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Mechanism of DPD (N,N-diethyl-p-phenylenediamine) reaction with free chlorine
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Khanifa Mzee
Mechanism of DPD (N,N-diethyl-p-phenylenediamine) reaction with free chlorine
Note that hydrogen peroxide is slow to oxidize DPD except at high concentrations but using peroxidase cleaves at the oxygens to form hydroxyl radicals. As noted in the cited paper [1], this can be used to quantify hydrogen peroxide as it becomes activated to oxidize DPD.
So the hydroxyl radical from hypochlorous acid can form water from the hydrogen on DPD leaving the radical on DPD. This leaves a chlorine radical which can react with another chlorine radical from a second parallel reaction. The resulting molecular chlorine in water forms hypochlorous acid and hydrochloric acid. The net result is the following (the HOCl doesn't normally split to form radicals except by UV photolysis so I show a direct attack instead):
$\ce{2x (HOCl + H-DPD -> H2O + DPD• + Cl•)}$
$\ce{2Cl• -> Cl2}$
$\ce{Cl2 + H2O -> HOCl + HCl}$
$\ce{HOCl + 2H-DPD -> H2O + 2DPD• + HCl}$
Normally hypochlorous acid ($\ce{HOCl}$) oxidizes in a 2-electron transfer but the above has it oxidize via a 1-electron transfer via its hydroxyl radical. Having two of these reactions occur creates molecular chlorine that then disproportionates into hypochlorous acid and hydrochloric acid but since you started with two hypochlorous acid the net effect is that two reacting DPD behave together like a 2-electron transfer reaction converting hypochlorous acid into hydrochloric acid.
Another way to think about the initial steps in this reaction is as a chlorine substitution reaction that then cleaves into radicals. That is,
$\ce{HOCl + H-DPD -> H2O + Cl-DPD}$
$\ce{Cl-DPD -> Cl• + DPD•}$
Normally chlorine substitution reactions occur with amine groups, but the nitrogen in this case is cationic so the hydrogen is more easily substituted but the chlorine being electronegative doesn't bond well with the cationic nitrogen.
Reference
[1] : Chang, Q., Deng, K., Zhu, L. et al. Determination of hydrogen peroxide with the aid of peroxidase-like Fe3O4 magnetic nanoparticles as the catalyst. Microchim Acta 165, 299 (2009). Doi: 10.1007/s00604-008-0133-z
Note that hydrogen peroxide is slow to oxidize DPD except at high concentrations but using peroxidase cleaves at the oxygens to form hydroxyl radicals. As noted in the cited paper [1], this can be used to quantify hydrogen peroxide as it becomes activated to oxidize DPD.
So the hydroxyl radical from hypochlorous acid can form water from the hydrogen on DPD leaving the radical on DPD. This leaves a chlorine radical which can react with another chlorine radical from a second parallel reaction. The resulting molecular chlorine in water forms hypochlorous acid and hydrochloric acid. The net result is the following (the HOCl doesn't normally split to form radicals except by UV photolysis so I show a direct attack instead):
$\ce{2x (HOCl + H-DPD -> H2O + DPD• + Cl•)}$
$\ce{2Cl• -> Cl2}$
$\ce{Cl2 + H2O -> HOCl + HCl}$
$\ce{HOCl + 2H-DPD -> H2O + 2DPD• + HCl}$
Normally hypochlorous acid ($\ce{HOCl}$) oxidizes in a 2-electron transfer but the above has it oxidize via a 1-electron transfer via its hydroxyl radical. Having two of these reactions occur creates molecular chlorine that then disproportionates into hypochlorous acid and hydrochloric acid but since you started with two hypochlorous acid the net effect is that two reacting DPD behave together like a 2-electron transfer reaction converting hypochlorous acid into hydrochloric acid.
Another way to think about the initial steps in this reaction is as a chlorine substitution reaction that then cleaves into radicals. That is,
$\ce{HOCl + H-DPD -> H2O + Cl-DPD}$
$\ce{Cl-DPD -> Cl• + DPD•}$
Normally chlorine substitution reactions occur with amine groups, but the nitrogen in this case is cationic so the hydrogen is more easily substituted but the chlorine being electronegative doesn't bond well with the cationic nitrogen.
Reference
[1] : Chang, Q., Deng, K., Zhu, L. et al. Determination of hydrogen peroxide with the aid of peroxidase-like Fe3O4 magnetic nanoparticles as the catalyst. Microchim Acta 165, 299 (2009). Doi: 10.1007/s00604-008-0133-z
Note that hydrogen peroxide is slow to oxidize DPD except at high concentrations but using peroxidase cleaves at the oxygens to form hydroxyl radicals. As noted in the cited paper [1], this can be used to quantify hydrogen peroxide as it becomes activated to oxidize DPD.
So the hydroxyl radical from hypochlorous acid can form water from the hydrogen on DPD leaving the radical on DPD. This leaves a chlorine radical which can react with another chlorine radical from a second parallel reaction. The resulting molecular chlorine in water forms hypochlorous acid and hydrochloric acid. The net result is the following (the HOCl doesn't normally split to form radicals except by UV photolysis so I show a direct attack instead):
$\ce{2x (HOCl + H-DPD -> H2O + DPD• + Cl•)}$
$\ce{2Cl• -> Cl2}$
$\ce{Cl2 + H2O -> HOCl + HCl}$
$\ce{HOCl + 2H-DPD -> H2O + 2DPD• + HCl}$
Normally hypochlorous acid ($\ce{HOCl}$) oxidizes in a 2-electron transfer but the above has it oxidize via a 1-electron transfer via its hydroxyl radical. Having two of these reactions occur creates molecular chlorine that then disproportionates into hypochlorous acid and hydrochloric acid but since you started with two hypochlorous acid the net effect is that two reacting DPD behave together like a 2-electron transfer reaction converting hypochlorous acid into hydrochloric acid.
Another way to think about the initial steps in this reaction is as a chlorine substitution reaction that then cleaves into radicals. That is,
$\ce{HOCl + H-DPD -> H2O + Cl-DPD}$
$\ce{Cl-DPD -> Cl• + DPD•}$
Normally chlorine substitution reactions occur with amine groups, but the nitrogen in this case is cationic so the hydrogen is more easily substituted but the chlorine being electronegative doesn't bond well with the cationic nitrogen.
Reference
[1] : Chang, Q., Deng, K., Zhu, L. et al. Determination of hydrogen peroxide with the aid of peroxidase-like Fe3O4 magnetic nanoparticles as the catalyst. Microchim Acta 165, 299 (2009). Doi: 10.1007/s00604-008-0133-z
Note that hydrogen peroxide is slow to oxidize DPD except at high concentrations but using peroxidase cleaves at the oxygens to form hydroxyl radicals. As noted in the cited paper [1], this can be used to quantify hydrogen peroxide as it becomes activated to oxidize DPD.
So the hydroxyl radical from hypochlorous acid can form water from the hydrogen on DPD leaving the radical on DPD. This leaves a chlorine radical which can react with another chlorine radical from a second parallel reaction. The resulting molecular chlorine in water forms hypochlorous acid and hydrochloric acid. The net result is the following (the HOCl doesn't normally split to form radicals except by UV photolysis so I show a direct attack instead):
$\ce{2x (HOCl + H-DPD -> H2O + DPD• + Cl•)}$
$\ce{2Cl• -> Cl2}$
$\ce{Cl2 + H2O -> HOCl + HCl}$
$\ce{HOCl + 2H-DPD -> H2O + 2DPD• + HCl}$
Normally hypochlorous acid ($\ce{HOCl}$) oxidizes in a 2-electron transfer but the above has it oxidize via a 1-electron transfer via its hydroxyl radical. Having two of these reactions occur creates molecular chlorine that then disproportionates into hypochlorous acid and hydrochloric acid but since you started with two hypochlorous acid the net effect is that two reacting DPD behave together like a 2-electron transfer reaction converting hypochlorous acid into hydrochloric acid.
Another way to think about the initial steps in this reaction is as a chlorine substitution reaction that then cleaves into radicals. That is,
$\ce{HOCl + H-DPD -> H2O + Cl-DPD}$
$\ce{Cl-DPD -> Cl• + DPD•}$
Normally chlorine substitution reactions occur with amine groups, but the nitrogen in this case is cationic so the hydrogen is more easily substituted but the chlorine being electronegative doesn't bond well with the cationic nitrogen.
Reference
[1] : Chang, Q., Deng, K., Zhu, L. et al. Determination of hydrogen peroxide with the aid of peroxidase-like Fe3O4 magnetic nanoparticles as the catalyst. Microchim Acta 165, 299 (2009). Doi: 10.1007/s00604-008-0133-z
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