Rate-determining intermediate

As we have just seen the rate determining intermediate m the reaction of tert butyl alco hoi with hydrogen chloride is the carbocation (CH3)3C Convincing evidence from a variety of sources tells us that carbocations can exist but are relatively unstable When carbocations are involved m chemical reactions it is as reactive intermediates formed slowly m one step and consumed rapidly m the next one... 

FIGURE 23 4 Struc ture of the rate determining intermediate in the reaction of p fluoronitrobenzene with methoxide ion... 

Write equations describing the addition-elimination mechanism for the reaction of hexafluorobenzene with sodium methoxide clearly showing the structure of the rate determining intermediate j... 

The rate determining intermediate is a cyclohexadienyl anion and is sta bilized by electron withdrawing substituents... 

Ethyl 2,2-dimethylpropanoate Hydroxide Rate-determining intermediate ... 

The tert-butyl group of ethyl 2,2-dimethylpropanoate causes more crowding than the methyl group of ethyl acetate the rate-determining intermediate is less stable and is formed more slowly. 

Until now several groups have failed to identify the elusive metallaoxetane and the extensive set of available kinetic data provide no indication for the existence of the species. But the possibility that it might be a non-rate-determining intermediate could not be excluded experimentally. Different mechanisms were proposed to explain the variety of experimental results available, but the mechanistic issues remain unresolved. 

Certain systems show structure insensitivity over a region of surface states for which it is claimed that coverage of the rate-determining intermediate is high or for which the reaction takes place on an overlayer. 

Rate-determining intermediate from methyl m-nitrobenzoate... 

The models of these surface reactions (more details are given in Section 6.7) imply a number of assumptions (I) there is equilibrium in adsorption and desorption (2) there is only one rate-determining intermediate reaction between adsorbed species (3) the species on the surface are well mixed and (4) there is a thermal probability of a transition of the physisorbed to the chemisorbed state before subsequent reaction and diffusion. These assumptions are not independently proved but are justified by the degree of success of the models in predicting the kinetics. 

Since the early transition state does not involve covalent interaction with the nucleophile, the rate-determining intermediate also does not involve covalent interaction with the nucleophile, and is best represented as an ion pair. A structural representation of this ion pair cannot be specified precisely, but certainly there has been massive electron reorganization with little covalent interaction between the nuclei in the bond being broken. Mutual polarization of the carbonium ion and counter-ion can be assumed, as well as coulombic attraction. The intermediate formed in the rate-determining step is then rapidly converted to an intermediate which does involve covalent interaction with solvent or other nucleophiles present. Thus, the rate-determining step is unimolecular, and the observed kinetics are first-order. As will be seen shortly, the stereochemical course of the reaction can also be accommodated. 

In the presence of. 06 M sodium azide, both 2-octanol and 2-octyl azide are formed with complete inversion of configuration. Clearly, in the presence of the very nucleophilic azide ion, the weakly nucleophilic dioxane cannot compete effectively, and the racemization pathway is eliminated. On the basis of these results and related kinetic studies, it was proposed that the rate-determining intermediate was the intimate ion pair, and that the displacement of the anion from the intimate ion pair occurred stereospecifically with inversion. " ... 

For consideration of chiral recognition by the syn-iy t 12, the stability of the dipolar tetrahedral structure (26) was considered as the rate-determining intermediate for aminolysis of thioester [8]. The tetrahedral intermediate (26) formed across... 

Nucleation is an active process of generating within a meta-stable mother phase the initial fragments, called the nucleus, of a new of more stable phase. Once the nuclei (the critical rate-determining intermediates) are formed, the... 

Judgment must be exercised in cases in which two of the resonance criteria are in conflict. For example, the Lewis structure B is an important contributor to the resonance hybrid for the rate-determining intermediate formed in the nitration of... 

The results cited in entry 5 serve to illustrate the importance of solvation of the ion pair intermediates in nucleophilic substitution reactions of secondary substrates. The results show that partial racemization occurs in aqueous dioxane but that an added nucleophile (azide ion) results in complete inversion, both in the product resulting from reaction with azide ion and from the reaction with water. In aqueous dioxane, the ion-pair intermediate may be solvated by water molecules or by dioxane. Inverted product results from solvation by water, but solvation by dioxane cannot collapse to a stable product. Subsequent solvent reorganization can lead to symmetrical solvation with water displacing dioxane. Alternatively, attack of water on the dioxane-solvated ion could lead to retained configuration, since two inversions would have occurred. In the presence of. 06 M sodium azide, both 2-octanol and 2-octyl azide are formed with complete inversion of configuration. In the presence of the very nucleophilic azide ion, the weakly nucleophilic dioxane does not compete effectively and the route to retained alcohol is precluded. Only inverted alcohol formed by way of the initial ion-pair intermediate by back-side solvation with water is observed. These results are consistent with the intimate ion pair being the rate-determining intermediate. " ... 

TDI and TDTS are new terms that only have sense in catalysis. In noncatalytic reactions, it is more accurate to speak about rate-determining intermediate (RDI) and rate-determining intermediate transition state (RDTS) (or collectively, RDStates) [37, 54, 55]. However, the concept is not new (as always), and has been called by other names. The determining intermediate is also known as the resting state or the most abundant reaction intermediate MARI) [2, 22-24, 56]. Unfortunately, the kinetic importance of the TDI has been disregarded, rarely... 

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