Ionic Reactions—Stereochemistry

Molecular Orbitals and Organic Chemical Reactions Reference Edition Ian Fleming 2010 John Wiley Sons, Ltd 

It is not helpful to use the word stereospecific to mean 100% stereoselective, as many people thoughtlessly do—a useful distinction is lost, and understanding suffers. Furthermore, it is arguable that there are no reactions that give not even a trace of the stereoisomer, and so all stereospecific reactions, when it is ill-defined as those reactions which are completely stereoselective, are at risk of losing their status when a better analytical method comes along. 

This chapter is divided into two sections, largely separating stereospecific reactions from the merely stereoselective. The first (Section 5.1) deals largely with stereospecific reactions, and the explanations based on molecular orbital theory for the sense of that stereospecificity. The second (Section 5.2) deals with stereoselective reactions, in which a new stereocentre is created selectively under the influence of one or more existing stereocentres or stereochemical features. The way in which a resident stereocentre controls which of two surfaces of a n bond is attacked is also sometimes a question of how the orbitals interact. The stereospecificity that is such a striking feature of pericyclic reactions is covered in the next chapter. 

To achieve control of stereochemistry, understanding is vital, and understanding requires a feeling for all the factors that influence the stereochemistry of organic reactions. We begin with two adjectives, stereoselective and stereospecific, which, with their derived adverbs, are much used and misused. They are defined following Zimmerman below, and used carefully in this book, because the distinction between them is useful. 

Unfortunately, there is a grey area. There are reactions that are, in their fundamental nature, the same as those we call stereospecific, but for which it is not possible to have two stereoisomers either of the starting material or of the product. Thus the addition of bromine to an isolated double bond is stereospeci-fically anti, but the corresponding addition to an acetylene cannot be proved to be stereospecifically anti by the usual criterion, because there is no possibility of having two stereoisomers of an acetylene. The same problem arises for reactions taking place in the opposite direction—in elimination reactions producing acetylenes, one vinyl bromide may react faster than the other, but they both produce the same acetylene. 

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