Substitution mechanisms and

Much less work has been done on electrophilic aliphatic substitution mechanisms than on nucleophilic substitutions, and the exact mechanisms of many of the reactions in this chapter are in doubt. For many of them, not enough work has been done to permit us to decide which of the mechanisms described in this chapter is operating, if indeed any is. There may be other electrophilic substitution mechanisms, and some of the reactions in this chapter may not even be electrophilic substitutions at all. 

The number of reactions in this chapter may seem overwhelming at first. The key to success is to remember that nucleophiles react with electrophiles. If you can identify the nucleophile or base and the electrophilic carbon (the one bonded to the leaving group) in each reaction and recall the factors that affect the competition between the two substitution mechanisms and the two elimination mechanisms, the material you have to learn will be much more manageable. 

Octahedral substitution Mechanisms and reactive intermediates (Co111)... 

For the compound below, draw an SnI substitution mechanism and show both enantiomers of the product that is formed. 

Complexes with a configuration often form square planar complexes (see Section 20.3), especially when there is a large crystal field Rh(I), Ir(I), Pt(II), Pd(II), Au(III). However, 4-coordinate complexes of Ni(II) may be tetrahedral or square planar. The majority of kinetic work on square planar systems has been carried out on Pt(II) complexes because the rate of ligand substitution is conveniently slow. Although data for Pd(II) and Au(III) complexes indicate similarity between their substitution mechanisms and those of Pt(II) complexes, one cannot justifiably assume a similarity in kinetics among a series of structurally related complexes undergoing similar substitutions. 

Mother G, Robert J-L (1986) Titanium in muscovites from two mica granites substitutional mechanisms and partition with coexisting biotite. N Jahrb Mineral Abh 153 147-161 Morandi N, Nannetti MC, Pirani R, Resmi U (1984) La mica verde delle rocce di contatto nelTarea Predazzo-Monzoni. Rend Soc It Mineral Petrol 39 677-693 Muller F, Drits VA, Plangon A. Besson G (2000b) Dehydration of Fe, Mg-rich dioctahedral micas. (I) Structural transformation. Clay Mineral 35 491-504... 

Compositions of the Apatite-Group Minerals Substitution Mechanisms and Controlling Factors... 

The apatite-group minerals of the general formula, Mio(Z04)6X2 (M = Ca, Sr, Pb, Na..., Z = P, As, Si, V..., and X = F, OH, Cl...), are remarkably tolerant to structural distortion and chemical substitution, and consequently are extremely diverse in composition (e.g., Kreidler and Hummel 1970 McConnell 1973 Roy et al. 1978 Elliott 1994). Of particular interest is that a number of important geological, environmental/paleoenvironmental, and technological applications of the apatite-group minerals are directly linked to their chemical compositions. It is therefore fundamentally important to understand the substitution mechanisms and other intrinsic and external factors that control the compositional variation in apatites. 

Some reversible polyhedral rearrangements have been described. Although most are of academic curiosity, others provide insight into the substitution mechanisms and methods by which ligand rearrangements might occur. This is clearly evident in certain examples and merely proposed (based on surrounding evidence) in others. 

The relative magnitudes of these rate constants lead to different scenarios. If and are both large with respect to ki, the first step can be considered an equilibrium (with = ki/k-i) that can be treated independently from the second step. The intermediate preassociation complex may be detectable on the basis of the relative magnitudes of k and k-i. Detection of this intermediate provides supporting evidence for this substitution mechanism, and may allow calculation of K. ... 

This chapter is concerned with reactions that introduce or interchange substituent groups on aromatic rings. The most important group of such reactions are the electrophilic aromatic substitutions, but there are also significant reactions that take place by nucleophilic substitution mechanisms, and still others that involve radical mechanisms. Examples of synthetically important reactions from each group will be discussed. Electrophilic aromatic substitution has also been studied in great detail from the point of view of reaction mechanism and structure-reactivity relationships these mechanistic studies received considerable attention in Part A, Chapter 9. In this chapter, the synthetic aspects of electrophilic aromatic substitutions will be emphasized. 

Scordari,F.,Ventruti,G.,Sabato,A., Bellatreccia,F,Della Ventura, G, Pedrazzi, G. (2006) Ti-rich phlogopite from Monte Vulture (Potenza, Italy) investigated by a multi-analytical approach substitutional mechanisms and orientation of the OH dipoles. Eur. J. Mineral., 18,379-391. 

Table 6.2 should be helpful. It summarizes what we have said so far about the two substitution mechanisms, and it compares them with respect to two other variables, solvent and nucleophile structure, which we will discuss here. 

Starting from solution as in sol—gel processes, the key to design the desired dispersion and mixing of the different metal centers lies in an understanding of the chemical reactivity and structural chemistry of the precursors. Typical precursors for sol-gel processes are metal alkoxides, for example, Ti(OR)4 for titania or Si(OR)4 for silica. A challenge in hydrolytic sol-gel processing of these different alkoxides is the different reaction rate for the hydrolysis reaction. This reaction typically proceeds via a nucleophilic substitution mechanism and therefore depends on the partial charge, 5, on the metal atom (Si or Ti). The... 

According to [6,7], N and O can occur in crystalline-lattice nodes by the substitution mechanism and H and other elements, by the mechanism of diffuse impurities, by forming defects in diamond crystals. 

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