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.

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.

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.

Kinetics of hydrolyses of o-methylbenzylideneaniline and o-hydroxybenzylideneaniline

1968 ◽  
Vol 46 (9) ◽  
pp. 1589-1592 ◽  
Author(s):  
Alfred V. Willi ◽  
José F. Siman
Keyword(s):  
Hydrogen Bond ◽  
Schiff Base ◽  
Rate Constants ◽  
Experimental Error ◽  
Steric Effect ◽  
Hydrolysis Rate ◽  
Aqueous Methanol ◽  
Buffer Solutions ◽  
Acid Catalyzed ◽  
Kinetics Of

Rates of hydrolysis have been measured for o-methylbenzylideneaniline, o-hydroxybenzylideneaniline, and benzylideneaniline in various buffer solutions in 20% (by volume) aqueous methanol at 29.9 °C. Rate constants for the o-CH3 compound and the unsubstituted Schiff base agree within experimental error which indicates that there is no appreciable rate retarding steric effect. The o-OH group decreases the hydrolysis rate at pH = 5.6 – 6.6 by approximately one power of ten. This effect is caused by the hydrogen bond between the OH group and the azomethine N, which renders the Schiff base less accessible to acid-catalyzed hydrolysis.

ortho-Effect on the acid-catalyzed hydration of 2-substituted α-methylstyrenes

2009 ◽  
Vol 74 (1) ◽  
pp. 85-99 ◽  
Author(s):  
Ondřej Prusek ◽  
Filip Bureš ◽  
Oldřich Pytela
Keyword(s):  
Sulfuric Acid ◽  
Benzene Ring ◽  
Rate Constants ◽  
Reaction Centre ◽  
Acidity Function ◽  
Isopropyl Group ◽  
Kinetic Acidity ◽  
Reaction Constant ◽  
Acid Catalyzed ◽  
Catalytic Rate

α-Methylstyrene and nine ortho-substituted analogs have been synthesized and the kinetics of their acid-catalyzed hydration in aqueous solutions of sulfuric acid at 25 °C have been investigated. The kinetic acidity function HS has been constructed from the dependence of the observed rate constants kobs on the sulfuric acid concentration. The catalytic rate constants of the acid-catalyzed hydration kortho have been calculated as well. The identical shape of the kinetic acidity functions for ortho- and para-derivatives confirms what the consistent mechanism A-SE2 of the acid-catalyzed hydration has already proved for the corresponding para-derivatives. The A-SE2 mechanism involves a rate-determining proton transfer of the hydrated proton to the substrate. From the dependence of the catalytic rate constants of the ortho-derivatives on the catalytic rate constants of the para-derivatives, it is seen that the logarithm of the catalytic rate constant for hydrogen as a substituent is markedly out of the range of the other substituents and, simultaneously, that the ortho-derivatives react significantly slower than the corresponding para-derivatives. In correlation with the substitent constants σp+, a reaction constant of ρ+ = –1.45 have been found. The constant is, in absolute value, considerably smaller than that for para-derivatives (ρ+ = –3.07). In parallel, the steric effects are enforced more significantly for the monoatomic substituents (slope of the Charton’s constants 3.92) than for substituents including more atoms (slope of the Charton’s constants 2.09). A small value of the reaction constant ρ+ has been elucidated due to the lower conjugation between the reaction centre and the benzene ring as a consequence of the geometric twist of the reaction centre out of the main aromatic plane accompanied by fading mesomeric interaction between the reaction centre and the substituents attached to the benzene ring. The isopropyl group in the carbocation is twisted less out of the aromatic plane for the monoatomic substituents and, therefore, also a small difference in the bulk of substituents has considerable steric influence on the conjugation between the carbocation and the benzene ring bearing substituents. On the contrary, the isopropyl group in the carbocations with polyatomic substituents is twisted to such a degree that changes in the bulk of substituents affect the resonant stabilization negligibly. Similar conclusions were also deduced from the correlations of the substitution constants σI and σR+.

General acid-catalyzed addition of methanol to (E)-N-benzylideneanilines

1999 ◽  
Vol 77 (5-6) ◽  
pp. 760-773 ◽  
Author(s):  
Sadjia Bennour ◽  
Jean Toullec
Keyword(s):  
Carboxylic Acid ◽  
Transition State ◽  
Rate Constants ◽  
Bond Cleavage ◽  
Substituent Effects ◽  
Bond Forming ◽  
General Acid ◽  
Acid Catalyzed ◽  
Catalytic Rate ◽  
Solvent Molecules

The reaction of equilibrium addition of methanol (α-amino ether formation) to benzylideneanilines (C6H5=NC6H4Y, with Y = H (1a), 3-Cl (1b), 3-NO2 (1c), 4-CN (1d), and 4-NO2(1e)) in methanol is shown to be general acid-catalyzed in carboxylic acid buffers. The mechanism involves fast iminium ion formation followed by base-assisted addition of methanol. The α Brønsted exponents are in the 0.67-0.88 range, and α increases with the electron-withdrawing ability of Y. The same mechanism is valid for MeOH2+-catalysis, meaning that two solvent molecules are involved in the addition process, one of them playing the role of base. The equilibrium constant, K, is increased by electron-withdrawing substituents, log K depending linearly on the σ- substituent parameters. The substituent effects on the forward and reverse catalytic rate constants are analyzed by means of the log k = ρnσn + ρr(σ- - σn) + constant (Young-Jencks) equation. For carboxylic acid catalysis, the ρn and ρr parameters are in keeping with ca. half C—O bond forming or breaking at the transition state. The catalytic rate constants and α exponent for elimination of ClCH2CH2OH in methanol from the C6H5CH(OCH2CH2Cl)NH(4-CNC6H4) chloroethyl adduct are compared with those for the elimination of methanol from C6H5CH(OCH3)NH(4-CNC6H4). The chloromethyl group makes the reaction slower and α lower. This indicates that proton transfer is a little ahead of C—O bond cleavage at the transition state. Y substituent effects, α values, and the effects of the CH2Cl group are interpreted on the basis of a More O'Ferrall - Jencks diagram.Key words: imine, free energy linear relationship, nucleophilic addition, More O'Ferrall - Jencks diagram, Schiff base

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.

Syntheses, rate constants, and x-ray structures of alkylrhodoximes with .sigma.-donating alkyl groups methyl, ethyl isopropyl. A comparison with the analogous alkylcobaloximes, a vitamin B12 model

1990 ◽  
Vol 29 (18) ◽  
pp. 3437-3441 ◽  
Author(s):  
Nevenka Bresciani Pahor ◽  
Renata Dreos-Garlatti ◽  
Silvano Geremia ◽  
Lucio Randaccio ◽  
Giovanni Tauzher ◽  
...  
Keyword(s):  
Vitamin B12 ◽  
Rate Constants ◽  
Methyl Ethyl ◽  
X Ray ◽  
Alkyl Groups

Détermination cinétique de la nature du conformère réactif dans l'hydrolyse des orthoesters

1983 ◽  
Vol 61 (12) ◽  
pp. 2643-2650 ◽  
Author(s):  
Gérard Lamaty ◽  
Claude Moreau ◽  
Zéphirin Mouloungui
Keyword(s):  
Rate Constants ◽  
Rate Determining Step ◽  
Catalytic Rate ◽  
Hydrolysis Of

The rates of hydrolysis of the following orthoesters, 6 or 7-substituted 1,1-diethoxy 3,4-dihydrobenzo-2-pyrans (R = 6-MeO, H, 7-MeO, 7-NO2 and 6-NO2) were determined at 27 °C in water/dioxan (2:1 by volume). The catalytic rate constants as well as the Hammett ρ constants show that the rate-determining step is the formation of a cyclic carboxonium ion intermediate and constitute a kinetic piece of evidence for the determination of the nature of the reactive conformer in hydrolysis of 2,2-dialkoxy tetrahydropyran orthoesters.

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.

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.

Sign in / Sign up

Export Citation Format

Share Document