14309-96-7Relevant articles and documents
A kinetic study of the reaction between noradrenaline and iron(III): An example of parallel inner- and outer-sphere electron transfer
El-Ayaan, Usama,Jameson, Reginald F.,Linert, Wolfgang
, p. 1315 - 1319 (1998)
In anaerobic acid solution noradrenaline [norepinephrine, 4-(2-amino-1-hydroxyethyl)benzene-1,2-diol, H2LH+ (in which the phenolic protons are written on the left of L)] reacts with iron(III) [in the form of Fe(OH)2+] to yield iron(II) and the semiquinone form of noradrenaline which is in turn oxidised rapidly by more iron(III) to 'noradrenoquinone'. This reaction proceeds both directly (i.e. via an 'outer-sphere' reaction) and after prior formation of the complex Fe(LH)2+ which then decomposes via intramolecular electron transfer. [The observed rate of formation of the complex (monitored at 714 nm) is faster than the rate of its decomposition by a factor of about 200.] The quinone then cyclises by an intramolecular Michael addition giving the (UV transparent) leuconoradrenochrome (indoline-3,5,6-triol), which is able to react with iron(III) at high pH to give noradrenochrome (3,5-dihydro-3,6-dihydroxy-2H-indol-5-one). At lower pH values the presence of chloride ions shows a marked effect on the rate of complex formation because the species FeCl2+ is also able to react with noradrenaline to form the complex although chloride is not involved in the reverse reaction. The stability constant for the formation of FeCl2+ (K1Cl) was found to be 35, i.e. log K1Cl = 1.54 (identical to the value obtained from previous work with dopamine). The ring-closure reaction was studied by following the rate of quinone decomposition monitored at 380 nm, and a mechanism for this cyclisation is proposed. The following rate constants have been evaluated: (i) for the reversible formation of the iron-noradrenaline complex [via Fe(OH)2+ + H2LH+] k1 = 2170 ± 20 dm3 mol-1 s-1 and k-1 = 21 ± 2 dm3 mol-1 s-1, from which the stability constant of the Fe(LH)2+ complex has been calculated (log K1M = 21.2), (ii) rate of formation of the complex via FeCl2+ + H2LH+, kCl = 48 ± 3 dm3 mol-1 s-1, (iii) rate of decomposition of the complex Fe(HLH)3+, k2 = 2.6 ± 0.1 s-1 [protonation constant for Fe(LH)2+, KMH = 34 ± 1 dm3 mol-1], (iv) rate of outer-sphere redox reaction, k2′ = 100 ± 2 dm3 mol-1 s-1, (v) rate of indole formation (ring-closure reaction), kcyc = 1400 ± 20 s-1 (for quinone) and kcycH = (2.0 ± 0.1) × 105 s-1 (for protonated quinone). All measurements were carried out at 25.0°C in solutions of ionic strength 0.10 mol dm-3 (KNO3 serving as inert electrolyte).
Spectroelectrochemical Examination of Charge Transfer between Chlorpromazine Cation Radical and Catecholamines
Mayausky, J. S.,McCreery, R. L.
, p. 308 - 312 (2007/10/02)
With chlorpromazine (CPZ) and dopamine (DA) in their reduced forms in solution, the potential of a platinum electrode was stepped to a value where both species were oxidized at diffusion controlled rates.The CPZ+. cation radical so generated then diffused away from the electrode and encountered reduced DA, which it oxidized.The rate of oxidation was determined by monitoring the absorbance due to CPZ+. or dopamine quinone (DPQ) using the glancing incidence reflection spectroelectrochemical technique.The kinetics of the process indicate that the rate law is first order in both CPZ+. and DA, implying that the first encouter of the two molecules is the rate limiting step, rather than some subsequent process.The rate constant for dopamine was compared with those for three other catechols, and the rate constant increased monotonically with driving force, as measured by the difference in redox potentials between catechol and CPZ.The advantages of the technique for studying homogeneous charge transfer reactions are noted, as is the implication of the results on the CPZ+./DA reaction on the biological effects of CPZ.
