A New Insight into Using Chlorine Leaving Group and Nucleophile Carbon Kinetic Isotope Effects To Determine Substituent Effects on the Structure of SN2 Transition States

The effects of substituents on the structure of SN2 transition states suggested by kinetic isotope effects and Hammett ρ values are often different and, moreover, often do not agree with substituent effects predicted by current theories whether the change in substituent is made in the nucleophile, in the leaving group, or at the α-carbon. The importance of the strength of the reacting bonds in determining the effects of substituents on transition-state structure is investigated. A bond strength hypothesis that suggests there will be a significant change in the weaker reacting bond but little or no change in the stronger reacting bond in an SN2 transition state when a substituent in the nucleophile, the substrate, or the leaving group is altered in an SN2 reaction, predicts a high percentage of the experimental results.

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

Canadian Journal of Chemistry ◽

10.1139/v89-004 ◽

1989 ◽

Vol 67 (1) ◽

pp. 21-26 ◽

Cited By ~ 12

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.

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

Canadian Journal of Chemistry ◽

10.1139/v79-221 ◽

1979 ◽

Vol 57 (11) ◽

pp. 1354-1367 ◽

Cited By ~ 27

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.

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Kinetics and mechanism of oxidation of N,N′-dimethyl-9,9′-biacridanyl by some π acceptors and a one-electron oxidant

Canadian Journal of Chemistry ◽

10.1139/v85-073 ◽

1985 ◽

Vol 63 (2) ◽

pp. 445-451 ◽

Cited By ~ 6

Author(s):

Allan K. Colter ◽

Charles C. Lai ◽

A. Gregg Parsons ◽

N. Bruce Ramsey ◽

Gunzi Saito

Keyword(s):

Electron Transfer ◽

Reduction Potential ◽

Isotope Effects ◽

Substituent Effects ◽

Kinetic Isotope Effects ◽

Spin Trapping ◽

Single Electron Transfer ◽

Kinetic Isotope ◽

The One ◽

Rule Out

Oxidation of N,N′-dimethyl-9,9′-biacridanyl (DD) has been investigated as a model for single electron transfer (SET)-initiated oxidation of NADH coenzyme models such as N-methylacridan (DH). Oxidants investigated cover a 1010-fold range of reactivity in acetonitrile and include the π acceptors 1,4-benzoquinone (BQ), 2,6-dichloro-1,4-benzoquinone (DCIBQ), p-chloranil (CA), 2,3-dicyanobenzoquinone (DCBQ), 2,3-dicyano-1,4-naphthoquinone (DCNQ), 2,3-dicyano-5-nitro-1,4-naphthoquinone (DCNNQ), 9-dicyanomethylene-2,4,7-trinitrofluorene (DCMTNF), 9-dicyanomethylene-2,4,5,7-tetranitrofluorene (DCMTENF), 7,7,8,8-tetracyanoquinodimethane (TCNQ), and tetracyanoethylene (TCNE), and the one-electron oxidant tris(2,2′-bipyridyl)cobalt(III), [Formula: see text] The oxidation product is, in every case, N-methylacridinium ion (D+). A mechanism involving a rate-determining electron transfer with simultaneous fragmentation to D+ and N-methyl-9-acridanyl radical (D•) is proposed. This mechanism is supported by the observed dependence of the rate on oxidant reduction potential, by spin-trapping experiments, by kinetic isotope effects in oxidation of 9,9′-dideuterio-DD, and by substituent effects in oxidation of 2,2′- and 3,3′-dimethoxy-DD. The rate of oxidation of DD relative to that of DH is 3.4 × 102 with [Formula: see text] and with the π acceptors varies from ea. 0.3 (BQ) to 8.1 × 104 (DCMTENF). The results rule out a SET-initiated mechanism for oxidation of DH by all of the oxidants studied except TCNQ and DCMTENF.

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Substituent Effects on Temperature Dependence of Kinetic Isotope Effects in Hydride-Transfer Reactions of NADH/NAD+ Analogues in Solution: Reaction Center Rigidity Is the Key

Organic Letters ◽

10.1021/acs.orglett.0c02049 ◽

2020 ◽

Vol 22 (15) ◽

pp. 5963-5967

Author(s):

Peter Maness ◽

Shailendra Koirala ◽

Pratichhya Adhikari ◽

Nasim Salimraftar ◽

Yun Lu

Keyword(s):

Temperature Dependence ◽

Reaction Center ◽

Isotope Effects ◽

Substituent Effects ◽

Hydride Transfer ◽

Kinetic Isotope Effects ◽

Transfer Reactions ◽

Kinetic Isotope ◽

Solution Reaction ◽

Hydride Transfer Reactions

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Reactive mode composition factor analysis of transition states: the case of coupled electron–proton transfers

Physical Chemistry Chemical Physics ◽

10.1039/c9cp05131g ◽

2019 ◽

Vol 21 (45) ◽

pp. 24912-24918 ◽

Cited By ~ 2

Author(s):

Mauricio Maldonado-Domínguez ◽

Daniel Bím ◽

Radek Fučík ◽

Roman Čurík ◽

Martin Srnec

Keyword(s):

Factor Analysis ◽

Kinetic Energy ◽

Isotope Effects ◽

Transition States ◽

Kinetic Isotope Effects ◽

Composition Factor ◽

Kinetic Isotope ◽

Proton Transfers ◽

Relative Kinetic ◽

Reactive Mode

The kinetic energy distribution in the reactive mode in transition states correlates the asynchronicity of CPET with relative kinetic isotope effects.

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