Kinetics and mechanistic studies: I. Decomposition of copper (III)-tripeptide complexes in basic solution. II. Reactions of hypochlorous acid and trichloramine with iodide studied by the pulsed-accelerated-flow method

Julius Charles Nagy, Purdue University

Abstract

The decomposition of Cu(III)-tripeptide complexes is catalyzed by hydroxide ion over the $-$log (H$\sp{+}$) range from 8 to 13. The observed pseudo-first-order rate constants increase up to a $-$log (H$\sp{+}$) value of 11, and then decrease as $-$log (H$\sp{+}$) is increased further. The formation of amine-deprotonated and amine-deprotonated hydroxy Cu(III) species at higher $-$log (H$\sp{+}$) values is responsible for the decrease in the rates, as both of these species are more stable than the parent complex. The rate of decomposition increases almost 7.5 orders of magnitude as the reduction potential of the respective Cu(III/II) couple increases from 0.37 V to 0.92 V (vs. NHE). Complexes with alanine in the third position decompose faster than those with glycine in the third position, contrary to previous observations that increased $\alpha$-carbon substitution leads to greater kinetic stability. The observed behavior is explained by a mechanism that invokes a ligand-centered radical. The Pulsed-Accelerated-Flow (PAF) method is a kinetic technique of great utility. Still in development, several discoveries have been made. These include the proper amount of backpressure to prevent cavitation, a method to obtain accurate static absorbance measurements, the utility of deceleration profiles, and guidelines to follow for the study of parallel and series reactions. The oxidation of iodide by hypochlorous acid and by trichloramine was studied with the PAF method. The reaction of iodide and hypochlorous acid is general-acid-assisted. An intermediate species, HOClI$\sp{-}$, forms (K = 225 M$\sp{-1}$) and then decomposes with or without the assistance of a general acid. In the transition state a proton from the general acid species is transferred to HOClI$\sp{-}$ as ICl is liberated. The iodine monochloride (ICl) formed quickly reacts with excess iodide to form triiodide. The Bronsted $\alpha$ value for the reaction is 0.11. The reaction of iodide with trichloramine proceeds via the same type of association complex, NCl$\sb3$I$\sp{-}$ (K$\sb1$ = 6 $\times$ 10$\sp3$ M$\sp{-1}$). This complex subsequently decomposes (k$\sb2$ = 1.5 $\times$ 10$\sp4$ s$\sp{-1}$), forming NHCl$\sb2$. The rate of the reaction is unaffected by (H$\sp{+}$) and (Buffer) $\sb{\rm T}$.

Degree

Ph.D.

Advisors

Margerum, Purdue University.

Subject Area

Chemistry

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