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+.

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.

Synthesis and cyclization kinetics and mechanism of 1-(2-ethoxycarbonylphenyl)-3-aryltriazenes. Kinetic acidity function of sodium methoxide in methanol

1990 ◽  
Vol 55 (10) ◽  
pp. 2468-2474 ◽  
Author(s):  
Oldřich Pytela ◽  
Vladimír Dlouhý
Keyword(s):  
Rate Constants ◽  
Sodium Methoxide ◽  
Substituent Effects ◽  
Acidity Function ◽  
Subsequent Reaction ◽  
Limiting Step ◽  
Kinetic Acidity ◽  
Substituent Constants ◽  
Catalytic Rate ◽  
Cyclization Kinetics

Eight 1-(2-ethoxycarbonylphenyl)-3-aryltriazenes have been synthetized and the rate constants of their sodium-methoxide-catalyzed cyclization have been measured in methanol at 25 °C. The experimental rate constants kobs have been adopted to construct the kinetic acidity function HKM which has been shown to be identical with the -log[CH3O-] values. Two mathematical procedures have been used to determine the catalytic rate constants and their dependence on the Hammett substituent constants. A closer dependence is obtained with the σ values than with the σp- values. The ρ value found (0.3) indicates a compensation of the substituent effects upon the dissociation of the starting triazene and upon the subsequent reaction of the conjugated base. Out of the two mechanistic alternatives - E1cB and BAc2 - the latter appears to be more probable, the splitting of tetrahedral intermediate being its limiting step.

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.

Kinetic acidity function and solvolysis of 3-hydroxy-1,3-diphenyltriazenes

1986 ◽  
Vol 51 (3) ◽  
pp. 564-572 ◽  
Author(s):  
Oldřich Pytela ◽  
Stanislava Štumrová ◽  
Miroslav Ludwig ◽  
Miroslav Večeřa
Keyword(s):  
Sulphuric Acid ◽  
Substituent Effects ◽  
Acidity Function ◽  
Kinetic Acidity ◽  
Acid Catalyzed ◽  
Kinetics Of

Ten 3-hydroxy-1-(X-phenyl)-3-phenyltriazines have been synthesized, and kinetics of their solvolysis have been measured in 40% (v/v) ethanol and sulphuric acid. The concept of kinetic acidity function has been generalized, its construction has been suggested, and the procedure has been applied to the solvolysis of 3-hydroxy-1,3-diphenyltriazenes. The kinetic acidity function found has been confronted with the H0 acidity function. The substituent effects have been evaluated with respect to mechanism of the acid catalyzed solvolysis.

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.

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

Acid-catalyzed enolization of acetophenone: catalysis by bisulfate ion in sulfuric acid solutions

1986 ◽  
Vol 64 (6) ◽  
pp. 1224-1227 ◽  
Author(s):  
J. R. Keeffe ◽  
A. J. Kresge ◽  
J. Toullec
Keyword(s):  
Aqueous Solution ◽  
Sulfuric Acid ◽  
Acidity Constant ◽  
Acidity Function ◽  
Acid Solutions ◽  
Dilute Aqueous Solution ◽  
Acid Catalyzed ◽  
The Difference ◽  
Carbon Acid ◽  
Sulfuric Acid Solutions

Rates of acid-catalyzed enolization of acetophenone in dilute aqueous solution, measured under conditions where the solvated proton is the only acidic species present, give a hydrogen ion catalytic coefficient, [Formula: see text], that is 35% smaller than the value obtained by X acidity function extrapolation of measurements made in moderately concentrated sulfuric acid solutions. The difference may be attributed to catalysis by bisulfate ion in the sulfuric acid solutions; this is supported by direct measurement of the bisulfate ion catalytic coefficient in dilute sulfuric acid. This revised value of [Formula: see text] leads to new, but only slightly different, values of the keto–enol equilibrium constant for acetophenone in aqueous solution, pKE = 7.96 ± 0.04, the acidity constant for acetophenone ionizing as a carbon acid, [Formula: see text] and the encounter-controlled rate constant for the reaction of acetophenone enol with molecular bromine, k = (3.2 ± 0.4) × 109 M−1 s−1.

Sign in / Sign up

Export Citation Format

Share Document