Nonmetal redox kinetics of chlorine and iodine atom transfer reactions

Boudin Shin-Geun Yiin, Purdue University

Abstract

The reaction between HOCl and SO$\sb3\sp{2-}$ is shown to take place in two steps. First, OH$\sp-$ is generated very rapidly and then H$\sp+$ is generated at rates measurable by stopped-flow indicator methods. The two reactions are due to the formation of ClSO$\sb3\sp-$ (by Cl$\sp+$-transfer from HOCl to SO$\sb3\sp{2-}$) and its subsequent hydrolysis. The first-order rate constant for ClSO$\sb3\sp-$ hydrolysis at 25.0$\sp\circ$C equals 270 s$\sp{-1}$ with $\Delta$H$\sp\ddagger$ = 49 kJ mol$\sp{-1}$ and $\Delta$S$\sp\ddagger$ = $-$32 J mol$\sp{-1}$ K$\sp{-1}$. The negative $\Delta$S$\sp\ddagger$ value corresponds to the uptake of a water molecule to give H$\sb2$OSO$\sb3$Cl$\sp-$ as the transition state. Pulsed-accelerated-flow (PAF) and stopped-flow techniques are used to study the kinetics of sulfite with HNCl$\sb2$, CH$\sb3$NCl$\sb2$, and NCl$\sb3$. The second-order rate constants (M$\sp{-1}$ s$\sp{-1})$ of SO$\sb3\sp{2-}$ reactions with HNCl$\sb2$, CH$\sb3$NCl$\sb2$, and NCl$\sb3$ are 5.8 $\times$ 10$\sp6$, 2.4 $\times$ 10$\sp7$, and 4.5 $\times$ 10$\sp9$, respectively. The rate constant for the reaction of SO$\sb3$H$\sp-$ with NCl$\sb3$ is 1.4 $\times$ 10$\sp7$ M$\sp{-1}$ s$\sp{-1}$. A Cl$\sp+$-transfer mechanism is proposed for all the chloramines and sulfite reactions. The reactions of I$\sb3\sp-$ and I$\sb2$ with SO$\sb3$H$\sp-$ and SO$\sb3\sp{2-}$ are studied with the PAF technique. Pseudo-first-order rate constants are measured in the range from 6,300 to 74,000 s$\sp{-1}$ (25.0$\sp\circ$C, $\mu$ = 0.50). An I$\sp+$-transfer mechanism is proposed for each reaction. The second-order rate constants (M$\sp{-1}$ s$\sp{-1}$) are 1.4 $\times$ 10$\sp7$ (SO$\sb3$H$\sp-$ + I$\sb3\sp-$), 1.8 $\times$ 10$\sp9$ (SO$\sb3$H$\sp-$ + I$\sb2$), 2.8 $\times$ 10$\sp8$ (SO$\sb3\sp{2-}$ + I$\sb3\sp-$), and 3.2 $\times$ 10$\sp9$ (SO$\sb3\sp{2-}$ + I$\sb2$). The proposed intermediate, ISO$\sb3\sp-$, hydrolyzes with a first-order rate constant of 298 s$\sp{-1}$ at 25.0$\sp\circ$C with $\Delta$H$\sp\ddagger$ = 65 kJ mol$\sp{-1}$ and $\Delta$S$\sp\ddagger$ = 21 J mol$\sp{-1}$ K$\sp{-1}$. The positive $\Delta$S$\sp\ddagger$ value indicates a dissociative mechanism without the addition of H$\sb2$O in the transition state of ISO$\sb3\sp-$ hydrolysis. The kinetics of the reaction between NH$\sb3$ and NCl$\sb3$ in basic solutions are studied. The result shows that the reaction rate is not only general-base but also general-acid dependent. In the proposed transition state, Cl$\sb2$NClNH$\sb3$ reacts with an acid or a base to give HNCl$\sb2$ and NH$\sb2$Cl via Cl$\sp+$-transfer. The dichloramine so formed reacts rapidly with another NCl$\sb3$ by a base-catalyzed mechanism to give N$\sb2$, Cl$\sp-$, and HOCl.

Degree

Ph.D.

Advisors

Margerum, Purdue University.

Subject Area

Analytical chemistry

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