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. 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. VI. The effect of ion pairing on the transition state structure of SN2 reactions

1988 ◽  
Vol 66 (5) ◽  
pp. 1263-1271 ◽  
Author(s):  
Kenneth Charles Westaway ◽  
Zhu-Gen Lai
Keyword(s):  
Transition State ◽  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Isotope Effects ◽  
Ion Pair ◽  
State Structure ◽  
Transition State Structure ◽  
Substitution Reactions ◽  
Sn2 Reactions ◽  
Kinetic Isotope

Spectroscopic and conductivity studies of sodium thiophenoxide solutions in four different solvents and the secondary α-deuterium kinetic isotope effects found in the presence of 15-crown-5 ether demonstrate that the secondary α-deuterium kinetic isotope effect and transition state structure for the SN2 reaction between sodium thiophenoxide and n-butyl chloride are significantly different, depending on whether the ionic reactant is a solvent-separated ion-pair complex or a free ion. In all three solvents in which the form of the ionic reactant changes, a smaller isotope effect and tighter transition state are found for the reaction with the ion-pair complex.

Solvent effects on SN2 transition state structure. II: The effect of ion pairing on the solvent effect on transition state structure

1989 ◽  
Vol 67 (2) ◽  
pp. 345-349 ◽  
Author(s):  
Kenneth Charles West Away ◽  
Zhu-Gen Lai
Keyword(s):  
Transition State ◽  
Solvent Effects ◽  
Isotope Effects ◽  
Ion Pairs ◽  
Kinetic Isotope Effects ◽  
Ion Pair ◽  
State Structure ◽  
Butyl Chloride ◽  
Transition State Structure ◽  
Kinetic Isotope

Identical secondary α-deuterium kinetic isotope effects (transition state structures) in the SN2 reaction between n-butyl chloride and a free thiophenoxide ion in aprotic and protic solvents confirm the validity of the Solvation Rule for SN2 Reactions. These isotope effects also suggest that hydrogen bonding from the solvent to the developing chloride ion in the SN2 transition state does not have a marked effect on the magnitude of the chlorine (leaving group) kinetic isotope effects. Unlike the free ion reactions, the secondary α-deuterium kinetic isotope effect (transition state structure) for the SN2 reaction between n-butyl chloride and the solvent-separated sodium thiophenoxide ion pair complex is strongly solvent dependent. These completely different responses to a change in solvent are rationalized by an extension to the Solvation Rule for SN2 Reactions. Finally, the loosest transition state in the reactions with the solvent-separated ion pair complex is found in the solvent with the smallest dielectric constant. Keywords: ion pairs, transition state, solvent effects, nucleophilic substitution, isotope effects.

SECONDARY KINETIC ISOTOPE EFFECTS IN THE SOLVOLYSES OF 2-(2,4-DIMETIIOXYPHENYL)-1,1-d2-ETHYL AND 2-(3,5-DIMETHOXYPHENYL)-1,1-d2-ETIIYL p-BROMOBENZENESULFONATES

1966 ◽  
Vol 44 (21) ◽  
pp. 2491-2495
Author(s):  
C. C. Lee ◽  
L. Noszkó
Keyword(s):  
Transition State ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Ion Pair ◽  
Similar Data ◽  
Kinetic Isotope ◽  
Ion Stability ◽  
Transition State Structures

The secondary α-deuterium kinetic isotope effects in the acetolyses and formolyses of 2-(2,4-dimethoxyphenyl)-ethyl and 2-(3,5-dimethoxyphenyl)-ethyl p-bromobenzenesulfonates (I-OBs and II-OBs, respectively) and their corresponding 1,1-dideuterio analogues (I-OBs-1-d2 and II-OBs-1-d2) were determined. The observed kH/kD values are compared with similar data from the literature for 2-phenylethyl and 2-p-anisylethyl p-toluenesulfonates (III-OTs and IV-OTs, respectively). In the formolyses of III-OTs, IV-OTs, and I-OBs, which proceed either chiefly or exclusively by way of bridged ions as intermediates, the isotope effects appear to increase slightly with increasing bridged-ion stability. For I-OBs and I-OBs-1-d2, acetolyses gave smaller kH/kD values than formolyses because of deuterium scrambling caused by ion-pair returns during the acetolysis. Acetolyses and formolyses of II-OBs and II-OBs-1-d2 gave lower isotope effects than the corresponding reactions with I-OBs and I-OBs-1-d2. The magnitudes of the observed isotope effects in relation to transition-state structures are discussed.

Kinetic evidence for the importance of solvent-separated ion pairs during the solvolyses of adamantylideneadamantyl derivatives

2004 ◽  
Vol 82 (9) ◽  
pp. 1336-1340
Author(s):  
Xicai Huang ◽  
Andrew J Bennet
Keyword(s):  
Ionic Strength ◽  
Transition State ◽  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Isotope Effects ◽  
Ion Pairs ◽  
Kinetic Isotope Effects ◽  
Ion Pair ◽  
Salt Effects ◽  
Kinetic Isotope

The aqueous ethanolysis reactions of adamantylideneadamantyl tosylate, -bromide, and -iodide (1-OTs, 1-Br and 1-I) were monitored as a function of ionic strength. Special salt effects are observed during the solvolyses of both homoallylic halides, but not in the case of the tosylate 1-OTs. The measured α-secondary deuterium kinetic isotope effects for the solvolysis of 1-Br in 80:20 and 60:40 v/v ethanol–water mixtures at 25 °C are 1.110 ± 0.018 and 1.146 ± 0.009, respectively. The above results are consistent with the homoallylic halides reacting via a virtual transition state in which both formation and dissociation of a solvent-separated ion pair are partially rate-determining. While the corresponding transition state for adamantylideneadamantyl tosylate involves formation of the solvent-separated ion pair.Key words: salt effects, kinetic isotope effect, internal return, solvolysis, ion pairs.

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.

Model Calculations of Kinetic Isotope Effects in Nucleophilic Substitution Reactions

1974 ◽  
Vol 52 (6) ◽  
pp. 903-909 ◽  
Author(s):  
Jan Bron
Keyword(s):  
Transition State ◽  
Isotope Effects ◽  
Benzyl Bromide ◽  
Kinetic Isotope Effects ◽  
Substitution Reactions ◽  
Model Calculations ◽  
Kinetic Isotope ◽  
Proposed Model ◽  
Standard Reaction ◽  
Deuterium Isotope Effects

The results of calculations indicate that a previously proposed model for the transition state in "borderline" substitution reactions can be generalized and, as a result, the observed differences in the carbon-13 and deuterium isotope effects of SN1, SN2, and "borderline" reactions rationalized. Although the conclusions may apply more generally, the standard reaction investigated is the solvolysis of benzyl bromide. The importance of resonance interaction with the phenyl ring, the significance of the product- or reactant-like character of the transition state, and the influence of the magnitude of force constants in determining isotope effects are examined. The temperature dependence of kinetic isotope effects in solvolysis is also investigated.

Solvent effects on nucleophilic substitution reactions. III. The effect of adding an inert salt on the structure of the SN2 transition state

1996 ◽  
Vol 74 (12) ◽  
pp. 2528-2530 ◽  
Author(s):  
T.V. Pham ◽  
K.C. Westaway
Keyword(s):  
Ionic Strength ◽  
Transition State ◽  
Isotope Effects ◽  
Nitrogen Isotope ◽  
Kinetic Isotope Effects ◽  
Substitution Reactions ◽  
Kinetic Isotope ◽  
Ionic Transition ◽  
Hydrogen Deuterium ◽  
Inert Salt

The nitrogen and secondary α-hydrogen–deuterium kinetic isotope effects found for the SN2 reaction between thiophenoxide ion and benzyldimethylphenylammonium ion at different ionic strengths in DMF at 0 °C indicate that the structure of the transition state changes markedly with the ionic strength of the reaction mixture. In fact, a more reactant-like, more ionic, transition state is found at the higher ionic strength. This presumably occurs because a more ionic transition state is more stable in the more ionic solvent. Key words: transition state, ionic strength, secondary α deuterium kinetic isotope effects, nitrogen isotope effects, SN2.

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