Bimolecular rate-determining step

Bromide ion forms a bond to the primary carbon by pushing off a water molecule This step IS bimolecular because it involves both bromide and heptyloxonium ion Step 2 IS slower than the proton transfer m step 1 so it is rate determining Using Ingold s ter mmology we classify nucleophilic substitutions that have a bimolecular rate determining step by the mechanistic symbol Sn2... 

Second order kinetics is usually interpreted m terms of a bimolecular rate determining step In this case then we look for a mechanism m which both the aryl halide and the nucleophile are involved m the slowest step Such a mechanism is described m the fol lowing section... 

Better fits were obtained for n = 1 which gave linear reduced rate-conversion plots up to 20-30 % conversion, followed by a downward curvature. The apparent preexponential factors and activation energies associated with Kt and K2 were A, = 6.53 x 10s s-1, E, = 80.4kJ/mole, A2 = 3.01 x 10s s 1, and E2 = 71.3 kJ/mole. These kinetics can be explained in terms of a bimolecular rate-determining step between hydroxylic catalyst species and either amine or a rapidly-formed amine-epoxide adduct. An analysis similar to that of Horie et al. yields the kinetic Eq. 

The latter reaction clearly demands a bimolecular rate-determining step, and will be slow at low temperatures and low pressures. Thus PH3F2 can be obtained as a reasonably stable gaseous substance at room temperature. 

The bimolecular rate-determining step implies a second-order reaction. The intermediate can often be identified (e.g. spectroscopically) or even isolated and characterised in the case of a free radical, it can sometimes be trapped by reagents which quickly bind such species to give a readily-identifiable product. 

On the basis of independent studies, Williams and Brown (152) have suggested that the data indicating a bimolecular rate-determining step for... 

The E2 reaction is a concerted process, with a bimolecular rate-determining step. In this case, concerted means that bonding of the base with a proton, formation of a double bond, and departure of the leaving group all occur in one step. 

The hahde nucleophile helps to push off a water molecule from the alkyloxonium ion. According to this mechanism, both the halide ion and the alkyloxonium ion are involved in the same bimolecular elementary step. In Ingold s terminology, introduced in Section 4.11 and to be described in detail in Chapter 8, nucleophilic substimtions characterized by a bimolecular rate-determining step are given the mechanistic symbol Sn2. 

They found that the rate of hydrolysis depends only on the concentration of tert-butyl bromide. Adding the stronger nucleophile hydroxide ion, moreover, causes no change in the rate of substitution, nor does this rate depend on the concentration of hydroxide. Just as second-order kinetics was interpreted as indicating a bimolecular rate-determining step, first-order kinetics was interpreted as evidence for a unimolecular rate-determining step—a step that involves only the alkyl halide. 

Such an A2 mechanism (A stands for acid, 2 indicates a bimolecular rate-determining step) results in a stereospecific reaction. Thus ( )-butane-2,3-diol is formed from cz.s-2,3-dimethyloxirane and meso-butane-2,3-diol from ra 5-2,3-dimethyloxirane. The oxirane obtained by epoxidation of cyclohexene is ra .s -cyclohexane-l,2-diol. 

Solvent effects on and for reaction of Co(NO)(CO)3 have been studied. Effects on k are fairly small and are consistent with the effect of varying dielectric constant on the bimolecular rate-determining step. Values of ki for these reactions conducted in the poor solvents cyclohexane, toluene, and nitromethane are similar. This similarity, and the determined activation parameters, are consistent with a dissociative mechanism for this reaction pathway. But values for reaction in tetrahydrofuran, acetonitrile, and dimethyl sulphoxide are different from the former k values and from each other, indicating an associative mechanism involving the solvent for the k term. The balance between the k and k pathways for substitution in Co(NO)(CO)2L depends both on the 7r-bonding and steric characteristics of L. This dependence on the bulk of L is perhaps surprising in view of the tetrahedral stereochemistry and consequent lack of crowding in the substrate. The reverse reaction of Co(NO)(CO)2L with carbon monoxide has also been studied, so that rates in both directions and equilibrium constants are known for ... 

Equilibrium constants Ki reflect the degree of substitution, decreasing from 2.7 for mercaptoacetic acid to 1.31 for 2-mercaptopropanoic acid, and to 0.35 for 2-mercapto-2-methylpropanoic acid (T = 20 °C, / = 1.0 mol 1 ). The trend reflects steric interaction in the five-membered ring. These complexes decompose to yield the corresponding disulphides and iron(m) with a bimolecular rate-determining step. The overall mechanism,... 

The reaction rate depends only on the concentration of tert-butyl bromide. Just as Hughes and Ingold interpreted a second-order rate law in terms of a bimolecular rate-determining step, they saw first-order kinetics as evidence for a unimoZecM/ar rate-determiiung step—one that involves only the alkyl halide and is independent of both the concentration and identity of the nucleophile. Like the mechanism for the reaction of alcohols with hydrogen halides (Section 4.8), this pathway is classified as SnI (substitution-nucleophilic-unimolecular) and is characterized by the formation of a carbocation in the rate-determining step. 

The mechanism for this reaction involves a bimolecular rate-determining step in which the nucleophilic group approaches the electrophilic carbon atom and, simultaneously, the leaving group departs. The entering and leaving groups act at the same time in what is known as a concerted step, as depicted in this schematic representation ... 

The photolysis of HN3 in polar, organic solvents like alcohols and ethers yields mainly N2 and NH4N3 the amounts of H2 and N2H4 formed are small. NH4N3 results from a secondary reaction at T>220 K only. The speed of the liberation of N2 strongly depends on the concentration of HN3 and indicates a bimolecular rate-determining step [63]. The photolysis in hydrocarbons yields products resulting from reactions with the solvents in addition to NH3 and N2 see p. 149. 

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