Nucleophilic Catalysis by HPO42- in the Hydrolysis of Cr2O72-. Formation and Decay of HO3POCrO32-

1994 ◽  
Vol 47 (12) ◽  
pp. 2283
Author(s):  
NE Brasch ◽  
DA Buckingham ◽  
CR Clark
Keyword(s):  
Coordination Number ◽  
Phosphate Buffer ◽  
Initial Reaction ◽  
Nucleophilic Catalysis ◽  
Stepwise Mechanism ◽  
General Base ◽  
Chromium Vi ◽  
Rapid Formation ◽  
Hydrolysis Of ◽  
Spontaneous Reaction

The initial reaction of Cr2O72- in phosphate buffer (pH 6.03-8.54, 25.0°C, I = 1.0 M NaClO4) follows the rate law kobs = kK [HPO42]/(1+K[HPO42-]). This is interpreted as arising from the reversible and rapid formation of a chromium(VI)- phosphato intermediate of increased coordination number (K = 5.5�1.3M-1), and rate-determining loss of CrO42- from this species (k = 4.4�0.5 s-1) to give HO3POCrO32- ( pKc a= 6.96). This appears to be the first clear demonstration of an addition-elimination (stepwise) mechanism for substitution at chromium (VI). Subsequent equilibration of HO3POCrO32- to give HCrO4- and H2PO4- (K = 5.95�1.90 M-1) is seen as a separate process which is subject to specific H+ and OH-, and general base (HPO42-) catalysis, in addition to a spontaneous reaction.

The aminolysis of p-nitrophenyl acetate by aminopyridines. Mechanisms in aqueous and aprotic solvents

1980 ◽  
Vol 33 (11) ◽  
pp. 2441 ◽  
Author(s):  
LW Deady ◽  
WL Finlayson
Keyword(s):  
Acetic Anhydride ◽  
Dimethyl Sulfoxide ◽  
Amino Nitrogen ◽  
Base Catalysis ◽  
Aprotic Solvents ◽  
Nucleophilic Catalysis ◽  
General Base ◽  
Amide Formation ◽  
Ring Nitrogen ◽  
Hydrolysis Of

In dimethyl sulfoxide, the aminolysis of p-nitrophenyl acetate by aminopyridines results in amide formation, through nucleophilic catalysis by the ring nitrogen for 4-aminopyridine, but by direct amino nitrogen attack for 2-aminopyridine (as previously found for acetic anhydride). In water, the aminopyridines catalyse the hydrolysis of the ester (unlike aniline, which still gives acetanilide). In general, this occurs by nucleophilic catalysis by the ring nitrogen. Even 4-amino-2- methylpyridine reacts by this route (though 2-picoline does not) and, of the compounds studied, only for 2-amino- 6-methylpyridine does general base catalysis occur instead. Reasons for these mechanism changes are discussed.

scholarly journals Renal sodium-potassium adenosine triphosphatase. Optical localization and x-ray microanalysis.

1975 ◽  
Vol 23 (11) ◽  
pp. 828-839 ◽  
Author(s):  
R Beeuwkes ◽  
S Rosen
Keyword(s):  
Atpase Activity ◽  
Electron Probe ◽  
Sequential Analysis ◽  
Relative Sensitivity ◽  
Initial Reaction ◽  
Initial Product ◽  
Sodium Potassium ◽  
Nitrophenyl Phosphate ◽  
Hydrolysis Of ◽  
Convoluted Tubules

The distribution of sodium-potassium adenosine triposphatase (Na-K-ATPase) activity in kidney sections has been studied by a method based on the hydrolysis of p-nitrophenyl phosphate in alkaline medium containing dimethyl sulfoxide. The products at each stage in the reaction sequence have been subjected to electron probe microanalysis. The initial product was identified as a mixture of KMgPO4 and Mg(PO4)2, and sequential analysis demonstrated the linearity of conversion of this product to a visible form. In human, rabbit and rat kidneys the distribution of activity was found to be essentially identical, with highest levels located in thick ascending limbs and distal convoluted tubules. The initial reaction was completely potassium dependent and was inhibited by ouabain in concentrations reflecting the relative sensitivity of microsomal Na-K-ATPase in each species. Measurement of initial product phosphorus by means of the electron probe is presented as a practical technique for direct quantitation of Na-K-ATPase activity in identified tubule segments.

Micellar catalysis of organic reactions. VIII. Kinetic studies of the hydrolysis of 2-acetyloxybenzoic acid (aspirin) in the presence of micelles

1982 ◽  
Vol 35 (7) ◽  
pp. 1357 ◽  
Author(s):  
TJ Broxton
Keyword(s):  
Aqueous Solution ◽  
Cetyltrimethylammonium Bromide ◽  
Cetylpyridinium Chloride ◽  
Kinetic Studies ◽  
Base Catalysis ◽  
Plateau Region ◽  
General Base ◽  
Mechanism Of Hydrolysis ◽  
Ph Range ◽  
Hydrolysis Of

The hydrolysis of 2-acetyloxybenzoic acid in the pH range 6-12 has been studied in the presence of micelles of cetyltrimethylammonium bromide (ctab) and cetylpyridinium chloride (cpc). In the plateau region (pH 6-8) the hydrolysis is inhibited by the presence of micelles, while in the region where the normal BAC2 hydrolysis (pH > 9) occurs the reaction is catalysed by micelles of ctab and cpc. The mechanism of hydrolysis in the plateau region is shown to involve general base catalysis by the adjacent ionized carboxy group both in the presence and absence of micelles. This reaction is inhibited in the presence of micelles because the substrate molecules are solubilized into the micelle and water is less available in this environment than in normal aqueous solution.

Effect of Medium on Reactivity for Alkaline Hydrolysis of p-Nitrophenyl Acetate and S-p-Nitrophenyl Thioacetate in DMSO-H2O Mixtures of Varying Compositions: Ground State and Transition State Contributions

2021 ◽  
Author(s):  
Ik-Hwan Um ◽  
Seungjae Kim
Keyword(s):  
Transition State ◽  
Alkaline Hydrolysis ◽  
Ground State ◽  
Rate Constants ◽  
Kinetic Data ◽  
Reaction Medium ◽  
Stepwise Mechanism ◽  
Rate Determining Step ◽  
Leaving Group ◽  
Hydrolysis Of

Second-order rate constants (kN) for reactions of p-nitrophenyl acetate (1) and S-p-nitrophenyl thioacetate (2) with OH‒ have been measured spectrophotometrically in DMSO-H2O mixtures of varying compositions at 25.0 ± 0.1 oC. The kN value increases from 11.6 to 32,800 M‒1s‒1 for the reactions of 1 and from 5.90 to 190,000 M‒1s‒1 for those of 2 as the reaction medium changes from H2O to 80 mol % DMSO, indicating that the effect of medium on reactivity is more remarkable for the reactions of 2 than for those of 1. Although 2 possesses a better leaving group than 1, the former is less reactive than the latter by a factor of 2 in H2O. This implies that expulsion of the leaving group is not advanced in the rate-determining transition state (TS), i.e., the reactions of 1 and 2 with OH‒ proceed through a stepwise mechanism, in which expulsion of the leaving group from the addition intermediate occurs after the rate-determining step (RDS). Addition of DMSO to H2O would destabilize OH‒ through electronic repulsion between the anion and the negative-dipole end in DMSO. However, destabilization of OH‒ in the ground state (GS) is not solely responsible for the remarkably enhanced reactivity upon addition of DMSO to the medium. The effect of medium on reactivity has been dissected into the GS and TS contributions through combination of the kinetic data with the transfer enthalpies (ΔΔHtr) from H2O to DMSO-H2O mixtures for OH‒ ion.

scholarly journals A study of the alkaline hydrolysis of fractionated reticulocyte ribosomal ribonucleic acid and its relevance to secondary structure

1968 ◽  
Vol 106 (3) ◽  
pp. 733-741 ◽  
Author(s):  
R A Cox ◽  
Hannah J. Gould ◽  
K Kanagalingam
Keyword(s):  
Secondary Structure ◽  
Phosphate Buffer ◽  
Potassium Hydroxide ◽  
Chain Scission ◽  
Average Chain Length ◽  
Guanidinium Chloride ◽  
Stable Species ◽  
Hairpin Loops ◽  
Ribosomal Ribonucleic Acid ◽  
Hydrolysis Of

1. RNA isolated from the sub-units of rabbit reticulocyte ribosomes was hydrolysed by 0·4n-potassium hydroxide at 20°. The probability of main-chain scission was calculated from the number-average chain length, which was obtained from S25,w in 0·01m-phosphate buffer. 2. The fraction, f, of the original secondary structure that the fragments re-formed at neutral pH in 4m-guanidinium chloride, as well as in 0·01m- and 0·1m-phosphate buffer, was derived from changes in extinction over the range 220–310mμ on thermal denaturation. 3. The secondary structure of RNA is regarded as an assembly of hairpin loops each of 2N+b residues on average, where N is the number of base-paired residues and b is the number of unpaired residues. 4. If chain scission takes place at random then 2N+b=logf/log(1–p). 5. For RNA from the smaller sub-unit 2N+b was estimated as 25±5 residues, compared with 30±5 residues for the less stable species and 35±5 residues for the more stable species of hairpin loop of RNA from the larger sub-unit.

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