Thermolysis of 3-methyl-3-alkyl-1,2-dioxetanes: steric effects on the activation parameters

1983 ◽  
Vol 48 (21) ◽  
pp. 3713-3716 ◽  
Author(s):  
Alfons L. Baumstark ◽  
Tambra Dunams ◽  
Luiz H. Catalani ◽  
Etelvino J. H. Bechara
Keyword(s):  
Activation Parameters ◽  
Steric Effects

Steric Effects. Internal-Rotation of 1-Aryl-8-Phenylnaphthalenes

1987 ◽  
Vol 40 (6) ◽  
pp. 1107 ◽  
Author(s):  
R Cosmo ◽  
S Sternhell
Keyword(s):  
Internal Rotation ◽  
Activation Parameters ◽  
Steric Effects

A general, albeit low-yielding, synthesis of 1-aryl-8-phenylnaphthalenes was devised and used to prepare three l-(3'-X-5'-isopropylphenyl)-8-phenylnaphthalenes (X = H, F, Br). Activation parameters for the rotation of the 3'-X-5'4sopropylphenyl group were determined by dynamic m.m.r. spectroscopy.

ChemInform Abstract: THERMOLYSIS OF 3-METHYL-3-ALKYL-1,2-DIOXETANES: STERIC EFFECTS ON THE ACTIVATION PARAMETERS

1984 ◽  
Vol 15 (11) ◽  
Author(s):  
A. L. BAUMSTARK ◽  
T. DUNAMS ◽  
L. H. CATALANI ◽  
E. J. H. BECHARA
Keyword(s):  
Activation Parameters ◽  
Steric Effects

The kinetics and mechanism of acid catalysed hydrolysis of lactams

1980 ◽  
Vol 58 (23) ◽  
pp. 2423-2432 ◽  
Author(s):  
Peter Wan ◽  
Tomasz A. Modro ◽  
Keith Yates
Keyword(s):  
Transition State ◽  
Water Activity ◽  
Activation Parameters ◽  
Steric Effects ◽  
Ring Size ◽  
Intermediate Type ◽  
Positive Rate ◽  
Activity Dependence ◽  
Ion Species ◽  
Hydrolysis Of

The acid-catalysed hydrolysis of lactams of ring size 4–7, and of N-ethylacetamide, has been studied at various temperatures in 0–70% aqueous sulfuric acids. The kinetics have been analysed by means of the r-hydration parameter treatment and the transition-state activity coefficient (TSAC) approach. Except for the β-lactam, all substrates show a strong positive rate dependence on water activity, indicating a tetrahedral intermediate type of mechanism [Formula: see text], similar to that found for simple (acyclic) amide hydrolyses; only the β-lactam shows a negative rate – water activity dependence, which is interpreted in terms of a unimolecular [Formula: see text] mechanism, involving rate-determining acylium ion formation. Similarly, all substrates except the β-lactam show a pronounced destabilization of the transition-state as acidity is increased, typical of oxonium ion species. The behaviour of the β-lactam resembles that of methyl mesitoate hydrolysis. Basicity constants for lactams of ring size 4–9 show a definite dependence on ring size, with the δ-lactam being unusually basic. Both rate and pK dependence on ring size are discussed in terms of ring strain and steric effects. The activation parameters, particularly ΔS≠, are fully consistent with the conclusions drawn concerning transition-state hydration.

Studies on the kinetics of racemization of 2,4-disubstituted-5(4H)-oxazolones

1988 ◽  
Vol 66 (10) ◽  
pp. 2540-2544 ◽  
Author(s):  
Marek Slebioda ◽  
Marc A. St-Amand ◽  
Francis M. F. Chen ◽  
N. Leo Benoiton
Keyword(s):  
Nitrogen Atom ◽  
Tertiary Amine ◽  
Thermodynamic Data ◽  
Activation Parameters ◽  
Steric Effects ◽  
Tertiary Amines ◽  
Electronic Effects ◽  
Catalyzed Reactions ◽  
Kinetics Of ◽  
Benzyl Substituent

The kinetics of racemization of 2,4-disubstituted-5(4H)-oxazolones obtained from N-acetyl, N-benzoyl, and N-benzyloxycarbonylglycyl-L-leucine, -valine, and -phenylalanine have been studied in several solvents alone and in the presence of tertiary amines. The influence of the nature of the substituents on the rate of the reaction has been evaluated, and the values of the activation parameters ΔH≠ and ΔS≠ for the base-catalyzed reactions have been determined. The racemization process is governed by electronic effects of the C(2)-substituents and steric effects of the C(4)-substituents of the oxazolones. The thermodynamic data suggest that the 2-substituted-4-benzyl-5(4H)-oxazolones racemize more readily than the 4-alkyl analogues (alkyl ≠ benzyl) because of the unique conformation assumed by the benzyl substituent. The rate of the base-catalyzed reaction depends on the extent of hindrance about the nitrogen atom of the tertiary amine as well as on its basicity. The data suggest that the relative rates of racemization of oxazolones are not a true reflection of the relative tendencies to racemize of the pertinent residues during couplings of the parent acids.

Substituent and steric effects in the oxidation of alkyl aryl sulfides by peroxydisulfate

1978 ◽  
Vol 56 (24) ◽  
pp. 3043-3046 ◽  
Author(s):  
C. Srinivasan ◽  
P. Kuthalingam ◽  
N. Arumugam
Keyword(s):  
Aqueous Ethanol ◽  
Activation Parameters ◽  
Steric Effects ◽  
Reaction Site ◽  
Electrophilic Attack ◽  
Alkyl Aryl ◽  
Kinetics Of Oxidation ◽  
Potassium Peroxydisulfate ◽  
Aryl Sulfides ◽  
Kinetics Of

The kinetics of oxidation of a number of alkyl aryl sulfides with potassium peroxydisulfate in aqueous ethanol have been investigated. The effect of substituents on the oxidation has been studied by employing a number of p- and m-substituted phenyl methyl sulfides: the reaction is accelerated by electron-releasing and retarded by electron-withdrawing substituents, indicating a rate-determining electrophilic attack by the peroxydisulfate ion at the reaction site, sulfur. A good correlation is found to exist between the rate constants and the Hammett σ+/σ− constants, the ρ+ value for the reaction being −0.56 with a correlation coefficient of 0.979. The activation parameters have also been determined.Studies with different alkyl phenyl sulfides clearly indicate that the reaction is quite sensitive to steric congestion around the reaction site. The results obtained in this study are compared with those found for the oxidation of the same set of sulfides with different oxidants.

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