The ion-pair mechanism and bimolecular displacement at saturated carbon. VI. Racemization and radio-bromide exchange for substituted 1-phenylbromoethanes; solvent effects

1987 ◽  
Vol 65 (2) ◽  
pp. 363-371
Author(s):  
Allan R. Stein
Keyword(s):  
Transition State ◽  
Nucleophilic Substitution ◽  
Solvent Effects ◽  
Late Stage ◽  
Ion Pairs ◽  
Tetrabutylammonium Bromide ◽  
Empirical Support ◽  
Ion Pair ◽  
Exchange Kinetics ◽  
Substituted 1

Racemization and radio-bromide exchange kinetics for 1-phenylbromoethanes in acetonitrile and in nitromethane using tetrabutylammonium bromide are reported. The results, together with those previously reported for acetone solutions, provide direct empirical support for the ion-pair mechanism for nucleophilic substitution at saturated carbon. Changing the substituents on the phenyl from the 4-nitro through to the 3,4-dimethyl substrate and the solvent from acetone to the more polar acetonitrile and nitromethane shifts the transition state for bromide substitution from an early to a late stage of the equilibria series substrate [Formula: see text] intimate ion pair [Formula: see text] various solvated ion pairs [Formula: see text] free or dissociated ions. For all the substrates in acetone and, for the species giving the less stable carbocations, in acetonitrile and nitromethane, both racemizations and exchanges are bimolecular. In the latter solvents, the substrates giving the more stable carbocations show mixed kinetics.

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.

Eliminations promoted by weak bases. IX. Stereochemistry of olefin formation from trans-2-Phenylcyclopentyl p-bromobenzenesulfonate promoted by lithium chloride in acetone

1983 ◽  
Vol 36 (9) ◽  
pp. 1821 ◽  
Author(s):  
DJ McLennan ◽  
C Lim
Keyword(s):  
Transition State ◽  
Kinetic Analysis ◽  
Lithium Chloride ◽  
Ion Pairs ◽  
Chloride Ion ◽  
Ion Pair ◽  
Current Idea ◽  
Alternative Models ◽  
Weak Bases ◽  
Rate Limiting

Parker, Winstein, and their coworkers have previously established that in the E2C elimination of trans-2-phenylcyclopentyl p-bromobenzenesulfonate induced by Bu4NCl in acetone some 9% of the olefinic product is produced by a syn-elimination. In view of the current idea that syn-eliminations in solution are assisted by association of the base with its counterion, the stereochemistry of the reaction induced by lithium chloride in acetone has been studied. There is no increase in the amount of syn-elimination, and kinetic analysis reveals that lithium chloride ion pairs are completely unreactive. 1-Phenylcyclopentene is not produced by rate-limiting attack of chloride ion on a preformed symmetrical phenonium ion pair. These results do not serve to distinguish between two alternative models of the E2C transition state.

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.

REACTION MECHANISM STUDIES: 3. SOLVENT EFFECTS AND THE NATURE OF THE TRANSITION STATE IN THE DIAXIAL → DIEQUATORIAL REARRANGEMENT

1965 ◽  
Vol 43 (4) ◽  
pp. 847-861 ◽  
Author(s):  
J. F. King ◽  
R. G. Pews
Keyword(s):  
Reaction Mechanism ◽  
Transition State ◽  
Solvent Effects ◽  
Ion Pair ◽  
Polar Solvents ◽  
Concerted Mechanism ◽  
Opening Ratio ◽  
Mechanism Of The Reaction

The rates of the diaxial → diequatorial rearrangement of 2β,3α-dibromocholestane (Ia), and the analogous bromohydrin p-toluenesulfonates (Id and Ie) and anisates (If and Ig) have been measured in various solvents. The change in rate with variation in solvent was found to correlate with the change in the ionizing power of the solvents. The sensitivity of the rate of rearrangement to changes in solvent ionizing power, as measured by the ratio of the rate of rearrangement in nitromethane to that in decalin, was found to be smaller for the dibromide (Ia) than for the esters (Id to Ig). A detailed discussion of the mechanism of the reaction is presented. It is proposed that the following factors (either singly or simultaneously) could lead to the smaller sensitivity to solvent change shown by the dibromide (Ia) as compared with the esters (Id to Ig): (1) a difference in a postulated change of the axial: equatorial opening ratio with change in solvent, and (2) the development of a smaller charge in the transition state for the rearrangement of the dibromide (Ia) as compared with that in the rearrangement of the esters, when the reactions are carried out in the less polar solvents. It is argued that the operation of the latter factor would be most simply interpreted in terms of a merged ion-pair, cyclic-concerted mechanism, as initially suggested by Grob and Winstein.

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.

The ion-pair mechanism and bimolecular displacement at saturated carbon. VII. Racemization of substituted 1-phenylbromoethanes; ionic strength effects

1989 ◽  
Vol 67 (2) ◽  
pp. 297-304
Author(s):  
Allan R. Stein
Keyword(s):  
Ionic Strength ◽  
Rate Constant ◽  
Salt Effect ◽  
Ion Pairs ◽  
Tetrabutylammonium Bromide ◽  
Ion Pair ◽  
Nucleophilic Attack ◽  
Unimolecular Reaction ◽  
Salt Effects ◽  
Bromide Ion

No, or at most very small, salt effects have been detected for second-order racemizations of various 1-phenylbromoethanes, confirming that the "mixed kinetics", previously reported for 4-methyl- and 3,4-dimethylphenylbromoethanes with tetrabutylammonium bromide in acetonitrile and nitromethane, did not reflect a salt effect on a unimolecular reaction. Thus the uni- and bimolecular rate components can be evaluated using the equation:[Formula: see text]where k1 = first-order or unimolecular rate constant, the intercept, and [Formula: see text], the latter being the rate constant for the bimolecular substitution. A "special salt effect" was found for the unimolecular component; it was especially pronounced for 3,4-dimethyl substrate in acetonitrile with Bu4NClO4 as the electrolyte. In acetone, where no unimolecular reaction component was detected even with those substrates giving the most stable carbocations, the only salt effect was a common ion, Bu4N+, effect on the incomplete dissociation of Bu4NBr. From an iterative best fit for plots of observed rate versus calculated bromide ion activity for unsubstituted, 4-methyl, and 3,4-dimethyl substrates, Kassoc ~ 15 ± 1 at 40 °C. The results are interpreted as additional support for a progression of mechanism with nucleophilic attack possible at any stage of the series of equilibria: substrate [Formula: see text] contact ion pair [Formula: see text] various solvated ion pairs [Formula: see text] dissociated ions. Keywords: special salt effects, ionic strength effects, racemization of 1-phenylbromoethanes, mechanism of nucleophilic substitution, ion pair mechanism.

Ion pairs and heat capacity of activation for 2,2-disubstituted cyclopropyl bromides

1976 ◽  
Vol 54 (8) ◽  
pp. 1246-1252 ◽  
Author(s):  
Surendra Singh ◽  
Ross Elmore Robertson
Keyword(s):  
Heat Capacity ◽  
Transition State ◽  
Relative Rate ◽  
Ion Pairs ◽  
Water Structure ◽  
Ion Pair ◽  
External Effects ◽  
Enthalpy Of Activation ◽  
Rate Ratios ◽  
Hydrolysis Of

The temperature dependence of the rates of hydrolysis of 2,2-dimethylcyclopropyl bromide, 2,2-cis-vinyl-trans-methylcyclopropyl bromide and 2,2-cis-methyl-trans-vinylcyclopropyl bromide have been determined in water. The temperature coefficient of the enthalpy of activation (ΔCp≠) for these compounds was determined to be −52, −27 and −37 cal deg−1 mol−1 respectively. The relative rate ratios for hydrolysis of the 2,2-methylvinylcyclopropyl bromides with respect to the appropriate 2-vinylcyclopropyl bromide isomer indicate a considerable progress towards allyl cations at the transition state in contrast to the indications of the ΔCp≠ values.The ΔCp≠ term for such reactions in water depends to an important degree on the external effects of charge development on water structure but is insensitive to internal electrostatic effects. In the three examples of ΔCp≠ reported in this study, all tend to show small external effects in spite of evidence which might suggest larger. The differences in ΔCp≠ are attributed to the particular shape and charge distribution of the quasi-ion pair.

Stereospecific 1,3-H Transfer of Indenols Proceeds via Persistent Ion-Pairs Anchored By NH···Pi Interactions

2018 ◽  
Author(s):  
David Ascough ◽  
Fernanda Duarte ◽  
Robert Paton
Keyword(s):  
Computational Analysis ◽  
Ion Pairs ◽  
Ion Pair ◽  
Hydrogen Atom Transfer ◽  
Polar Solvents ◽  
Chirality Transfer ◽  
Activation Barriers ◽  
Sense And Avoid ◽  
Phase Dynamics ◽  
Pi Interactions

The base-catalyzed rearrangement of arylindenols is a rare example of a suprafacial [1,3]-hydrogen atom transfer. The mechanism has been proposed to proceed via sequential [1,5]-sigmatropic shifts, which occur in a selective sense and avoid an achiral intermediate. A computational analysis using quantum chemistry casts serious doubt on these suggestions: these pathways have enormous activation barriers and in constrast to what is observed experimentally, they overwhelmingly favor a racemic product. Instead we propose that a suprafacial [1,3]-prototopic shift occurs in a two-step deprotonation/reprotonation sequence. This mechanism is favored by 15 kcal mol<sup>-1</sup> over that previously proposed. Most importantly, this is also consistent with stereospecificity since reprotonation occurs rapidly on the same p-face. We have used explicitly-solvated molecular dynamics studies to study the persistence and condensed-phase dynamics of the intermediate ion-pair formed in this reaction. Chirality transfer is the result of a particularly resilient contact ion-pair, held together by electrostatic attraction and a critical NH···p interaction which ensures that this species has an appreciable lifetime even in polar solvents such as DMSO and MeOH.

scholarly journals Tripodal, Squaramide-Based Ion Pair Receptor for Effective Extraction of Sulfate Salt

Molecules ◽  
2021 ◽  
Vol 26 (9) ◽  
pp. 2751
Author(s):  
Damian Jagleniec ◽  
Marcin Wilczek ◽  
Jan Romański
Keyword(s):  
Ion Pairs ◽  
Binding Mode ◽  
Selective Extraction ◽  
Ion Pair ◽  
Hofmeister Series ◽  
Sulfate Salt ◽  
Lower Affinity ◽  
High Affinity ◽  
Binding Domains ◽  
Hydrated Sulfate

Combining three features—the high affinity of squaramides toward anions, cooperation in ion pair binding and preorganization of the binding domains in the tripodal platform—led to the effective receptor 2. The lack of at least one of these key elements in the structures of reference receptors 3 and 4 caused a lower affinity towards ion pairs. Receptor 2 was found to form an intramolecular network in wet chloroform, which changed into inorganic–organic associates after contact with ions and allowed salts to be extracted from an aqueous to an organic phase. The disparity in the binding mode of 2 with sulfates and with other monovalent anions led to the selective extraction of extremely hydrated sulfate anions in the presence of more lipophilic salts, thus overcoming the Hofmeister series.

On the Calculation of Transition State Activity Coefficient and Solvent Effects on Chemical Reactions

1998 ◽  
Vol 63 (12) ◽  
pp. 1969-1976 ◽  
Author(s):  
Alvaro Domínguez ◽  
Rafael Jimenez ◽  
Pilar López-Cornejo ◽  
Pilar Pérez ◽  
Francisco Sánchez
Keyword(s):  
Activity Coefficient ◽  
Transition State ◽  
Chemical Reactions ◽  
Solvent Effects ◽  
Transition State Theory ◽  
Reaction Medium ◽  
State Theory ◽  
Precursor Complex ◽  
State Activity ◽  
Classical Transition

Solvent effects, when the classical transition state theory (TST) holds, can be interpreted following the Brønsted equation. However, when calculating the activity coefficient of the transition state, γ# it is important to take into account that this coefficient is different from that of the precursor complex, γPC. The activity coefficient of the latter is, in fact, that calculated in classical treatments of salt and solvent effects. In this paper it is shown how the quotients γ#/γPC change when the reaction medium changes. Therefore, the conclusions taken on the basis of classical treatments may be erroneous.

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