NONMETAL REDOX KINETICS - HYPOBROMITE AND HYPOIODITE REACTIONS WITH CYANIDE AND THE HYDROLYSIS OF CYANOGEN HALIDES

Pulsed-accelerated-flow spectroscopy is used to measure second-order rate constants (where the initial half-lives are 3-9 mu s) for the reactions of cyanide ion with OBr- and with OI- (25.0 degrees C, mu = 1.00 M). The proposed mechanism includes parallel paths with halogen-cation transfer to CN- by solvent-assisted reaction with OX- (X = Br, I) and by direct reaction with HOX: [GRAPHICS] The relative reactivities of the hypohalites with CN-(k(OX)) are as follows: OI- (6 X 10(7) M(-1) s(-1)) approximate to OBr- (5.7 x 10(7) M(-1) s(-1)) >> OCl-(310 M(-1) s(-1)). The rate constants for the hypohalous acid reactions with CN-(k(HOX)) are as follows: HOBr (4.2 x 10(9) M(-1) s(-1)) > HOCl (1.22 x 10(9) M(-1) s(-1)). The base hydrolysis of ICN is studied spectrophotometrically by the appearance of I- at 225 nm (epsilon = 12 070 M(-1) cm(-1)). Saturation kinetics are observed with increased OH-concentration. This is attributed to rapid equilibration to give HOICN-(K-OH = 3.2 M(-1)), which inhibits the OH-attack at the carbon atom in ICN to form OCN- (k(4) = 1.34 X 10(-2) M(-1) s(-1)). The base hydrolysis of BrCN is studied by following the disappearance of the 105 amu peak with membrane introduction mass spectrometry. Rate constants for the reactions of BrCN with OH- (k(OH) = 0.53 +/- 0.01 M(-1) s(-1)) and with CO32- are determined (k(CO3), = (7.5 +/- 0.3) x 10(-3) M(-1) s(-1)). The relative reactivities of cyanogen halides for the base hydrolysis are as follows: ClCN >> BrCN >> ICN.