Nonmetal inorganic reaction kinetics: Bromine transfer reactions of iodine monobromide and hypobromite

Robert Christopher Troy, Purdue University

Abstract

Kinetics of the reaction of IBr with I$\sp-$ ($\mu$ = 1.0 M, 25$\sp\circ$C) is studied by pulsed-accelerated-flow spectroscopy. Pseudo-first-order rate constants with excess I$\sp-$ are measured between 2100-41000 s$\sp{-1}$. The reaction IBr(aq) + I$\sp{-}\ \sbsp{\gets}{\to}$ I$\sb2$Br$\sp-$ is reversible with a forward rate constant k$\sb{\rm f}$ = 2.0 $\times$ 10$\sp9$ M$\sp{-1}$ s$\sp{-1}$ and a reverse rate constant k$\sb{\rm r}$ = 800 s$\sp{-1}$. The dissociation equilibrium constant of IBr$\sb2\sp-\ \sbsp{\gets}{\to}$ IBr(aq) + Br$\sp-$ is measured spectrophotometrically and is 0.0035 M. The hydrolysis of iodine monobromide ($\mu$ = 0.50 M, 25$\sp\circ$C) to form HOI and Br$\sp-$ is extremely rapid (k = 9 $\times$ 10$\sp5$ s$\sp{-1}$). The reaction is accelerated by hydroxide ion (k = 6.0 $\times$ 10$\sp9$ M$\sp{-1}$ s$\sp{-1}$). Two forms of IBr(aq) are proposed: IBr (45%) and H$\sb2$OIBr (55%). The pulsed-accelerated-flow method is used to study the reactions of OBr$\sp-$ with I$\sp-$ and with SO$\sb3\sp{2-}$ ($\mu$ = 0.50 M, 25$\sp\circ$C). Pseudo-first-order rate constants for the OBr$\sp-$ with I$\sp-$ reaction are measured in the range 5930-12900 s$\sp{-1}$. The OBr$\sp-$ with So$\sb3\sp{2-}$ reaction is studied under second-order-unequal concentration conditions. Parallel paths are proposed with the reactions of OBr$\sp-$ (k$\sb{\rm OBr}$) and of HOBr (k$\sb{\rm HOBr}$) with the two nucleophiles. Nucleophilic attack at bromine (Br$\sp+$-transfer reactions) is much faster than corresponding attack at chlorine (Cl$\sp+$-transfer reactions). For I$\sp-$, k$\sb{\rm OBr}$ = 6.8 $\times$ 10$\sp5$ M$\sp{-1}$ s$\sp{-1}$ and k$\sb{\rm HOBr}$ = 5.0 $\times$ 10$\sp9$ M$\sp{-1}$ s$\sp{-1}$. For SO$\sb3\sp{2-}$, k$\sb{\rm OBr}$ = 1.0 $\times$ 10$\sp8$ M$\sp{-1}$ s$\sp{-1}$ and k$\sb{\rm HOBr}$ = 5 $\times$ 10$\sp9$ M$\sp{-1}$ s$\sp{-1}$. A rapid aqueous equilibration with HOBr formation (pK$\sb{\rm a}$ = 8.8) precedes the HOBr path. The BrSO$\sb3\sp-$ that is formed in the reaction of OBr$\sp-$ with SO$\sb3\sp{2-}$ hydrolyzes with a rate constant k$\sb{\rm h}$ = 290 s$\sp{-1}$ at 2.0$\sp\circ$C. A narrow temperature dependent (0.0-4.0$\sp\circ$C) stopped-flow study gives $\Delta$H$\sp\ddagger$ = 70 kJ mol$\sp{-1}$ and $\Delta$S$\sp\ddagger$ = 60 J mol$\sp{-1}$ K$\sp{-1}$ for the hydrolysis reaction.

Degree

Ph.D.

Advisors

Margerum, Purdue University.

Subject Area

Chemistry

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