Stereochemical consequences

Other empirical guidelines have been formulated that attempt to predict and rationalise the addition of HY to such a system. However, for all of these it must be remembered that only the relative direction of addition of the anionic component with respect to the rest of the substrate is considered. The orientation of the addition of the proton is ignored in all cases. 

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Surfactants have also been of interest for their ability to support reactions in normally inhospitable environments. Reactions such as hydrolysis, aminolysis, solvolysis, and, in inorganic chemistry, of aquation of complex ions, may be retarded, accelerated, or differently sensitive to catalysts relative to the behavior in ordinary solutions (see Refs. 205 and 206 for reviews). The acid-base chemistry in micellar solutions has been investigated by Drummond and co-workers [207]. A useful model has been the pseudophase model [206-209] in which reactants are either in solution or solubilized in micelles and partition between the two as though two distinct phases were involved. In inverse micelles in nonpolar media, water is concentrated in the micellar core and reactions in the micelle may be greatly accelerated [206, 210]. The confining environment of a solubilized reactant may lead to stereochemical consequences as in photodimerization reactions in micelles [211] or vesicles [212] or in the generation of radical pairs [213]. 

The nonplanarity of allenes has an interesting stereochemical consequence 1 3 Disubstituted allenes are chiral they are not superimposable on their mirror images Even an allene as simple as 2 3 pentadiene (CH3CH=C=CHCH3) has been obtained as sep arate enantiomers... 

In this chapter, configurational relationships will be emphasized. Both structural and dynamic aspects of stereochemical relationships will be considered. We will be concerned both with the fimdamental principles of stereochemistry and the conventions which have been adopted to describe the spatial arrangements of molecules. We will consider the stereochemical consequences of chemical reactions so as to provide a basis for understanding the relationships between stereochemistry and reaction mechanism that will be encountered later in the book. 

The concerted displacement mechanism implies both kinetic and stereochemical consequences. The reaction will exhibit second-order kinetics, first-order in both reactant... 

Absolute configurations of the amino acids are referenced to D- and L-glyceraldehyde on the basis of chemical transformations that can convert the molecule of interest to either of these reference isomeric structures. In such reactions, the stereochemical consequences for the asymmetric centers must be understood for each reaction step. Propose a sequence of reactions that would demonstrate that l( —)-serine is stereochemically related to l( —)-glyceraldehyde. 

A stereochemical consequence of the way A T and G C base pairs form is that the sugars of the respective nucleotides have opposite orientations, and thus the sugar-phosphate backbones of the two chains run in opposite or... 

In his valence bond theory (VB), L. Pauling extended the idea of electron-pair donation by considering the orbitals of the metal which would be needed to accommodate them, and the stereochemical consequences of their hybridization (1931-3). He was thereby able to account for much that was known in the 1930s about the stereochemistry and kinetic behaviour of complexes, and demonstrated the diagnostic value of measuring their magnetic properties. Unfortunately the theory offers no satisfactory explanation of spectroscopic properties and so was... 

The pentacoordinate molecules of trigonal bipyramidal form, like PF5, are a very nice example for the study of the formal properties of stereoisomerizations. They are characterized by an appreciable nonrigidity and they permit the description of kinetics among a reasonable number of isomers, at least in particular cases (see below). Therefore the physical and chemical properties of these molecules have been thoroughly investigated in relation to stereoisomerization. Recent reviews may be found in the literature on some aspects of this problem. Mislow has described the role of Berry pseudorotation on nucleophilic addition-elimination reactions and Muetterties has reviewed the stereochemical consequences of non-rigidity, especially for five- and six-atom families as far as their nmr spectra are concerned. 

A number of photochemically induced 4 + 2 cycloadditions have been observed/131-150 This reaction can be either a concerted ( 4, + 20) or a ( 40 4- 2S) addition. The stereochemical consequences of the allowed reactions are as follows ... 

The first interaction has favorable orbital phase overlap for a concerted (2ns + vizs) reaction. The interaction integral y, Eqs. 3—6, for a concerted process would have a maximum value if the two molecules approached each other so that the reacting orbitals could overlap in the most efficient manner. The best geometry would involve a face-to-face reaction of the two reactant species. The stereochemical consequence of such a reaction would be specific retention of substituent relative geometries. 

There are readily observable stereochemical consequences that serve as a test of the bromonium ion mechanism ... 

By chance rather than by design, the third chapter in this volume also emanates from Israel. Bernard S. Green, Rina Arad-Yellin, and Mendel D. Cohen have surveyed organic reactions in the solid state from the standpoint of the stereochemist. In the first part of the chapter, the authors discuss the stereochemical consequences of the crystallization of conformationally mobile systems. Conformational, crystal-field, and hydrogen-bonding effects, among others, are responsible for the selective crystallization of stereoisomers that may not be dominant in solution. The second part of the chapter is concerned with the stereochemical consequences of chemical, and especially photochemical, reactions in the solid state. 

The concept that the stereochemistry of reduction of an unsaturated hydrocarbon is determined at the adsorption stage of the reaction is evident in each of the above accounts, and others could be cited. However, the development of techniques which permit the identification of different product-controlling reactions directs one to consider the stereochemical consequences of various postulated reaction sequences and this will be discussed in Section IV. 

At the same time, it is possible that one-electron transfer creates conditions favorable for the existence of a definite conformer, which leads to corresponding stereochemical consequences. 

The stereochemical consequences of the electrophilic addition of, say, bromine to certain alkenes can be predicted as follows ... 

In the formation of 1-chlorobutane, an intermediate primary radical is involved, and there are no stereochemical consequences. However, the secondary radical involved in 2-chlorobutane formation is planar, and when it abstracts a chlorine atom from a chlorine molecule it can do so from either side with equal probability. The result is formation of a racemic product, ( )-2-chlorobutane. 

The stereochemical consequences of this are illustrated in the following examples. 

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