Kinetics of hypobromite and hypoiodite reactions with cyanide and base hydrolysis of cyanogen bromide and cyanogen iodide

Cynthia M Gerritsen, Purdue University

Abstract

Pulsed-accelerated-flow (PAF) spectroscopy is used in the ultraviolet region to measure second-order rate constants with initial half-lives as small as 3.1 $\mu$s for the reactions of excess cyanide with OBr$\sp-$ and OI$\sp-$ (25.0$\sp\circ$C, $\mu$ = 1.00 M). The proposed mechanism includes parallel paths with halogen$\sp+$-transfer to CN$\sp-$ via solvent-assisted reaction with OX$\sp-$ (X = Br or I) as well (UNFORMATTED TABLE OR EQUATION FOLLOWS)$$\eqalign{\rm OX\sp- + CN\sp- + H\sb2O\ &\rm{\buildrel{k\sb{OX}}\over\longrightarrow}\ XCN + 2OH\sp-\cr \rm OX\sp- + H\sb2O &\sbsp{\gets}{\to}\ \rm HOX + OH\sp-\cr \rm HOX + CN\sp-\ &\rm{\buildrel{k\sb{HOX}}\over\longrightarrow}\ XCN + OH\sp-\cr}$$(TABLE/EQUATION ENDS)as reactions that approach diffusion-controlled limits between HOX and CN$\sp-$. The relative reactivities of the hypohalites with CN$\sp-$ (k$\sb{\rm ox}$) are: $\rm OI\sp-\ (6 \times 10\sp7\ M\sp{-1}\ s\sp{-1}) \approx OBr\sp-\ (5.7 \times 10\sp7\ M\sp{-1}\ s\sp{-1}) \gg OCl\sp-\ (310\ M\sp{-1}\ s\sp{-1}).$ The hypohalous acids are more reactive than the corresponding hypohalites; the reactivity of HOBr is greater than that of OBr$\sp-$ (by a factor of 73 for CN$\sp-$). The increased reactivity of HOBr over OBr$\sp-$ is associated with the greater ease of the Br$\sp+$-transfer accompanied by the rapid loss of OH$\sp-$. The relative rates for hypohalous acids reactions with CN$\sp-$ are: $\rm HOBr\ (4.2 \times 10\sp9\ M\sp{-1}\ s\sp{-1}) > HOCl\ (1.22 \times 10\sp9\ M\sp{-1}\ s\sp{-1}).$ The base hydrolysis of ICN was studied spectrophotometrically by the appearance of I$\sp-$ at 225 nm ($\epsilon$ = 12070 M$\sp{-1}$ cm$\sp{-1}$). In the ICN mechanism, saturation kinetics are observed with OH$\sp-$. This is attributed to rapid equilibration with HOICN$\sp-$ (K$\sb{\rm OH}$ = 3.2 M$\sp{-1}$), which inhibits the OH$\sp-$ reaction at carbon to form OCN$\sp-$ (k$\sb4$ = 1.34 $\times$ 10$\sp{-2}$ M$\sp{-1}$ s$\sp{-1}$). In the present work the hydroxide ion concentrations are extremely high and the proposed HOICN$\sp-$ species is relatively stable. (UNFORMATTED TABLE OR EQUATION FOLLOWS)$$\eqalign{&\rm CNI + OH\sp-\ {\buildrel{k\sb 4}\over\longrightarrow}\ HOCN + I\sp-\cr&\rm K\sb{OH}\ \downarrow\uparrow\sk{75}\downarrow rapid + OH\sp-\cr&\rm HOICN\sp-\sk{60}OCN\sp- + H\sb2O + I\sp-\cr}$$(TABLE/EQUATION ENDS)Hydroxide addition at the iodine atom to give HOICN$\sp-$ makes OH$\sp-$ attack at the carbon to give cyanate more difficult. Therefore at high (OH$\sp-$), k$\sb{\rm obsd}$ = k$\sb4$/K$\sb{\rm OH}$ and no longer depends on (OH$\sp-$). $\sp{13}$C-NMR is used to verify the formation of BrCN and ICN and also to show that the final product of the reaction of Na$\sp{13}$CN with OCl$\sp-$, OBr$\sp-$, and OI$\sp-$ in high base concentrations is OCN$\sp-$. The base hydrolysis of BrCN was studied by following the disappearance of the 105 M/Z peak with membrane introduction mass spectrometry. The pseudo-first-order rate constant for the base hydrolysis of BrCN is 1.81 $\times$ 10$\sp{-3}$ s$\sp{-1}$ (p(H$\sp+$) = 10.53, (Co$\sb3\sp{2-}\rbrack\sb{\rm T}$ = 0.2 M, $\mu$ = 1.0 M, 25$\sp\circ$C). The relative reactivities for the base hydrolysis of cyanogen halides at pH = 10.5 are in the order ClCN $>$ BrCN $>$ ICN.

Degree

Ph.D.

Advisors

Margerum, Purdue University.

Subject Area

Analytical chemistry|Chemistry

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