Alpha-Effect nucleophiles and azide ion: effective charge studies of displacement reactions at esters

1998 ◽  
Vol 76 (6) ◽  
pp. 678-685 ◽  
Author(s):  
Matthew J Colthurst ◽  
Angeline JSS Kanagasooriam ◽  
Michael SO Wong ◽  
Christian Contini ◽  
Andrew Williams
Keyword(s):  
Key Words ◽  
Rate Constants ◽  
Effective Charge ◽  
Primary Amine ◽  
Phenyl Ester ◽  
Bimolecular Reactions ◽  
Anionic Nucleophiles ◽  
Azide Ion ◽  
Displacement Reactions ◽  
Alpha Effect

The bimolecular reactions of azide, 2-iodosobenzoate, and acetohydroxamate ions and methoxyamine nucleophiles with substituted phenyl acetates possess Yukawa-Tsuno type correlations with substantial resonance interaction. The rate constants for hydroperoxide ion and the oxyanion of N,N-diethylhydroxylamine obey simple Hammett equations. The reactivities of azide, acetohydroxamate, hydroperoxide, and 2-iodosobenzoate ion nucleophiles exhibit modest alpha -effects when the reactivity in substitution at 4-nitrophenyl acetate is compared with that of phenoxide ions of similar pKa values. The N,N-diethylhydroxylamine oxyanion is less reactive than its phenoxide ion analogue due to steric requirements, and the reactivities of the anionic nucleophiles exhibit changes in effective charge on the leaving oxygen similar to those for regular nucleophiles of similar pKa. Methoxyamine possesses only a small enhancement compared with the reactivity of a normal primary amine of similar pKa.Key words: alpha -effect, nucleophile, Yukawa-Tsuno, Leffler, Hammett, phenyl ester, effective charge, Brønsted.

Remarkable kinetic stability of α-thiocarbamoyl substituted 4-methoxybenzylcations

1998 ◽  
Vol 76 (12) ◽  
pp. 1910-1915 ◽  
Author(s):  
Robert A McClelland ◽  
Victoria E Licence ◽  
John P Richard ◽  
Kathleen B Williams ◽  
Shrong-Shi Lin
Keyword(s):  
Key Words ◽  
Strong Interaction ◽  
Rate Constants ◽  
Flash Photolysis ◽  
Visible Absorption ◽  
Methyl Group ◽  
Thioamide Group ◽  
Azide Ion ◽  
Uv Visible ◽  
Leaving Groups

4-Methoxybenzyl cations bearing α-(N,N-dimethylcarbamoyl) and α-(N,N-dimethylthiocarbamoyl) substituents have been generated photochemically upon irradiation of precursors with pentafluorobenzoate or 4-methoxybenzoate leaving groups. The ions have been observed with flash photolysis in 40:60 acetonitrile:water and in 50:50 methanol:water, and rate constants were measured for their decay in solvent alone and for their capture by azide ion. The cations so studied and their lifetimes in 40% acetonitrile are 6, ArC+H-CONMe2, 0.6 μs; 2, ArC+H-CSNMe2, 7 ms; and 4, ArC+(CH3)-CSMe2, 6 ms, where Ar = 4-MeOC6H4. The cation 4 reacts with solvent by elimination of a proton from the α-methyl group, and the rate constant for solvent addition must be less than 1 s-1. The CSNMe2 substituted cations are 105-107-fold longer lived than analogs where the thioamide group has been replaced with an α-methyl. The UV-visible absorption spectra of these two cations also show significant differences from those of typical 4-methoxybenzyl cations. Thus, both the lifetimes and spectra point to a strong interaction of the benzylic centre with the thioamide group. Key words: flash photolysis, thiocarbamoyl stabilized carbocation, photosolvolysis.

An ICR mass spectrometry study of ion/molecule reactions between ethyl chloride and alkylamines

1991 ◽  
Vol 69 (2) ◽  
pp. 363-367
Author(s):  
Guoying Xu ◽  
Jan A. Herman
Keyword(s):  
Mass Spectrometry ◽  
Key Words ◽  
Rate Constant ◽  
Rate Constants ◽  
Ethyl Chloride ◽  
Ion Molecule Reactions ◽  
Mass Spectrometry Study ◽  
Ethyl Cation

Ion/molecule reactions in mixtures of ethyl chloride with C1–C4 alkylamines were studied by ICR mass spectrometry. Ethyl cation transfer to C1–C4 alkylamines proceeds mainly through diethylchloronium ions with rate constants ~3 × 10−10cm3 s−1. In the case of s-butylamine the corresponding rate constant is 0.5 × 10−10 cm3 s−1. Key words: ICR mass spectrometry, ion/molecule reactions, ethylchloride, methylamine, ethylamine, propylamines, butylamines

Formation and nucleophilic capture of N-nitrosiminium ions

1999 ◽  
Vol 77 (5-6) ◽  
pp. 1148-1161 ◽  
Author(s):  
Latifa Chahoua ◽  
Alain Vigroux ◽  
Yvonne Chiang ◽  
James C Fishbein
Keyword(s):  
Ionic Strength ◽  
Rate Constants ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Aqueous Media ◽  
Laser Flash ◽  
Azide Ion ◽  
Iminium Ions ◽  
Nucleophilic Trapping

A study of the solvolysis of a series of (N-nitrosomethylamino)arylmethyl esters and azides and the products of nucleophilic trapping of the corresponding N-nitrosiminium ion intermediates in aqueous media, 25°C, ionic strength 1 M is reported. Structure-reactivity data for the forward and reverse reactions have been obtained. In three cases, the rate constants for reactions of the cations with nucleophiles have been measured directly by laser flash photolysis. The data allow a comparison of the degree to which the N-methyl-N-nitroso functionality enhances cation stability from a thermodynamic and kinetic perspective. It has been possible to deduce that the carbon basicity of azide ion is less than 1 kcal/mol greater than that of acetate ion.Key words: nitrosiminium ions, α-acetoxynitrosamines, carbocations, iminium ions, nucleophilicity.

Gas-phase reactions of anions with halogenated methanes at 297 ± 2 K

1976 ◽  
Vol 54 (10) ◽  
pp. 1643-1659 ◽  
Author(s):  
K. Tanaka ◽  
G. I. Mackay ◽  
J. D. Payzant ◽  
D. K. Bohme
Keyword(s):  
Gas Phase ◽  
Rate Constants ◽  
Hydrogen Atom Transfer ◽  
Gas Phase Reactions ◽  
Phase Measurements ◽  
Displacement Reactions ◽  
Nucleophilic Reactivity ◽  
Apparent Activation Energies

The rate constants for a number of exothermic displacement (SN2) reactions of the type X− + CH3Y → Y− + CH3X where X− = H−, O−, C−, F−, S−, Cl−, OH−, C2−, CN−, SH−, S2−, C2H−, NH2−, NO2−, CHF−, CH2Cl−, CH2Br−, CH3O−, CH3S−, and CH3NH− and Y = F, Cl, and Br, have been measured in the gas phase at 297 ± 2 K using the flowing after-glow technique. These gas-phase measurements provided an opportunity to determine the intrinsic nucleophilic reactivity of 'nude' anions and hence to assess the role of solvation in the kinetics of SN2 reactions proceeding in solution. Comparisons of the experimental rate constants with rate constants calculated using classical theories of capture indicate that several displacement reactions may possess large intrinsic energies of activation, [Formula: see text]. Correlations were found between apparent activation energies and the heats of reaction. These correlations provided a convenient classification of the various anion nucleophiles. Displacement was observed to compete with proton transfer in reactions involving nucleophiles of high intrinsic basicity and with hydrogen atom transfer and H2+ transfer in the reactions of the O− radical anion.

Comparison of the nucleophilicities of alcohols and alkoxides

2005 ◽  
Vol 83 (9) ◽  
pp. 1554-1560 ◽  
Author(s):  
Thanh Binh Phan ◽  
Herbert Mayr
Keyword(s):  
Free Energy ◽  
Key Words ◽  
Rate Constants ◽  
Correlation Equation ◽  
Linear Free Energy Relationship ◽  
Hydroxide Solution ◽  
Linear Free Energy ◽  
S Parameters ◽  
Nucleophilic Reactivity ◽  
Kinetics Of

The kinetics of the reactions of benzhydrylium ions with some alcohols and alkoxides dissolved in the corresponding alcohols were photometrically investigated. Using the correlation equation log k (20 °C) = s(E + N), the N and s parameters of methoxide, ethoxide, n-propoxide, and isopropoxide in alcohol–acetonitrile (91:9, v/v) were determined. The cosolvent acetonitrile has only a little influence on the rate constants of the reactions of alcohols and alkoxides. The order of N values (OH– << MeO– < EtO– < n-PrO– < i-PrO–) shows that alkoxides differ only moderately in reactivity but are considerably more nucleophilic than hydroxide. As a consequence, the nucleophilic reactivity of a 0.5 mmol/L aqueous hydroxide solution increases by a factor of 13 when 10% (v/v) methanol is added. In 1–10 mmol/L alkoxide solutions in alcohols, weak electrophiles react considerably faster with alkoxides than with the corresponding alcohols. With increasing electrophilicity, the preference for alkoxides decreases, and electrophiles of –3 < E < 3 react with alkoxides (1–10 mmol/L) and alcohols with comparable rates. Stronger electrophiles will react with alcohols exclusively when alkoxides are present in concentrations ≤10 mmol/L. Key words: kinetics, alcohol, alkoxide, linear free energy relationship, nucleophilicity.<

Method for determination of the ratio of rate constants, secondary to primary amine, in epoxy-amine systems

Polymer ◽  
1997 ◽  
Vol 38 (12) ◽  
pp. 3117-3120 ◽  
Author(s):  
Senen Paz-Abuin ◽  
Arturo Lopez-Quintela ◽  
Montse Varela ◽  
Mercedes Pazos-Pellin ◽  
Pilar Prendes
Keyword(s):  
Rate Constants ◽  
Primary Amine ◽  
Epoxy Amine
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