Two-stage mechanism

Addition-elimination mechanism (Section 23 6) Two stage mechanism for nucleophilic aromatic substitution In the addition stage the nucleophile adds to the carbon that bears... 

Nucleophilic acyl substitutions follow a two-stage mechanism and proceed by way of a tetrahedral intermediate. 

It is quite reasonable to expect the bimolecular two-stage mechanism Sj Ar ) to predominate in most aromatic nucleophilic substitutions of activated substrates. However, only in rare instances is there adequate evidence to rule out the simultaneous occurrence or predominance of other mechanisms. The true significance of the alternative mechanisms in azines needs to be determined by trapping the intermediates or by applying modem separation and characterization methods to the identification of at least the major portion of the products, especially in kinetic studies. 

The lack of a uniform order of relative reactivity of the halogens in reactions of certain nucleophiles with nitro- and polynitro-phenyl halides led Parker and Read to propose a one-stage mechanism for some aromatic nucleophilic substitutions. An alternative explanation within the framework of the two-stage S Ar2 mechanism had been proposed earlier. A range of mechanisms has been considered in the past by Chapman, who properly points out that only in a limited number of examples is the evidence for the two-stage mechanism compelling even though the balance of evidence favors it. 

The results obtained, i.e., agreement between the directions of cyclization and acetylenic cyclocondensation of iodazolecarboxylic acids and much shorter duration of the first reaction, confirm the assumption of the two-stage mechanism of cyclocondensation (66JOC4071 69JA6464). 

With a two-stage mechanism the propagation rate constant is ... 

The reaction kinetics In the case of HMDS suggests a two stage mechanism for the major dichloride consuming reaction. First a slow Na surface dependent build up of long lived active centers then a propagation step In which the Na surface is not rate determining. The reaction which occurs on the surface must be fast. 

It seems very probable that the epoxidation reaction proceeds through a two-stage mechanism. Hydroperoxide oxidizes the catalyst to peroxo complex and the this complex performs epoxidation of olefins. 

These Co(II) complexes react with molecular oxygen by a two stage mechanism a first step with rapid formation of a 1 1 metal-dioxygen complex, followed by the slower formation of a 2 1 complex (believed to be responsible for the irreversible coordination of oxygen) according to the sequence ... 

A two-stage mechanism was proposed by Takeuchi et al. based on these observations, wherein the... 

The vacuum line used in the following preparations is similar to that described by Shriver.14 It consists of a pump station, a main reaction manifold with six reaction stations, a fractionation manifold with four U-traps and a reaction station at each end, a McLeod Guage, and a Topler pump. The pump station employs a two-stage mechanical forepump and a two-stage mercury diffusion pump. Operating vacuum is 1.0 X 10"S torr. Teflon valves are employed throughout. 

The radical (RO)2PS2 in indene would thus effectively act as a chaincarrying radical and is therefore not considered to be an intermediate in chain-breaking inhibition by dithiophosphates. A two-stage mechanism was therefore proposed, involving a stabilized zinc salt-peroxy complex, either a radical or an ion pair as illustrated in structures V and VI. 

As outlined in our previous review (5), coupling reactions of a nucleophilic reagent Nu with an organosilane RaSi—X occur only by a one-stage nucleophilic mechanism [Eq. (1)]. The halosilanes are not prone to one-electron transfer which could lead to a two-stage mechanism [Eq. (2)]. 

Here we limit ourselves to the application of (52) to a simple two-stage mechanism ... 

We shall confine ourselves to the consideration of a simple two-stage mechanism when the surface contains in a significant amount only intermediate particles of one kind, the degrees of coverage by other particles, if there are any, being small. Such mechanism may be described by the following scheme (48) ... 

The two-stage mechanism of CO conversion became a prototype of a general two-stage mechanism discussed in Sections VII and X that later proved applicable to numerous heterogeneous catalytic reactions as is evidenced by the review by Boudart (125) and the present article. 

---