Use of Fluorine Kinetic Isotope Effects in the Study of Steric Effects in Nucleophilic Aromatic Substitution Reactions

1998 ◽  
Vol 63 (25) ◽  
pp. 9348-9350 ◽  
Author(s):  
Jonas Persson ◽  
Olle Matsson
Keyword(s):  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Nucleophilic Aromatic Substitution ◽  
Steric Effects ◽  
Aromatic Substitution ◽  
Substitution Reactions ◽  
Kinetic Isotope

Isotope effects in nucleophilic substitution reactions. V. The mechanism of the decomposition of 1-phenylethyldimethylphenylammonium halides in chloroform

1986 ◽  
Vol 64 (6) ◽  
pp. 1206-1214 ◽  
Author(s):  
Helen Alma Joly ◽  
Kenneth Charles Westaway
Keyword(s):  
Ground State ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Steric Effects ◽  
Halide Ions ◽  
Substitution Reactions ◽  
Kinetic Isotope ◽  
Hydrogen Deuterium ◽  
Primary Substrate ◽  
Triple Ion

Secondary α and β hydrogen–deuterium kinetic isotope effects have been used together to show that the SN reaction between 1-phenylethyldimethylphenylammonium ion and bromide or iodide ion in chloroform occurs by way of an SN2 mechanism within a triple ion in spite of the fact that it reacts faster than the primary substrate, benzyldimethylphenylammonium bromide. The very loose transition state and steric effects in the ground state appear to be responsible for the unusually fast SN2 reactions between 1-phenylethyldimethylphenylammonium ion and halide ions in chloroform.

Steric Effects on the Primary Isotope Dependence of Secondary Kinetic Isotope Effects in Hydride Transfer Reactions in Solution: Caused by the Isotopically Different Tunneling Ready State Conformations?

2015 ◽  
Vol 137 (20) ◽  
pp. 6653-6661 ◽  
Author(s):  
Binita Maharjan ◽  
Mahdi Raghibi Boroujeni ◽  
Jonathan Lefton ◽  
Ormacinda R. White ◽  
Mortezaali Razzaghi ◽  
...  
Keyword(s):  
Isotope Effects ◽  
Hydride Transfer ◽  
Kinetic Isotope Effects ◽  
Transfer Reactions ◽  
Steric Effects ◽  
Kinetic Isotope ◽  
Hydride Transfer Reactions

AROMATIC SUBSTITUTION: PART VIII. SOME ASPECTS OF THE MECHANISM OF THE TSCHITSCHIBABIN REACTION

1965 ◽  
Vol 43 (4) ◽  
pp. 725-731 ◽  
Author(s):  
R. A. Abramovitch ◽  
F. Helmer ◽  
J. G. Saha
Keyword(s):  
Molecular Orbital ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Aromatic Substitution ◽  
Direct Amination ◽  
Detailed Mechanism ◽  
Methyl Group ◽  
Kinetic Isotope ◽  
Rule Out

The mechanism of the direct amination of pyridines is discussed in terms of both addition–elimination and elimination–addition pathways. The absence of deuterium kinetic isotope effects as well as the orientations observed in such reactions rule out the intervention of 2,3- or 2,6-dehydropyridines. The effect of a 3-methyl group upon the reactivities towards the amide ion of the various nuclear positions in pyridine has been determined. Predictions based on molecular orbital (m.o.) calculations are discussed in terms of the detailed mechanism of this reaction and of the orientations observed.

Isotope effects in nucleophilic substitution reactions. VII. The effect of ion pairing on the substituent effects on SN2 transition state structure

1989 ◽  
Vol 67 (1) ◽  
pp. 21-26 ◽  
Author(s):  
Zhu-Gen Lai ◽  
Kenneth Charles Westaway
Keyword(s):  
Nucleophilic Substitution ◽  
Isotope Effects ◽  
Transition States ◽  
Substituent Effects ◽  
Ion Pairing ◽  
Kinetic Isotope Effects ◽  
Ion Pair ◽  
Substitution Reactions ◽  
Kinetic Isotope ◽  
Free Ion

The secondary α-deuterium kinetic isotope effects and substituent effect found in the SN2 reactions between a series of para-substituted sodium thiophenoxides and benzyldimethylphenylammonium ion are significantly larger when the reacting nucleophile is a free ion than when it is a solvent-separated ion pair complex. Tighter transition states are found when a poorer nucleophile is used in both the free ion and ion pair reactions. Also, the transition states for all but one substituent are tighter for the reactions with the solvent-separated ion pair complex than with the free ion. Hammett ρ values found by changing the substituent on the nucleophile do not appear to be useful for determining the length of the sulphur–α-carbon bond in the ion pair and free ion transition states. Keywords: Isotope effects, ion pairing, nucleophilic substitution, SN2 reactions, transition states.

Isotope effects in nucleophilic substitution reactions. III. The effect of changing the leaving group on transition state structure in SN2 reactions

1979 ◽  
Vol 57 (11) ◽  
pp. 1354-1367 ◽  
Author(s):  
Kenneth Charles Westaway ◽  
Syed Fasahat Ali
Keyword(s):  
Transition State ◽  
Nucleophilic Substitution ◽  
Isotope Effects ◽  
Heavy Atom ◽  
Substituent Effects ◽  
Kinetic Isotope Effects ◽  
Substitution Reactions ◽  
Nucleophilic Substitution Reactions ◽  
Kinetic Isotope ◽  
Leaving Group

The nucleophilic substitution reactions of a series of 4-substituted phenylbenzyldimethyl-ammonium ions with thiophenoxide ions at 0 °C in N,N-dimethylformamide have been used to demonstrate how a change in the leaving group alters the structure of the SN2 transition state. Heavy atom (nitrogen) kinetic isotope effects, secondary α-deuterium kinetic isotope effects and Hammett ρ values provide qualitative descriptions of both the nucleophile–α-carbon and α-carbon–leaving group bonds in the transition states of these reactions. The results indicate that changing to a better leaving group causes the bond between the α-carbon and the nucleophile to be much more fully formed while the bond to the leaving group is essentially unchanged. The results are discussed in the light of current theories of substituent effects on SN2 reactions and a possible explanation for the surprising results (i) that the greatest effect is in the bond more remote from the point of structural change and (ii) that more nucleophilic assistance is required to displace a better leaving group is given.

Isotope effects in nucleophilic substitution reactions. IV. The effect of changing a substituent at the α carbon on the structure of SN2 transition states

1982 ◽  
Vol 60 (19) ◽  
pp. 2500-2520 ◽  
Author(s):  
Kenneth Charles Westaway ◽  
Zbigniew Waszczylo
Keyword(s):  
Nucleophilic Substitution ◽  
Isotope Effects ◽  
Transition States ◽  
Activation Parameters ◽  
Kinetic Studies ◽  
Kinetic Isotope Effects ◽  
Substitution Reactions ◽  
Nucleophilic Substitution Reactions ◽  
Kinetic Isotope ◽  
Benzyl Chlorides

Kinetic studies, secondary α-deuterium kinetic isotope effects, primary chlorine kinetic isotope effects (1), Hammett ρ values determined by changing the substituent in the nucleophile, and activation parameters have been used to determine the detailed (relative) structures of the transition states for the SN2 reactions between para-substituted benzyl chlorides and thiophenoxide ion. A rationale for the U-shaped Hammett ρ plots observed when para-substituted benzyl compounds react with negatively charged nucleophiles is also presented.

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