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.

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. II. Secondary α-deuterium kinetic isotope effects: a criterion of mechanism?

1979 ◽  
Vol 57 (9) ◽  
pp. 1089-1097 ◽  
Author(s):  
Kenneth Charles Westaway ◽  
Syed Fasahat Ali
Keyword(s):  
Transition State ◽  
Nucleophilic Substitution ◽  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Substitution Reactions ◽  
Nucleophilic Substitution Reactions ◽  
Kinetic Isotope ◽  
Steric Crowding

A very large secondary α-deuterium kinetic isotope effect of 1.179 ± 0.007 (1.086 ± 0.003 per α-deuterium) has been observed for the SN2 reaction of thiophenoxide ion with benzyldimethylphenylammonium ion in DMF at 0°C. This large isotope effect which is far outside the range reported for SN2 reactions, is attributed to the fact that the extraordinarily large steric crowding around the Cα—H bonds in the substrate is reduced in the SN2 transition state. The structure of the transition state is shown to be consistent with this hypothesis.

Concerted Pathway to the Mechanism of the Anilinolysis of Bis(N,N-diethylamino)phosphinic Chloride in Acetonitrile

2017 ◽  
Vol 70 (1) ◽  
pp. 101 ◽  
Author(s):  
Hasi Rani Barai
Keyword(s):  
Nucleophilic Substitution ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Substituted Anilines ◽  
Substitution Reactions ◽  
Nucleophilic Substitution Reactions ◽  
Kinetic Isotope ◽  
Kd Values ◽  
Kinetics Of

The kinetics of the nucleophilic substitution reactions of bis(N,N-diethylamino)phosphinic chloride with substituted anilines (XC6H4NH2) and deuterated anilines (XC6H4ND2) are investigated in MeCN at 65.0°C. The deuterium kinetic isotope effects (DKIEs) are secondary inverse (kH/kD < 1: 0.706–0.947) and the magnitudes of the secondary inverse DKIEs (kH/kD) increase constantly as the nucleophiles are changed from weakly basic to strongly basic anilines. The magnitudes of the selectivity parameters are ρX(H) = –6.34, and βX(H) = 2.24 with substituted anilines and ρX(D) = –6.13 and βX(D) = 2.17 with deuterated anilines. A concerted SN2 mechanism involving predominant backside attack is proposed based on the kH/kD values with substituent X.

Isotope Effects in Nucleophilic Substitution Reactions. I. The Mechanism of the Reaction of Phenylbenzyldimethylammonium Ion with Thiophenoxide Ion

1975 ◽  
Vol 53 (21) ◽  
pp. 3216-3226 ◽  
Author(s):  
Kenneth Charles Westaway ◽  
Raymond Alcide Poirier
Keyword(s):  
Nucleophilic Substitution ◽  
Isotope Effects ◽  
Phenyl Group ◽  
Bond Rupture ◽  
Substitution Reactions ◽  
Nucleophilic Substitution Reactions ◽  
Carbonium Ion ◽  
Kinetic Isotope ◽  
Nitrogen Bond ◽  
Mechanism Of The Reaction

A kinetic study of the nucleophilic substitution reactions of a series of para-substituted phenylbenzyldimethylammonium nitrates with sodium thiophenoxide at 0 °C in N,N-dimethylformamide containing a large excess of sodium nitrate, has shown the reaction to be a second-order process which is first-order in both the substrate and the nucleophile. A Hammett ρ value of +2.04 for different para-substitutents on the N-phenyl group of the quaternary ammonium salt and a large nitrogen kinetic isotope effect of 1.0200 ± 0.0007 have excluded any mechanism involving a carbonium ion intermediate but are in accord with an SN2 mechanism with a substantial amount of carbon–nitrogen bond rupture in the transition state.

Isotope effects in nucleophilic substitution reactions. IX. The effect of bond strength on substituent effects on the structure of SN2 transition states

1993 ◽  
Vol 71 (12) ◽  
pp. 2084-2094 ◽  
Author(s):  
Kenneth Charles Westaway
Keyword(s):  
Bond Strength ◽  
Transition State ◽  
Isotope Effects ◽  
Transition States ◽  
Substituent Effects ◽  
Kinetic Isotope Effects ◽  
Substitution Reactions ◽  
Kinetic Isotope ◽  
Leaving Group ◽  
Made In

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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