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.

Isotope effects in nucleophilic substitution reactions XI. The effect of ion-pairing, substituents, and the solvent on SN2 transition states

1999 ◽  
Vol 77 (5-6) ◽  
pp. 879-889 ◽  
Author(s):  
Kenneth Charles Westaway ◽  
W Jiang
Keyword(s):  
Transition State ◽  
Nucleophilic Substitution ◽  
Substituent Effect ◽  
Isotope Effects ◽  
Transition States ◽  
Ion Pairing ◽  
Ion Pair ◽  
Leaving Group ◽  
Free Ion ◽  
Alpha Carbon

The secondary alpha deuterium and primary leaving group nitrogen KIEs and Hammett ρ values found for the free ion and ion-pair SN2 reactions between benzyldimethylphenylammonium ion and sodium para-substituted thiophenoxides in methanol at 20.000°C show how (i) ion-pairing of the nucleophile, (ii) a change in substituent in the nucleophile, and (iii) a change in solvent alters the structure of a Type II SN2 transition state. Ion-pairing shortens the weaker sulfur - alpha carbon (S—Cα) transition state bond significantly but does not alter the stronger alpha carbon - leaving group (Cα—N) transition state bond as the bond strength hypothesis predicts. However, the effect of ion pairing, i.e., the decrease in the S—Cα bond on ion-pairing, decreases as a more electron-withdrawing substituent is added to the nucleophile, and the S—Cα bond actually increases when the nucleophile is the p-chlorothiophenoxide ion. The identical Hammett ρ values of -0.85 and -0.84 for the free ion and ion-pair reactions, respectively, may be observed because, on average, the S—Cα bonds are identical in the free ion and ion-pair transition states. When a more electron-donating substituent is added to the nucleophile, an earlier transition state is found in both the ion-pair and free ion reactions. However, the substituent effect is smaller in the ion-pair reactions, presumably because the change in the negative charge on the sulfur atom with substituent is greater in the free ion than in the ion-pair. The substituent effect on transition state structure suggested by the KIEs is not predicted by any of the theories that are used to predict substituent effects on SN2 reactions. Both the secondary alpha deuterium and primary leaving group nitrogen KIEs and the Hammett ρ values indicate that the transition state is earlier when the solvent is changed from DMF to methanol as the "solvation rule for SN2 reactions" predicts. This probably occurs because an earlier, more ionic, transition state is more highly solvated (more stable) in methanol.Key words: nucleophilic substitution, SN2, isotope effect, transition state, substituent, ion-pair.

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.

Isotope effects in nucleophilic substitution reactions. VIII. The effect of the form of the reacting nucleophile on the transition state structure of an SN2 reaction

1991 ◽  
Vol 69 (6) ◽  
pp. 1017-1021 ◽  
Author(s):  
Yao-Ren Fang ◽  
Kenneth Charles Westaway
Keyword(s):  
Transition State ◽  
Isotope Effects ◽  
Ion Pair ◽  
P Value ◽  
Complex Reaction ◽  
Butyl Chloride ◽  
Substitution Reactions ◽  
Kinetic Isotope ◽  
Contact Ion Pair ◽  
Free Ion

A spectroscopic investigation indicated that lithium thiophenoxide exists as a contact ion pair complex in dry diglyme whereas the other alkali metal thiophenoxides exist as a solvent-separated ion pair complex in diglyme. The addition of small amounts of water converts the lithium thiophenoxide contact ion pair complex into a solvent-separated ion pair complex. A smaller secondary α-deuterium kinetic isotope effect and a larger Hammett p value are observed when the nucleophile is the contact ion pair complex in the SN2 reaction between n-butyl chloride and thiophenoxide ion in diglyme. This indicates that the transition state for the contact ion pair complex reaction is tighter with a shorter nucleophile–α-carbon bond than the transition state for the solvent-separated ion pair complex reaction. The secondary α-deuterium kinetic isotope effects for the free ion and the solvent-separated ion pair complex reactions between sodium thiophenoxide and n-butyl chloride in DMF suggest that the loosest transition state is found when the nucleophile is the free ion. Key words: transition state, SN2, isotope, deuterium, Hammett ρ.

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.

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