Substitution of acyclic sugar acetals. Is the acid-catalyzed substitution of erythro and threo l,2,3,4,5-penta-O-acetyl-l-arabinose S-ethyl monothioacetals anchimerically assisted?

1967 ◽  
Vol 45 (13) ◽  
pp. 1475-1483 ◽  
Author(s):  
Edgar Page Painter ◽  
Norman H. Kurihara
Keyword(s):  
Rate Constants ◽  
Acetoxy Group ◽  
Rate Determining Step ◽  
Acid Catalyzed

The rate constants for the inversion and for the 14C-acetoxy exchange of the diastereomeric 1,2,3,4,5-penta-O-acetyl S-ethyl monothioacetals show that the substitution is not anchimerically assisted by the C2-acetoxy group. The substitution of acyclic poly-O-acetyl sugar monoacetals is best explained by the formation of the acyclic intermediate [Formula: see text] (X = S or O) in the rate-determining step.

SUBSTITUTION OF ACYCLIC SUGAR ACETALS: RATE OF PERCHLORIC ACID CATALYZED ACETOLYSIS OF 2,3,4,5,6-PENTA-O-ACETYL-D-GLUCOSE DIETHYL DITHIOACETAL AND 1,2,3,4,5,6-HEXA-O-ACETYL-D-GLUCOSE S-ETHYL MONOTHIOACETAL

1966 ◽  
Vol 44 (15) ◽  
pp. 1773-1782 ◽  
Author(s):  
Norman H. Kurihara ◽  
Edgar Page Painter
Keyword(s):  
Acetic Acid ◽  
Acetic Anhydride ◽  
Perchloric Acid ◽  
Rate Constants ◽  
Acetoxy Group ◽  
Rate Determining Step ◽  
Conjugate Acid ◽  
Acid Catalyzed ◽  
Sugar Derivatives

The rate constants for the perchloric acid catalyzed substitution of one ethylthio group of 2,3,4,5,6-penta-O-acetyl-D-glucose diethyl dithioacetal (Va) and the acetoxy group bonded to C1 of 1,2,3,4,5,6-hexa-O-acetyl-D-glucose S-ethyl monothioacetal (VIa) have been measured when the substrates were dissolved in solutions of acetic acid and acetic anhydride. The rate-determining step is interpreted to be the dissociation of the substrate conjugate acid to give a carbonium–sulfonium cation. The rate constants indicate that acyclic sugar derivatives are substituted faster than cyclic (pyranose) derivatives, and that an acetoxy group is substituted faster than an ethylthio group.

Kinetics and Mechanism of Acid Catalyzed Decomposition of 1-Phenyl-3,3-dialkyltriazenes

1994 ◽  
Vol 59 (2) ◽  
pp. 401-411 ◽  
Author(s):  
Miroslav Ludwig ◽  
Pavla Valášková ◽  
Oldřich Pytela
Keyword(s):  
Rate Constants ◽  
Steric Effect ◽  
Aqueous Ethanol ◽  
Pivalic Acid ◽  
Decomposition Kinetics ◽  
Methyl Ethyl ◽  
Rate Determining Step ◽  
Alkyl Groups ◽  
Acid Catalyzed ◽  
Catalytic Rate

Five model 1-phenyl-3,3-dialkyltriazenes (methyl, ethyl, 2-propyl, butyl, cyclohexyl) have been synthesized and their acid-catalyzed decomposition kinetics have been investigated spectrophotometrically in aqueous ethanol (40 vol.%) with pivalic acid as the catalyst. The results show that the rate-determining step is catalyzed by the proton. The decrease in the observed rate constant at higher concentrations of pivalic acid is explained by the formation of an unreactive complex of the nondissociated acid and respective triazene. The steric effect of alkyl groups on the catalytic rate constants is discussed.

Hydrolysis of trioxaadamantane ortho esters. II. Kinetic analysis and the nature of the rate-determining step

1984 ◽  
Vol 62 (6) ◽  
pp. 1074-1080 ◽  
Author(s):  
Robert A. McClelland ◽  
Patrick W. K. Lam
Keyword(s):  
Rate Constants ◽  
Product Formation ◽  
Low Ph ◽  
Acid Solutions ◽  
Ortho Ester ◽  
Rate Determining Step ◽  
Acid Catalyzed ◽  
Cation Hydration ◽  
Ortho Esters ◽  
Hydrolysis Of

A detailed kinetic study of the hydrolysis of a series of 3-aryl-2,4,10-trioxaadamantanes is reported. These ortho esters equilibrate with the ring-opened dialkoxycarbocation, in a very rapid process which could be studied using temperature-jump spectroscopy for aryl = 2,4-dimethylphenyl. Relaxation rate constants are of the order of 104 s−1; these could be analyzed to provide the rate constants for both the ring opening and the ring closing. Product formation from this equilibrating mixture is much slower. In acid solutions (0.01 M H+ −50% H2SO4), first-order rate constants for product formation initially increase with increasing acidity, but a maximum is reached at 20–35% H2SO4 and the rate then falls. This behavior is explained by a counterbalancing of two factors. Increasing acidity increases the amount of the dialkoxycarbocation in the initial equilibrium, but, outside the pH region, it decreases the rate of hydrolysis of this cation through a medium effect. Rate constants over a range of pH have been measured for two trioxaadamantanes and for the cation DEt+ derived by treatment of the ortho ester with triethyloxonium tetraafluoroborate. The latter models the cation formed in the ortho ester hydrolysis but it cannot ring close. Rate–pH profiles obtained in these systems are more complex than expected on the basis of rate-determining cation hydration. An interpretation is proposed with a change in rate-determining step between high pH and low pH. Cation hydration is rate determining at high pH but at low pH hemiorthoester decomposition becomes rate determining. Under these conditions the hemiorthoester equilibrates with both the dialkoxycarbocation and with the trioxaadamantane. The change in rate-determining step occurs because acid-catalyzed reversion of the hemiorthoester to dialkoxycarbocation is a faster process than acid-catalyzed hemiorthoester decomposition. This makes the latter rate-determining in acid solutions. Additional pathways available to the decomposition, however, make it the faster process at higher pH. A kinetic analysis furnishes all of the rate and equilibrium constants for the system, and provides support for the mechanistic interpretation. A comparison of these numbers with those obtained for the three stages in the hydrolysis of a simple monocyclic ortho ester underlines the novelty of the trioxaadamantane system.

Solvolysis kinetics and mechanism of substituted 3-acetyl-1,3-diphenyltriazenes in acid medium; Kinetic acidity function

1987 ◽  
Vol 52 (9) ◽  
pp. 2212-2216
Author(s):  
Oldřich Pytela ◽  
Martin Kaska ◽  
Miroslav Ludwig ◽  
Miroslav Večeřa
Keyword(s):  
Acid Medium ◽  
Sulphuric Acid ◽  
Rate Constants ◽  
Decomposition Kinetics ◽  
Acidity Function ◽  
Hammett Equation ◽  
Kinetic Acidity ◽  
Reaction Constant ◽  
Acid Catalyzed ◽  
Catalytic Rate

The decomposition kinetics has been measured of fourteen 3-acetyl-1,3-bis(subst. phenyl)triazenes in 40% (v/v) ethanol and sulphuric acid. The kinetic acidity function and catalytic rate constants have been determined from the rate constants observed. Mechanism has been suggested for the general acid-catalyzed solvolysis from comparison of the course of the kinetic acidity function and H0 function and from the reaction constant of the Hammett equation.

A kinetic standard for precise calibration of spectrophotometer cell temperature.

1981 ◽  
Vol 27 (5) ◽  
pp. 753-755 ◽  
Author(s):  
P A Adams ◽  
M C Berman
Keyword(s):  
Temperature Dependence ◽  
Rate Constants ◽  
Cell Temperature ◽  
First Order ◽  
Order Rate ◽  
Acid Catalyzed ◽  
Pseudo First Order ◽  
Hydrolysis Of

Abstract We describe a simple, highly reproducible kinetic technique for precisely measuring temperature in spectrophotometric systems having reaction cells that are inaccessible to conventional temperature probes. The method is based on the temperature dependence of pseudo-first-order rate constants for the acid-catalyzed hydrolysis of N-o-tolyl-D-glucosylamine. Temperatures of reaction cuvette contents are measured with a precision of +/- 0.05 degrees C (1 SD).

POWER RATE LAW BASED CHEMICAL KINETICS AND THERMODYNAMIC MODELING OF AFRICAN PEAR SEED OIL CONSECUTIVE IRREVERSIBLE BASE METHANOLYSIS FOR BIODIESEL PRODUCTION

2021 ◽  
Vol 36 (1) ◽  
pp. 53-66
Author(s):  
C. Esonye ◽  
O. D Onukwuli ◽  
S. O. Momoh
Keyword(s):  
Rate Constants ◽  
Seed Oil ◽  
Scale Up ◽  
Cetane Number ◽  
Calorific Value ◽  
Biodiesel Production ◽  
Rate Determining Step ◽  
Increase With Increase ◽  
African Pear ◽  
Kinetics And Thermodynamic

Currently the major challenge of biodiesel application as a replacement to petrodiesel is its industrial production sustainability.Consequently, the successful scale-up of laboratory results in transesterification requires so much information obtained through chemical kinetics.This paper presents the kinetics and thermodynamic study of alkali-homogeneous irreversible methanolysis of seed oil derived from African pear. The transesterification process was carried out from 0-100 minutes at temperature range of 55-65°C. The reaction mixture compositions were ascertained using gas chromatography- flame ionization detector (GC-FID) technique. Rate constants of the triglyceride (Tg), diglycerides (Dg) and monoglycerides(Mg) hydrolysis were in the range of 0.0140- 0.07810 wt%/min and increased with increase in temperature. The rate of reaction was found to increase with increase in temperature. Activation energies were found to be 6.14, 20.01 and 28.5kcal/mol at 55, 60 and 65oC respectively. Tg hydrolysis to Dg was observed asthe rate determining step while the reaction agreed with second order principles. A biodiesel yield of 93.02% was obtained with cloud point of 10°C , flash point of 125°C , pour point of 4°C , calorific value of 34.4MJ/kg, and cetane number of 54.90 which satisfy EN14214 and ASTM D 6751 standards. Results presented in this report would serve as idealized conditions for industrial scale up of biodiesel production from African pear seed oil. Keywords:Kinetics; methanolysis; rate constants; activation energy; African pear seed oil; biodiesel

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.

The decomposition of aliphatic N-nitro amines in aqueous sulfuric acid. Bisulfate as a nucleophile

1996 ◽  
Vol 74 (10) ◽  
pp. 1774-1778 ◽  
Author(s):  
Robin A. Cox
Keyword(s):  
Sulfuric Acid ◽  
Water Molecule ◽  
Rate Constants ◽  
Activation Parameters ◽  
Standard State ◽  
Aqueous Sulfuric Acid ◽  
Alkyl Groups ◽  
Different Temperatures ◽  
State Rate ◽  
Acid Catalyzed

In aqueous sulfuric acid, aliphatic N-nitro amines decompose to N2O and alcohols. An excess acidity analysis of the observed rate constants for the reaction shows that free carbocations are not formed. The reaction is an acid-catalyzed SN2 displacement from the protonated aci-nitro tautomer, the nucleophile being a water molecule at acidities below 82–85% H2SO4, and a bisulfate ion at higher acidities. Bisulfate is the poorer nucleophile by a factor of about 1000. Twelve compounds were studied, of which results obtained for nine at several different temperatures enabled calculation of activation parameters for both nucleophiles. The reaction appears to be mainly enthalpy controlled. The intercept standard-state rate constants are well correlated by the σ* values for the alkyl groups; the slopes are negative, with a more negative value for the slower bisulfate reaction. Interestingly the m≠m* slopes also correlate with σ*, although the scatter is bad. Key words: N-nitro amines, excess acidity, bisulfate, nucleophiles, acid-catalyzed, kinetics.

Hydrogen exchange and isomerization in ortho substituted benzamides. On the question of free rotation in an N-protonated amide

1979 ◽  
Vol 57 (22) ◽  
pp. 2896-2901 ◽  
Author(s):  
Robert A. McClelland ◽  
William F. Reynolds
Keyword(s):  
Exchange Reaction ◽  
Rate Constants ◽  
Hydrogen Exchange ◽  
Isomerization Reaction ◽  
Free Rotation ◽  
Bond Rotation ◽  
Diffusion Limited ◽  
Deprotonation Reaction ◽  
Substituted Benzamides ◽  
Acid Catalyzed

Rate constants have been obtained for the acid-catalyzed N–H exchange of N-methyl, 2,N-dimethyl, and 2,4,6,N-tetramethylbenzamide and the acid-catalyzed isomerization of the three corresponding N,N-dimethylbenzamides. The ratio [Formula: see text] increases significantly with increased number of ortho methyl substituents. This is explained in terms of a suggestion of Perrin, that C—N bond rotation is not completely free in the N-protonated amide, since it must compete with a diffusion limited deprotonation reaction. The isomerization reaction, which requires such a rotation, is therefore slowed by ortho methyl substituents which hinder rotation, relative to the exchange reaction, which does not require rotation.

ChemInform Abstract: An ab initio Calculation of the Acid-Catalyzed Hydrolysis of N-Nitrosoamines. A Hypothesis on the Rate-Determining Step.

ChemInform ◽  
1987 ◽  
Vol 18 (28) ◽  
Author(s):  
NGUYEN MINH THO NGUYEN MINH THO ◽  
A. F. HEGARTY
Keyword(s):  
Ab Initio ◽  
Ab Initio Calculation ◽  
Rate Determining Step ◽  
Acid Catalyzed ◽  
Hydrolysis Of
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