproportionation is called comproportionation.
Examples: Hydrogen peroxide converting into water and oxygen is a disproportionation reaction.
$$\ce{2 H2O2 -> H2O + O2}$$
Water dissociating into $\ce{H3O+}$ and $\ce{OH-}$ is an example of a disproportionation reaction that is not a redox reaction.
In intramolecular redox reaction, the molecule of a single substance undergoes oxidation-reaction by the process decomposition. e.g. Thermal decomposition of ammonium nitrite into $\ce{N2}$ and $\ce{H2O}$: $$\ce{NH4NO2 -> N2 + 2H2O}$$ Here, the oxidation number of nitrogen in $\ce{NH4+}$ ion changes from -3 to zero (oxidation), and that of nitrogen in $\ce{NO2-}$ ion changes from +3 to zero (reduction) in the formation of $\ce{N2}$. Another example is the thermal decomposition of $\ce{KClO3}$ into $\ce{KCl}$ and $\ce{O2}$. In this case, chlorine is reduced to -1 state and oxygen is oxidised to zero state.
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1$\begingroup$ I suppose your example for intramolecular redox is better than the one given in the previous answer. $\endgroup$ Commented May 5, 2017 at 15:02
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1$\begingroup$ @AaronJohnSabu I don’t; ammonium nitrite (apart from not existing due to immediately decomposing) is an ionic compound composed of ammonium molecular ions and nitrite molecular ions. The ammonium and nitrite ions do not form common molecules thus the redox reaction is not intramolecular. $\endgroup$– JanCommented Sep 13, 2017 at 6:42
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In intramolecular redox a single molecule is the substrate. But in case of disproportionation a single element is both reduced and oxidised.The element might be a part of a molecule with other elements.
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2$\begingroup$ Could you expand on this a little more? Usually when answering a question that has other answers, you want to make sure your answer is sufficiently different from the others or clarifies some point that other answers missed. Welcome to Chem SE! $\endgroup$– Tyberius ♦Commented Sep 12, 2017 at 19:46
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Intramolecular reactions are those in which different elements (or species), which are from the same molecule, undergo oxidation and reduction.
Disproportionation reactions are those in which the same element undergoes both oxidation and reduction to give different products.
