Ionic reactions rate-determining step

Nevertheless, chemical methods have not been used for determining ionization equilibrium constants. The analytical reaction would have to be almost instantaneous and the formation of the ions relatively slow. Also the analytical reagent must not react directly with the unionized molecule. In contrast to their disuse in studies of ionic equilibrium, fast chemical reactions of the ion have been used extensively in measuring the rate of ionization, especially in circumstances where unavoidable irreversible reactions make it impossible to study the equilibrium. The only requirement for the use of chemical methods in ionization kinetics is that the overall rate be independent of the concentration of the added reagent, i.e., that simple ionization be the slow and rate-determining step. 

In the proposed mechanism (Scheme 9), the rate-determining step is the reaction between aldehyde and enolate. In the absence of a solvent, a major issue with this reaction is the typical low rate and the need for a high concentration of catalyst (usually DABCO). It was reported recently that, under basic conditions, the ionic liquid [BDMIM][PF6] is inert and that the Baylis Hillman reaction in [BDMIMjPFg proceeds smoothly with better yields than in [BMIMjPFg (163). 

As the concentration of BH increases, the observed catalytic coefficient will decrease until, when 2[BH] > k, the catalytic coefficient equals ,[OH ] and the rate-determining step is the addition of hydroxide ion to the substrate. Choice may be made between a number of unsymmetrical mechanisms depending upon the rate dependence upon hydrogen ion, hydroxide ion or water concentrations at high buffer concentrations or [B] or [BH] at low buffer concentrations. Johnson has tabulated the 18 kinetic possibilities and the 13 different types of kinetic behaviour of general acid-base-catalysed reaction, pointing out that this tabulation uses only one ionic form for the tetrahedral intermediate. 

The rate-determining step in the ionic hydrogenation reaction of carbon-carbon double bonds involves protonation of the C==C to form a carbocation intermediate, followed by the rapid abstraction of hydride from the hydride source (equation 45). ° There is a very sensitive balance between several factors in order for this reaction to be successful. The proton source must be sufficiently acidic to protonate the C—C to form the intermediate carbocation, yet not so acidic or electrophilic as to react with the hydride source to produce hydrogen. In addition, the carbocation must be sufficiently electrophilic to abstract the hydride from the hydride source, yet not react with any other nucleophile source present, i.e. the conjugate anion of the proton source. This balance is accomplished by the use of trifluoroacetic acid as the proton source, and an alkylsilane as the hydride source. The alkene must be capable of undergoing protonation by trifluoroacetic acid, which effectively limits the reaction to those alkenes capable of forming a tertiary or aryl-substituted carbocation. This essentially limits the application of this reaction to the reduction of tri- and tetra-substituted alkenes, and aryl-substituted alkenes. 

In the box above, you can see acetone used as a solvent for an 5 2 reaction and formic acid (HCO2H) as solvent for the S l reaction. These are typical choices a less polar solvent for the Sn2 reaction (just polar enough to dissolve the ionic reagents) and a polar protic solvent for the S l reaction. The S l reaction fairly obviously needs a polar solvent as the rate-determining step usually involves the formation of ions and the rate of this process will be increased by a polar solvent. More precisely, the transition state is more polar than the starting materials and so is stabilized by the polar solvent. Hence solvents like water or carboxylic acids (RCO2H) are ideal. 

Conclusively the decomposition of methoxynaphthalene in SC water is a proton-catalyzed ionic reaction, all in agreement with proton-catalyzed hydrolysis. The mechanism proposed is Illustrated in Figure 9 The rate-determining step is assumed to be the decomposition of the protonated ether, either in a unimolecular A-1 mechanism or in a blmolecular A-2 mechanism where water is involved In the formation of the transition state complex. The essential difference between both mechanisms is that A-2 regenerates a protium ion in a cyclic process by proton transfer, while A-1 requires a new Ionization of water for every mole of ether converted. Which of both mechanisms dominates remains uncertain at this stage. 

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