The Nature and Transmission of Substituent Electrical Effects in Alicyclic Solvolysis

1999 ◽  
Vol 64 (11) ◽  
pp. 1849-1876 ◽  
Author(s):  
Marvin Charton
Keyword(s):  
Transition State ◽  
Rate Constants ◽  
Field Effect ◽  
Resonance Effect ◽  
Steric Effects ◽  
Reaction Site ◽  
Reliable Indication ◽  
Leaving Group ◽  
Electrical Effect ◽  
Substituted 1

The correlation of rate constants taken from the literature for 2-exo- and 2-endo-norbornyl derivatives substituted in positions 1, 4, 5-exo, 6-exo, 6-endo, and 7-anti with the triparametric LDR equation and relationships derived from it has been carried out. A delocalized (resonance) electrical effect has been found in all but the 4-substituted derivatives. The resonance effect may be somewhat greater when the leaving group is exo than when it is endo. Steric effects occur in the 6-endo-substituted derivatives. The transition state resembles a classical carbocation rather than the nonclassical norbornyl carbocation. The correlation of solvolysis rate constants for 2- and 3-substituted 1- and 4-exo-substituted 2-exo-adamantyl derivatives gave no reliable indication that a delocalized electrical effect existed in these systems. A study of solvolysis rate constants for 4-substituted 1-[2.2.2]bicyclooctanyl, 6-exo-substituted [2.2.2]bicyclooctan-2-exo-yl and 6-exo-substituted [2.2.2]bicyclo- octan-2-endo-yl derivatives showed a delocalized electrical effect only in the latter of these systems. Such an effect was also observed in 4-substituted 2-chloro-2-methylbutane solvolyses. The transmission of electrical effects is by a modified field effect. It is dependent on 1/n where n is the number of bonds intervening between substituent and reaction site.

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.

Kinetics of reactions of para-substituted phenyl isocyanates with amines and alcohols

1991 ◽  
Vol 56 (8) ◽  
pp. 1662-1670 ◽  
Author(s):  
Ivan Danihel ◽  
Falk Barnikol ◽  
Pavol Kristian
Keyword(s):  
Transition State ◽  
Solvent Effects ◽  
Rate Constants ◽  
Substituent Effects ◽  
Steric Effects ◽  
Stopped Flow ◽  
Partial Charges ◽  
Hammett Correlation ◽  
One Step ◽  
Kinetics Of

The reaction of para-substituted phenyl isocyanates with amines and alcohols was studied by stopped-flow method. The Hammett correlation obtained showed that the sensitivity of the above mentioned reactions toward substituent effects is the same as that of analogous reactions of phenyl isothiocyanates (ρ ~ 2). The rate constants of these reactions were found to be affected more by steric effects than by solvent effects. An one step multicentre mechanism with partial charges in transition state has been proposed for the title reactions.

Pyrolysis of aryl azides. III. Steric and electronic effects upon reaction rate

1975 ◽  
Vol 28 (10) ◽  
pp. 2147 ◽  
Author(s):  
LK Dyall
Keyword(s):  
Transition State ◽  
Rate Constants ◽  
Reaction Rate ◽  
Steric Effects ◽  
Electronic Effects ◽  
Aryl Azides ◽  
Phenyl Azide ◽  
Methyl Group ◽  
First Order ◽  
Ortho Substituent

First-order rate constants have been measured for the pyrolysis of 15 phenyl azides in decalin solution. The rate for phenyl azide is increased only slightly by all para and many ortho substituents; in these cases Eact and ΔSact values are related linearly. ��� The very large rate increases when the ortho substituent is phenylazo, nitro, acetyl or benzoyl cannot be from steric or normal electronic effects and therefore identify a specific involvement of these groups in the transition state. This rate enhancement is reduced to scarcely significant levels by a 6-chloro or 6-methyl group in 2- nitrophenyl azide, but not by a 6-nitro group. These results raise doubts about recent claims1 to establish mechanism by measuring polar effects on rates of pyrolysis of azides in which steric effects might also operate.

THE APPLICATION OF THE HAMMETT EQUATION TO ORTHO-SUBSTITUTED BENZENE REACTION SERIES

1960 ◽  
Vol 38 (12) ◽  
pp. 2493-2499 ◽  
Author(s):  
Marvin Charton
Keyword(s):  
Benzene Ring ◽  
Substituent Effects ◽  
Para Position ◽  
Steric Effects ◽  
Side Chain ◽  
Ortho Position ◽  
Reaction Site ◽  
Hammett Equation ◽  
Reaction Series ◽  
Electrical Effect

The Hammett equation is directly applicable to ortho-substituted benzene reaction series in which reaction site and benzene ring are separated by some group Z, apparently due to the absence of steric effects in these series. The σp values are used in the correlations. Fourteen ortho-substituted benzene reaction series have been correlated. The electrical effect of a substituent in the ortho position is found to be about 0.75 times its effect in the para position. The effect of the side-chain Z in the transmission of substituent effects is OCH2 > SCH2 > CH = CH > SeCH2 > CH2CH2.

ELECTRONIC EFFECTS IN E2 REACTIONS: II. t-BUTOXIDE-INDUCED ELIMINATIONS OF 2-METHYL-3-PENTYL ARENESULFONATES

1965 ◽  
Vol 43 (5) ◽  
pp. 1282-1292 ◽  
Author(s):  
Allan K. Colter ◽  
Donald R. McKelvey
Keyword(s):  
Transition State ◽  
Dimethyl Sulfoxide ◽  
Vapor Phase ◽  
Rate Constants ◽  
Equilibrium Mixture ◽  
Second Order ◽  
Electronic Effects ◽  
Electronic Nature ◽  
Leaving Group ◽  
Electron Withdrawal

As part of a study of the influence of electronic effects imposed by the leaving group on orientation in E2 reactions, a study of elimination in a series of 2-methyl-3-pentyl arenesulfonates (I) has been carried out. The compositions of the olefin mixtures resulting from reaction of selected members of this series with potassium t-butoxide in t-butanol (1a, Id), 50% (by volume) t-butanol – dioxane (Ia–Ig), 25% (by volume) t-butanol – dioxane (Id–If), and 25% (by volume) t-butanol – dimethyl sulfoxide (Ia, Id, If, Ig), at 50 °C, have been determined using vapor phase chromatography. Second-order rate constants were measured for these reactions in t-butanol (Ie), and 50% t-butanol – dioxane (Ia, Ic–Ig) at 50 °C and in 25% t-butanol – dimethyl sulfoxide (Ia, Id, Ie) at 25 °C.The compositions of the olefin mixtures vary in a fairly regular way with changes in the electronic nature of the leaving group, the fraction of 2-methyl-2-pentene and ratio of trans-to cis-4-methyl-2-pentene for the most part increasing with increasing electron withdrawal. The E2 reactions in 25% t-butanol – dimethyl sulfoxide produce trans- and cis-4-methyl-2-pentene in ratios (20 to 35) considerably higher than those in an equilibrium mixture (ca. 6).The results are discussed in terms of recent views on the E2 transition state.

Hydroboration of acyclic allenes with disiamylborane

1969 ◽  
Vol 47 (6) ◽  
pp. 1083-1086 ◽  
Author(s):  
D. S. Sethi ◽  
G. C. Joshi ◽  
D. Devaprabhakara
Keyword(s):  
Carbon Atom ◽  
Transition State ◽  
Steric Effects ◽  
Electrophilic Attack ◽  
Terminal Carbon Atom ◽  
Central Carbon Atom ◽  
Central Carbon

The present investigation demonstrates the hydroboration of 1,2-nonadiene, phenylpropadiene, 3-phenyl-1,2-butadiene, 4,5-nonadiene, and tetramethylallene with disiamylborane. All the allenes except tetramethylallene underwent 100% conversion. Examination of the products indicated preferential electrophilic attack of boron on the least substituted terminal carbon atom in the case of 1,2-nonadiene, phenylpropadiene, 3-phenyl-1,2-butadiene, and on the central carbon atom in 4,5-nonadiene. In tetramethylallene boron, attack was exclusively on the central carbon atom. These results have been explained in terms of steric effects on a four-centered transition state.

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