Reaction Paths for Nucleophilic Substitution SN2 Reactions

There are two general classes of reactions to change coordination at a reaction center addition/elimination and substitution. The words used to identify and further characterize these reactions are traditionally different in organic and in inorganic chemistry. In order to have a common basis for discussing analogous processes at such diverse reaction centers as Cd, Pb, Sn, Ge, Si, C, Al, and B, we start by giving a condensed summary of the respective classifications. 

Addition and elimination are simply different aspects of the same transformation followed either in the forward or backward directions. In the process the coordination number of the reaction center is increased or decreased by one. The transition state of both reactions is characterized by a partial bond between the reactants. In Chapter 6 several examples of addition/elimination reactions have been discussed, primarily those involving carbonyl carbon and other three-coordinate atoms as reaction centers. The inorganic analogs given there illustrate the relationship between the organic transformations and the more general concept of addition and elimination of Lewis bases to and from Lewis acids. 

The above summary shows that mechanistic parlance implies energetic and structural aspects. As discussed in Chapter 5, structure correlation does not allow detailed energetic deductions to be made and can usually not distinguish one step from multistep mechanisms, or early from late transition states. Other aspects of mechanism, however, can be addressed by structure correlation methods, e.g. the detailed structural changes necessary to deform the ground state of the reactants into a transition state, and the structural characteristics of the latter such as looseness or tightness. 

On this general background we shall discuss ligand interchange reactions at four-coordinate main group and d -transition elements. We start with Cd, proceed to Sn and Si, digress to Pb and Ge for purposes of comparison, regain the main alley with a short comparison of A1 and B, and end up with structural aspects of Sn2 reactions at C. 

Azmax was choscn as rf(Cd-Seq)/3, corresponding to Ox,max = 109.47°. The value of the constant a = 1.05 A was determined by a least-squares calculation on the observed values of Ax, Ay and Az. With these constants. Equations (7.1) and (7.2) satisfy the condition that the function passes through the points [Ax = 0, Ox = 109.47°] and [Ay = oo, Cy = 70.53°]. Equations 7.1 and 7.2 were inspired by Pauling s formula relating differences in interatomic distances Ad to the logarithm of a bond number n [3, 4] 

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