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 r ate constants (where the initial half-lives are 3-9 mu s) for the reac tions of cyanide ion with OBr- and with OI- (25.0 degrees C, mu = 1.00 M). The proposed mechanism includes parallel paths with halogen-catio n transfer to CN- by solvent-assisted reaction with OX- (X = Br, I) an d 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 spectrophot ometrically by the appearance of I- at 225 nm (epsilon = 12 070 M(-1) cm(-1)). Saturation kinetics are observed with increased OH-concentrat ion. 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 f orm OCN- (k(4) = 1.34 X 10(-2) M(-1) s(-1)). The base hydrolysis of Br CN is studied by following the disappearance of the 105 amu peak with membrane introduction mass spectrometry. Rate constants for the reacti ons 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 r elative reactivities of cyanogen halides for the base hydrolysis are a s follows: ClCN >> BrCN >> ICN.