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.

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.

Aromatic sulphonates and the hydrolysis of 2-(p-nitrophenoxy)tetrahydropyran

1984 ◽  
Vol 62 (7) ◽  
pp. 1320-1324 ◽  
Author(s):  
Stella O'Leary
Keyword(s):  
Ionic Strength ◽  
Rate Constant ◽  
Polyacrylic Acid ◽  
Order Rate Constant ◽  
High Ionic Strength ◽  
Buffer Concentration ◽  
Salt Effects ◽  
Low Ionic Strength ◽  
Hydrolysis Of ◽  
Aromatic Sulphonates

The rate of hydrolyis of 2-(p-nitrophenoxy)tetrahydropyran was measured in a variety of buffers in water at 30 °C. At low ionic strength (μ = 0.05), 3,6-disulphonaphthoxyacetic acid catalysed the reaction. The second-order rate constant was 20 times faster than predicted from pKa. At high ionic strength (μ = 0.5), plots of kobs vs. total buffer concentration for both 3,6-disulphonaphthoxyacetic acid and 6,8-disulphonaphthyoxyacetic acid go through a maximum. Polyacrylic acid catalysed the reaction. The results are discussed in terms of aggregation and salt effects.

ION PAIR EFFECTS IN THE REACTION BETWEEN POTASSIUM FERROCYANIDE AND POTASSIUM PERSULFATE

1966 ◽  
Vol 44 (4) ◽  
pp. 437-445 ◽  
Author(s):  
R. W. Chlebek ◽  
M. W. Lister
Keyword(s):  
Ionic Strength ◽  
Rate Constants ◽  
Equilibrium Constants ◽  
Ion Pairs ◽  
Ion Pair ◽  
Glass Electrode ◽  
Potassium Ferrocyanide ◽  
Potassium Persulfate ◽  
Ion Concentration ◽  
True Rate

The rate of the reaction between potassium ferrocyanide and potassium persulfate has been measured over a range of conditions. The rate is dependent on the potassium ion concentration, and it is shown that this is explained if it is assumed that KFe(CN)63− and KS2O8− are the reacting species. The equilibrium constants governing the formation of these ion pairs were measured with a cation-sensitive glass electrode. Similar constants for the products KFeCCN6)2− and KSO4−, and also for KNO3, were measured. From these equilibrium constants, the true rate constants of the reaction can be obtained, and it is shown that these vary with ionic strength in the manner predicted by Brönsted's equation.

DECOMPOSITION OF SODIUM HYPOCHLORITE: THE UNCATALYZED REACTION

1956 ◽  
Vol 34 (4) ◽  
pp. 465-478 ◽  
Author(s):  
M. W. Lister
Keyword(s):  
Sodium Chloride ◽  
Ionic Strength ◽  
Rate Constant ◽  
Sodium Hypochlorite ◽  
Rate Constants ◽  
Bimolecular Reaction ◽  
Activation Energies ◽  
Unimolecular Reaction ◽  
Uncatalyzed Reaction ◽  
Catalytic Effects

The decomposition of sodium hypochlorite has been re-examined. The results show that Foerster and Dolch’s mechanism of the decomposition to chlorate and chloride is correct; they postulated a slow bimolecular reaction to chlorite and chloride, followed by a faster reaction of chlorite with more hypochlorite. Values of the rate constants of both steps are reported; they make the activation energies 24.8 kcal./gm-molecule for the first step and 20.8 kcal./gm-molecule for the second. The rates are such that at 40 °C. a solution of sodium hypochlorite will contain about 1% as much chlorite as hypochlorite. The rate is strongly affected by changing ionic strength; at low ionic strengths it is nearly constant or falls slightly; above about 0.8, the rate rises and at high ionic strengths the rise is quite rapid. No signs of specific catalytic effects of sodium chloride, hydroxide, or carbonate could be observed, and it seems probable that earlier reports of this were due to variations in ionic strength. The decomposition to chloride and oxygen has been measured and is a unimolecular reaction, which is possibly, but not certainly, uncatalyzed. Values of its rate constant are reported; they also are much altered by changing the ionic strength.

Studies in solvolysis. Part II. Some comments on the ion-pair mechanism for displacements at a primary carbon atom

1970 ◽  
Vol 48 (24) ◽  
pp. 3807-3818 ◽  
Author(s):  
John M. W. Scott
Keyword(s):  
Carbon Atom ◽  
Equilibrium Constant ◽  
Rate Constant ◽  
Rate Constants ◽  
Ion Pairs ◽  
Ion Pair ◽  
Experimental Basis ◽  
Methyl Halides ◽  
Bimolecular Rate Constant ◽  
Substitution Process

The implication of Sneen and co-workers (2–5) that all SN2 substitutions at a primary carbon atom probably proceed via the intermediate production of an intimate ion-pair is examined with respect to the reactions of the methyl halides (MeX; X = F, Cl, Br, I) with various nucleophiles (H2O, OH−, F−, Cl−, Br−, I−, CN−, CSN−, S2O32−) in water. By establishing certain rules concerning the behavior of derived reactivity scales (essentially ρ values) as contrasted with absolute reactivity scales (observed rate constants), it is concluded that Sneen and Larsen's mechanistic description is consistent with the experimental facts, and that in such cases the substitution process involves a pre-equilibrium constant, Ke, which is independent of the attacking nucleophile. This is followed by a rate determining bimolecular rate constant, kn, which depends on the ion pair and the nature of the nucleophile. The observed rate (k°) is given by k = Kekn. A method of calculating Ke is described and values of kn for nine nucleophiles attacking the four methyl halide ion-pairs are reported along with a number of confirmatory calculations. It is concluded that the classical Hughes–Ingold SN2 Heitler–London description of these reactions is inadequate. Some further suggestions to place the new mechanistic description on a firmer experimental basis are made.

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.

Nature of Salt Effects and Mechanism of Covalent Bond Heterolysis

2007 ◽  
Vol 32 (2) ◽  
pp. 73-118 ◽  
Author(s):  
Genrih F. Dvorko ◽  
Engelsine A. Ponomareva ◽  
Mykola E. Ponomarev ◽  
Mikhailo V. Stambirsky
Keyword(s):  
Transition State ◽  
Salt Effect ◽  
Ion Pairs ◽  
Covalent Bond ◽  
Organic Substrates ◽  
Reaction Products ◽  
Salt Effects ◽  
Hsab Principle ◽  
Limiting Step ◽  
Acids And Bases

Data on the influence of neutral salts on the rates of unimolecular heterolyses of organic substrates, obtained mainly by the verdazyl method, are summarized here. It is assumed that heterolysis takes place with consecutive formation of four ion pairs: contact (CIP), cavity-separated (CSIP), one solvent moleculeseparated (SIP) and solvent-separated (SSIP). [Formula: see text] In the limiting step, the CIP interacts with a solvent cavity and the CSIP is formed, which converts quickly into the SIP and subsequently to the SSIP, which also quickly gives the reaction products. In the transition state, bonds between the molecules solvating the CIP are broken. In the absence of salt, the return from external ion pairs does not have much importance. The verdazyl indicator quickly and quantitatively reacts with the SSIP. The normal salt effect takes place due to the action of salt on the covalent substrate, which catalyses CIP formation. The special salt effect is caused by the association of the salt with the CIP, which prolongs the lifetime of the intermediate and increases the probability of its contact with a solvent cavity. The negative special salt effect is caused by association of the salt with the SIP or SSIP, which prolongs the lifetime of the intermediates and increases the probability of their contact with a solvent cavity to return to the covalent substrate. When the salt reacts with the SIP, the salt effect does not depend on the concentration and nature of verdazyl, but such a dependence takes place when the salt reacts with the SSIP. The site of the action of the salt is determined by the Hard and Soft Acids and Bases (HSAB) principle.

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.

Kinetic salt effects in the hydrolysis of ethyl malonate, methyl glutarate and ethyl adipate semiesters

1986 ◽  
Vol 51 (12) ◽  
pp. 2781-2785 ◽  
Author(s):  
M. Martín Herrera ◽  
J. J. Maraver Puig ◽  
F. Sánchez Burgos
Keyword(s):  
Salt Effect ◽  
Alkaline Media ◽  
Salt Effects ◽  
Coulombic Interaction ◽  
Hydroxide Ion ◽  
Ethyl Malonate ◽  
Hydrolysis Of

A study is made on the kinetic salt effect on the reaction of hydrolysis of several charged esters in alkaline media. The results are interpreted on the basis of the coulombic interaction, the salting in of hydroxide ion and a third component depending on size of the substrate.

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