Crowd-ion mechanism

Interstitially mechanism (Figure 8.3C). The atoms placed at the interstitial sites can move, pushing the nearest neighboring atom and displacing it. This kind of mechanism occurs in metallic solutions when solute atoms are the same size as matrix ones. The displacement of several atoms placed in a close-packed direction from their equilibrium position by means of an interstitially mechanism is called the crowd-ion mechanism (Figure 8.3D). This kind of distortion spreads out only along one dimension. 

Crowd ion mechanism refers to the displacement of an extra atom that can displace several atom positions, thus producing a diffusion flux. The diffusivity in a single crystal is always substantially smaller than that of a multicrystalline sample, because the latter has diffusion along the grain boundaries. 

Correlations of Nemst-Haskell [9] for electrolytes were mentioned. The effect of concentration, that is, dilnte versus concentrated solutions were separately discussed. Correlations of Wilkee-Chang, Siddiqi-Lucas, and Haydeek-Minhas were described. The diffusion mechanism in solids was discussed. The various mechanisms of diffusion such as vacancy mechanism, interstitial mechanism, snbstitu-tional mechanism, and crowd ion mechanism were outlined. Knudsen diffusion, when the mean free path of the molecule is greater than the diffusion path, as in pore diffusion, was discussed. Diffusion in polymers and the Arrhenius dependence of the diffusion coefficient with temperature were discussed. 

Crowd-ion mechanism. An extra atom in a chain of close-packed atoms can displace several atoms in the line from their equilibrium position, thus producing a diffusion flux. 

Fluorination with retention of configuration (such as in 327) has often been attributed to steric crowding on the opposite side of the hydroxyl group, hindering the back side of the fluoride ion for Sn2 attack. However, a possible Sn/ mechanism by loss of an OSp2NEt2 fragment to give a car-bonium cation, followed by front-side attack of fluoride ion, should be... 

In the mechanism preferred by the authors, the observed KIE is the product of the EIE for the reversible formation of the bromonium ion and the KIE for the rate-determining formation of the /3-bromocarbocation (Scheme 3). Because the steric crowding of the C-2, C-2, C-3 and C-3 endo-hydrogens in the bromonium ion would be relieved in going to the /3-bromocarbocation intermediate, one would expect the secondary deuterium KIE for the k2 step of the reaction to be normal, i.e. >1.00. If this is the case, the EIE for the formation of the bromonium ion must be significantly more inverse than the KIE for the k step of the reaction, i.e. the KIE for the formation of the... 

Conversion of the trimer (80) to the seven-membered ring system (81) occurs readily with cesium fluoride [3,75], whereas in the presence of TAS fluoride, the di-anion (82) is trapped. These observations lead to the most likely mechanism for rearrangement as that in Scheme 37 [3]. The cyclisation step shown in Scheme 37 is made more easily accepted by the fact that the diene (83) (Scheme 38), undergoes rapid rearrangement in the presence of fluoride ion, giving the cyclic system (86) [77]. The ready cyclisation of a crowded anion (84) to give what appears to be a sterically unfavourable intermediate (85) would not be easily predictable ... 

An 5 2 mechanism is uniikeiy at such a crowded carbon atom. However, the main reason why the Sm2 mechanism is wrong is that the S l mechanism is so very efficient, with a neighbouring MeO group whose ione pair can stabilize the carbocation intermediate. The 5 2 mechanism doesn t get a chance. 

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