The Transition State in Methyl Radical Formation. The Secondary α-Deuterium Isotope Effect

1964 ◽  
Vol 86 (18) ◽  
pp. 3836-3840 ◽  
Author(s):  
Andreas A. Zavitsas ◽  
Stanley. Seltzer
Keyword(s):  
Transition State ◽  
Isotope Effect ◽  
Radical Formation ◽  
Methyl Radical ◽  
Deuterium Isotope Effect

The transition State in Methyl Radical Formation. The α-Deuterium Effect

1964 ◽  
Vol 86 (6) ◽  
pp. 1265-1267 ◽  
Author(s):  
Andreas A. Zavitsas ◽  
Stanley. Seltzer
Keyword(s):  
Transition State ◽  
Radical Formation ◽  
Methyl Radical

Unequal primary kinetic isotope effects in the base-catalysed H–D exchange of diastereotopic protons

1980 ◽  
Vol 58 (1) ◽  
pp. 72-78 ◽  
Author(s):  
Robert R. Fraser ◽  
Philippe J. Champagne
Keyword(s):  
Transition State ◽  
Exchange Reaction ◽  
Isotope Effect ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Deuterium Isotope Effect ◽  
Kinetic Isotope

Primary kinetic isotope effects have been measured for the base-catalyzed exchange reaction of 4′,1″-dimethyl-1,2,3,4-dibenzcyclohepta-1,3-diene-6-one, 1. It was found that the isotope effects kH/kT and kD/kT for the faster exchanging protons (13.6 and 3.8 respectively) are significantly larger than the corresponding values for the slower exchanging protons (4.6 and 1.6 respectively). These differences could result from truly unequal isotope effects due to transition state differences or intrusion of a second pathway for exchange of the less reactive proton in the dedeuteration reaction. The data appear to support the latter interpretation. The secondary deuterium isotope effect was found to be 1.18.

Sterically Hindered Aromatic Compounds. VI. A Remote ε-Deuterium Kinetic Isotope Effect in the Solvolysis of Perdeutero-2,4,6-tri-tert-butylbenzyl Chloride

1975 ◽  
Vol 53 (21) ◽  
pp. 3171-3174 ◽  
Author(s):  
L. Ross C. Barclay ◽  
John R. Mercer ◽  
Peter J. MacAulay
Keyword(s):  
Transition State ◽  
Isotope Effect ◽  
Aromatic Compounds ◽  
Kinetic Isotope Effect ◽  
Inductive Effect ◽  
Steric Effects ◽  
Deuterium Isotope Effect ◽  
Tert Butyl ◽  
Kinetic Isotope ◽  
Sterically Hindered

2,4,6-Tri-tert-butylbenzyl chloride deuterated at the three tert-butyl groups was synthesized. Conductimetric solvolysis studies of the normal and perdeutero-2,4,6-tri-tert-butylbenzyl chloride at 30.06 °C in 80% ethanol–water provides evidence for an inverse remote ε-deuterium isotope effect, kH/kD = 0.873−0.874. Under the same conditions the α-deuterium isotope effect was kH/kαD = 1.166 per deuterium, indicative of limiting solvolytic behavior. The remote ε-deuterium isotope effect for the perdeutero compound is discussed in terms of the inductive effect of deuterium and steric effects on the transition state conformation.

Charge Development and Mechanism in Disrotatory Ring Opening of Cyclopropyl Bromides

1974 ◽  
Vol 52 (14) ◽  
pp. 2660-2665 ◽  
Author(s):  
Jan Han Ong ◽  
Ross Elmore Robertson
Keyword(s):  
Transition State ◽  
Temperature Dependence ◽  
Temperature Coefficient ◽  
Isotope Effect ◽  
Ring Opening ◽  
Deuterium Isotope Effect ◽  
Enthalpy Of Activation ◽  
Cis And Trans ◽  
Hydrolysis Of

The temperature dependence of the rates of hydrolysis of cis- and trans-2-vinylcyclopropyl bromides has been determined in water. The temperature coefficient of the enthalpy of activation (ΔCp≠) for both compounds was unusual (−27 and −35 cal mol−1 deg−1). From this fact, it was concluded that the charge development at the transition state was low, in agreement with the conclusions of Clark and Smale (19). The slightly inverse α-deuterium isotope effect (kH/kD = 0.994) is consistent with that conclusion.

The effect of steric crowding on an E2 transition state

1976 ◽  
Vol 54 (14) ◽  
pp. 2339-2341 ◽  
Author(s):  
George Stanley Dyson ◽  
Peter James Smith
Keyword(s):  
Transition State ◽  
Isotope Effect ◽  
Nitrogen Isotope ◽  
Bond Rupture ◽  
Deuterium Isotope Effect ◽  
Steric Crowding ◽  
Reaction Proceeds ◽  
Hydrogen Deuterium ◽  
Nitrogen Bond ◽  
Mechanism Of The Reaction

The mechanism of the reaction of 9-(4-substituted benzyl)fluorene-9-trimethylammonium ions with ethoxide is a normal E2 process. The magnitude of the primary hydrogen–deuterium isotope effect at 60 °C increased with increasing electron-donating ability of the 4-substituent, i.e., 4.15, 5.10, 5.34, 5.65, 5.75, and 5.91 for the 4-CF3, 4-Br, 4-Cl, 4-H, 4-CH3, and 4-OCH3 substituents, respectively. The magnitude of the nitrogen isotope effect at 70 °C decreased with increased electron-donating power of the 4-substituent, i.e., [(k14/k15)–1]100 = 1.24, 0.95, 0.92, 0.91, and 0.80 for the 4-CF3, 4-F, 4-H, 4-CH3, and 4-OCH3 substituents, respectively. A small Hammett ρ value of +1.33 was observed for the reaction. It is concluded that the reaction proceeds via a transition state where the proton is more than one-half transferred to base. It is further concluded that for a reaction in which the 4-substituents decrease the rate, both carbon–hydrogen and carbon–nitrogen bond rupture is more advanced in the transition state. This variance with Hammond's postulate is discussed in the light of steric crowding at the transition state.

A Primary Hydrogen-Deuterium Isotope Effect Study on the Carbonyl Elimination Reaction of 9-Fluorenyl Nitrate with Various Bases

1975 ◽  
Vol 53 (2) ◽  
pp. 263-268 ◽  
Author(s):  
Peter James Smith ◽  
Lorraine Marion Noble
Keyword(s):  
Transition State ◽  
Isotope Effect ◽  
Isotope Effects ◽  
Effect Study ◽  
Elimination Reaction ◽  
Deuterium Isotope Effect ◽  
Nitrogen Bases ◽  
Transition State Geometry ◽  
Hydrogen Deuterium ◽  
Deuterium Isotope Effects

The carbonyl elimination reaction of 9-fluorenyl nitrate with various nitrogen bases in anhydrous ethanol at 0 °C was examined. In all cases fluorenone was formed in 100% yield indicating that there was not any substitution. A reasonable Brønsted plot was obtained for reaction promoted by structurally similar bases with β = 0.84 which suggests a product-like transition state. As well, deviations from the Brønsted plot are discussed. Primary hydrogen-deuterium isotope effects were measured for reaction promoted by 11 different amine bases. A reasonable correlation was obtained for structurally similar bases when a plot of kH/kDvs. pKa was made. The conclusion is reached that when kH/kD reaches a maximum, ∼9.2 at 0 °C, it remains unchanged and hence is a poor measure of transition state geometry. As well, very poor correlations are found when the abstracting base is tertiary which leads to the conclusion that a comparison of kH/kD values is not warranted for structurally different bases.

The effect of the leaving group on the nature of the E2 transition state for reaction of 2-aryl-1-phenylethylammonium ions with ethoxide in ethanol

1981 ◽  
Vol 59 (20) ◽  
pp. 3016-3018 ◽  
Author(s):  
Shune-Long Wu ◽  
Peter James Smith
Keyword(s):  
Transition State ◽  
Isotope Effect ◽  
Deuterium Isotope Effect ◽  
Significant Dependence ◽  
Leaving Group ◽  
Rate Of Reaction ◽  
Hydrogen Deuterium ◽  
Leaving Groups

The reaction of 2-aryl-1-phenylethylammonium salts with ethoxide in ethanol at 40 °C for five different amine leaving groups has been investigated. A significant dependence of the rate of reaction on the basicity of the leaving group was found. The variation of the primary hydrogen–deuterium isotope effect and pKa of the leaving group was not linear and it is concluded that the proton is less than one-half transferred to base at the transition state for reactions involving the best two leaving groups. In support of this conclusion is the observed opposite variation of kH/kD with ring substituents for the various leaving groups, i.e., for N-methylpyrrolidine, kH/kD = 4.58, 4.72, and 5.01 for p-Me, H, and p-Cl, respectively; for N-methylmorpholine, kH/kD = 6.13, 5.73, and 5.50 for p-Me, H, and p-Cl, respectively.

scholarly journals Reactions of l-ergothioneine and some other aminothiones with 2,2′- and 4,4′-dipyridyl disulphides and of l-ergothioneine with iodoacetamide. 2-Mercaptoimidazoles, 2- and 4-thiopyridones, thiourea and thioacetamide as highly reactive neutral sulphur nucleophiles

1974 ◽  
Vol 139 (1) ◽  
pp. 221-235 ◽  
Author(s):  
Jan Carlsson ◽  
Marek P. J. Kierstan ◽  
Keith Brocklehurst
Keyword(s):  
Transition State ◽  
Isotope Effect ◽  
Rate Constant ◽  
Order Rate Constant ◽  
Base Catalysis ◽  
The Other ◽  
Deuterium Isotope Effect ◽  
General Base ◽  
Nucleophilic Reactivity ◽  
Ph Range

1. The reactions of 2,2′- and 4,4′-dipyridyl disulphide (2-Py–S–S–2-Py and 4-Py–S–S–4-Py) with l-ergothioneine (2-mercapto-l-histidine betaine), 2-mercaptoimidazole, 1-methyl-2-mercaptoimidazole, thiourea, thioacetamide, 2-thiopyridone (Py–2-SH) and 4-thiopyridone (Py–4-SH) were investigated spectrophotometrically in the pH range approx. 1–9. 2. These reactions involve two sequential reversible thiol–disulphide interchanges. 3. The reaction of l-ergothioneine with 2-Py–S–S–2-Py and/or with the l-ergothioneine–Py–2-SH mixed disulphide, both of which provide Py–2-SH, is characterized by at least three reactive protonic states. This provides definitive evidence that neutral l-ergothioneine is a reactive nucleophile, particularly towards the highly electrophilic protonated disulphides. 4. A similar situation appears to obtain in the reactions of l-ergothioneine and Py–2-SH with 4-Py–S–S–4-Py and in the reactions of the other 2-mercaptoimidazoles, thiourea and Py–4-SH with 2-Py–S–S–2-Py. The nucleophilic reactivity of Py–4-SH suggests that general base catalysis provided by the disulphide in a cyclic or quasi-cyclic transition state is not necessary to generate nucleophilic reactivity in the other amino-thiones whose geometry could permit such catalysis. 5. The existence of a positive deuterium isotope effect in the l-ergothioneine–2-Py–S–S–2-Py system at pH6–7 provides no evidence for general base catalysis but is in accord with a mechanism involving specific acid catalysis and post-transition-state proton transfer. 6. The pH-dependences of the overall equilibrium positions of the various thiol–disulphide interchanges are described. 7. Reaction of thioacetamide with a stoicheiometric quantity of 2-Py–S–S–2-Py at pH1 provides 2 molecules of Py–2-SH per molecule of thioacetamide and elemental sulphur; these findings can be accounted for by thiol–disulphide interchange to provide a thioacetamide–Py–2-SH mixed disulphide followed by fragmentation to provide CH3CN, S and Py–2-SH. 8. Provision of high reactivity in the neutral forms of the members of this series of sulphur nucleophiles by electron donation by the amino group is compared with the well known α effect that provides enhanced nucleophilicity in compounds containing an electronegative atom adjacent to the nucleophilic atom. 9. The decrease in the u.v. absorption of l-ergothioneine at 257nm consequent on transformation of its aminothione moiety into an S-alkyl-2-mercaptoimidazole moiety provides a convenient method of following the alkylation of l-ergothioneine by iodoacetamide. 10. The pH dependence of the extinction coefficient of l-ergothioneine at 257nm is described by ε257={8×103/(1+Ka/[H+]} +6×103m−1·cm−1 in which pKa=10.8. 11. In the pH range 3–11 the reaction is characterized by two reactive protonic states (X and XH). 12. The X state, reaction of the ionized 2-mercaptoimidazole moiety of the l-ergothioneine dianion with neutral iodoacetamide, is characterized by the second-order rate constant 4.0m−1·s−1 (25.0°C, I=0.05). The XH state, characterized by the second-order rate constant 0.03m−1·s−1, is interpreted as reaction of the thione form of the neutral 2-mercaptoimidazole moiety of the l-ergothioneine monoanion with neutral iodoacetamide. 13. The XH state of the alkylation reaction does not exhibit a deuterium isotope effect.

Deuterium isotope effect on the orientation dependence of cyanogen(B) radical formation in the reaction of argon(3P) with acetonitrile-d3

1991 ◽  
Vol 95 (21) ◽  
pp. 8159-8162 ◽  
Author(s):  
Dock Chil Che ◽  
Toshio Kasai ◽  
Hiroshi Ohoyama ◽  
Kazuhiko Ohashi ◽  
Tohru Fukawa ◽  
...  
Keyword(s):  
Isotope Effect ◽  
Orientation Dependence ◽  
Radical Formation ◽  
Deuterium Isotope Effect
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