The kinetics of the base hydrolysis of trans-Dichloro- and trans-Dibromo-(meso-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene)cobalt(III) perchlorate

1977 ◽  
Vol 30 (8) ◽  
pp. 1841 ◽  
Author(s):  
PL Kendall ◽  
GA Lawrance
Keyword(s):  
Base Hydrolysis ◽  
Steric Effects ◽  
Buffer System ◽  
Hydrolysis Rates ◽  
Analogous Complex ◽  
Rate Retardation ◽  
In Trans ◽  
Ph Range ◽  
Kinetics Of ◽  
Hydrolysis Of

The kinetics of the successive base hydrolysis of both halogen ligands in trans-[Co(hctd)Cl2]+ and trans-[Co(hctd)Br2]+ (hctd = meso- 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene) have been investigated at 25�C in formic acid-NaOH buffers (pH range 3.0-3.7) and 2,6-lutidine-perchloric acid buffers (pH range 6.4-7.4). Hydrolysis reactions in each buffer system, consistent with release of first and second coordinated halogen respectively, follow rate expressions of the type kobs = kaq+ kOH]OH-], with base hydrolysis rates kOH? = 7.8(�0.8) x 108 dm3 mol-1 s-1 and 1.9(�0.3)x109 dm3 mol-1 s-1 respectively for first chloride and bromide hydrolysis, and kOH? = 7.3(�0.4)x104 dm3 mol-1 s-1 and 2.4(�0.2)x105 dm3 mol-1 s-1 for second chloride and bromide hydrolysis respectively. Comparison with the analogous complex trans-[Co(dtcd)Cl2]+ (dtcd = meso-5,12-dimethyl- 1,4,8,11-tetraazacyclotetradeca-4,11-diene) shows a ninefold acceleration for base hydrolysis of the first chloride in the hctd complex over the dtcd complex, but an 80-fold rate retardation for base hydrolysis of the second chloride in the hctd complex compared with the dtcd complex, interpreted in terms of steric effects arising from gem- dimethyl substitution in the hctd ligand.

Kinetics of aquation and base hydrolysis of the macrocyclic complex trans-Bromo(N-meso-5,12-dimethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene)nitro-cobalt(III) perchlorate

1976 ◽  
Vol 29 (10) ◽  
pp. 2319 ◽  
Author(s):  
GA Lawrance ◽  
RW Hay
Keyword(s):  
Activation Parameters ◽  
Macrocyclic Complex ◽  
Base Hydrolysis ◽  
Hydrolysis Kinetics ◽  
Rate Expression ◽  
Entropy Of Activation ◽  
Ph Range ◽  
Kinetics Of ◽  
Hydrolysis Of

The macrocyclic complex trans-[Co(dtcd)(NO2)Br] [ClO4] (dtcd = 5,12-dimethyl-l,4,8,1l-tetraaza-cyclotetradeca-4,ll-diene) has been prepared and its hydrolysis kinetics investigated. At 25�C and 0.1 M HN03 the aquation occurs with kaq = 6.2 x 10-3 s-1 to give the trans-Co(dtcd)(NO2)- (OH2)]2+ cation. The activation parameters at 298 K are ΔH? = 75.0 kJ mol-1 and ΔS? = -35.6 J K-1 mol-1. Hydrolysis of the bromide in the pH range 7.5-8.8 follows the rate expression kobs = kaq + kOH[OH-]. At 25�C (I = 0.1 M, NaClO4) kOH = 1.21 x 103 1. mol-1 s-1 with the activation parameters for base hydrolysis being ΔH? = 74.2 kJ mol-1 and ΔS? = +63.2 J K-1 mol-1 at 298 K. Aquation and base hydrolysis of the bromo complex at 25�C occur at rates 14 and 5 times faster respectively than those previously reported for the analogous trans-[Co(dtcd)(NO2)Cl]+ complex, the acceleration being due to a more favourable entropy of activation in each case.

Reactivity of base catalysed hydrolysis of 2-pyridinylmethylene-8-quinolinyl-Schiff base iron(II) iodide complexes: solvent effects

2013 ◽  
Vol 67 (4) ◽  
Author(s):  
Ahmad Mohamad ◽  
Mohamed Adam
Keyword(s):  
Aqueous Solution ◽  
Schiff Base ◽  
Solvent Effects ◽  
Base Hydrolysis ◽  
Iodide Ions ◽  
Ligand Structure ◽  
Solvation Parameters ◽  
Kinetics Of ◽  
Hydrolysis Of ◽  
Direct Condensation

AbstractThree ligands of 2-pyridinylmethylene-8-quinolinyl (L1), methyl-2-pyridinylmethylene-8-quinolinyl (L2), and phenyl-2-pyridinylmethylene-8-quinolinyl (L3), Schiff bases were synthesised by direct condensation of 8-aminoquinoline with 2-pyridinecarboxaldehyde, 2-acetylpyridine, or 2-benzoylpyridine. They coordinated to Fe(II) ion in a 1: 2 mole ratio followed by treatment with iodide ions affording complexes with a general formula [Fe(L)2]I2·2H2O, (L = L1, L2, or L3). Spectrophotometric evaluation of the kinetics of base catalysed hydrolysis of these complex cations was carried out with an aqueous solution of NaOH in different ratios of water/methanol binary mixtures. Kinetics of the hydrolysis followed the rate law (k 2[OH−] + k 3[OH−]2)[complex]. Reactivity trends and their rate constants were compared and discussed in terms of ligand structure and solvation parameters. The methanol ratio affects the hydrolysis as a co-solvent which was analysed into initial and transition state components. The increase in the rate constant of the base hydrolysis of Fe(II) complexes, as the ratio of methanol increases, is predominantly caused by the strong effect of the organic co-solvent on the transition states.

Open-chain nitrogen compounds. Part XV. A kinetic study of the hydrolysis of 1-aryl-3-aryloxymethyl-3-methyltriazenes and related triazenes

1992 ◽  
Vol 70 (8) ◽  
pp. 2224-2233 ◽  
Author(s):  
Keith Vaughan ◽  
Donald L. Hooper ◽  
Marcus P. Merrin
Keyword(s):  
Rate Constant ◽  
Order Rate Constant ◽  
Buffer System ◽  
Open Chain ◽  
Nucleophilic Displacement ◽  
Ether Oxygen ◽  
Rate Of Hydrolysis ◽  
Electron Withdrawing Group ◽  
Ph Range ◽  
Kinetics Of

The kinetics of hydyrolysis of a series of 1-aryl-3-aryloxymethyl-3-methyltriazenes, Ar-N=N-NMe-CH2OAr′, was studied over the pH range 2–7.5. Reactions were followed by the change in UV absorbance spectra of the triazenes. The aryloxymethyltriazenes decompose more slowly at pH 7.5 than the hydroxymethyltriazenes, Ar-N=NMe-CH2OH; the hydrolysis is favoured by the presence of an electron-withdrawing group in Ar′. A mixed isopropanol/buffer system was developed in order to improve solubility of the aryloxymethyl triazenes. Lowering the pH caused an increase in the rate of hydrolysis and under strongly acidic conditions an electron-withdrawing group in Ar′ actually slows down the reaction. A Hammett plot of the pseudo-first-order rate constant, kobs, is curved, indicating that two or more mechanisms operate simultaneously and that the contribution of each mechanism is substituent-dependent. A plot of kobs vs. [buffer] is linear; the slope of the plot affords the rate constant, kb for the buffer-catalyzed reaction for each substituent. A Hammett plot of kb vs. σ is linear with ρ = +0.55, suggesting that the buffer-catalyzed reaction involves nucleophilic displacement of the phenoxy group by the buffer anion. Further analysis afforded the specific acid-catalyzed rate constants, [Formula: see text], for each substituent; this component of the reaction has a negative ρ, consistent with a mechanism involving protonation at the ether oxygen. The postulation that specific acid catalysis is a component of the reaction mechanism was confirmed by the observation of a solvent deuterium isotope effect, 2.28 > kH/kD > 1.60. Only the p-NO2 and p-CN phenyloxymethyltriazenes showed any spontaneous decomposition.

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