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Kinetics and mechanism of electron transfer to pyridinium chlorochromate (VI) from sulfur containing amino acid, L-cysteine in aqueous and micellar media

Senapati S., Das S., Mohanty P., Patnaik A.K.

Polish Journal of Chemical Technology

2011

Vol. 13, nr 2

6-10

The electron transfer reaction of L-cysteine (RSH) with pyridinium chlorochromate (PCC) has been studied spectrophotometrically over the range 2.0 ≤ 10^3 [RSH] ≤ 6.0; 0.01 ≤ [H+] ≤ 0.2; 298 ≤ T ≤ 318 K and I = 0.3 mol dm-3 (NaClO4). The electron transfer reaction has also been carried out in the presence of anionic, cationic and neutral micelle. The reaction in acid medium is strongly catalyzed by changing [SDS]T (sodium dodecyl sulfate) up to 3 × 10-2 mol dm-3, beyond this concentration of SDS, the rate is retarded. The cationic and neutral micelle has a small effect on the rate. ΔH≠ (kJ mol-1) and ΔS≠ (JK-1 mol-1) values for the k1 and k2 paths are 30.20 ± 0.25, -159.65 ± 0.83 and 29.60 ± 0.62, -127.09 ± 2.17, respectively. The negative activation entropy is indicative of the ordered transition state for the electron transfer reaction. Formation of 2-amino-3-(2-amino-2-carboxy-ethyl) disulfanyl-propanoic acid as product is strongly supported by IR spectra.

Kinetics and Mechanism of Base Hydrolysis of Some (Aminomonocarboxylato)(tetraethylenepentamine) Cobalt(III) Complexes in Aqueous Medium

Das S., Patnaik A. K.

Polish Journal of Chemistry

2009

Vol. 83, nr 3

455-466

Base hydrolysis of some (aminomonocarboxylato)(tetraethylenepentamine)Cobalt(III) complexes were studied in aqueous medium. The pseudo-first order rate constant fitted the relationship. kobs = kOH[OH-]T at 0.005 [OH-] mol dm-3 0.10 at 293.15 K-313.15 K, (I = 0.1 mol dm-3), where kOH denotes the second order base hydrolysis rate constant. The alfa beta R isomer of the alfa-alaninato complex reacted approximately 8 times faster than its alfa beta S analogue at 298.15 K. Also the reactivity difference kOH(pyridine- 2-carboxy lato)/kOH(beta-alaninato) asymptotic to 3 for the (alfa beta S) isomer is indicative of the relative electron displacement property of the pyridine ring and the aliphatic chain attached to the bound carboxylate function which influences partly the Co-O bond strength. The Dcb mechanism is valid for all these substrates. The ionic strength dependence of kOH in the range 0.01 Less-than or equal to I, mol dm-3 Less-than or equal to 0.1 for glycinato and beta-alaninato complex also confirmed the relationship log kOH = log kOH 0 + S1[I1/2/(1 + I1/2) + CI], where C is an adjustible parameter S1 = 2bZAZB, with b = 0.509 at 298.15 K, ZA and ZB denote charges of the reactant species. The acid catalyzed aquation indicated that alfa beta R-[(tetren)CoO2C-(CH3)NH3]3+ aquates via spontaneous (ko) and acid catalyzed (kaq) path. However, these reactions are extremely slow (ko = (1.74š0.17)×10-5 s-1, kaq = (2.72 + 0.05)×10-5 dm3 mol-1 s-1 at 333.15 K and I = 1.0 mol dm-3). Key words: kinetics mechanism, base hydrolysis, stopped flow, (aminomono carboxylato)(tetraethylenepentamine)cobalt(III)