Stereochemistry electrophilic addition reactions

Electrophilic Addition Reactions, Stereochemistry, and Electron Delocalization... 

The Lead-Off Reaction Addition of HBr to Alkenes Students usually attach great-importance to a text s lead-off reaction because it is the first reaction they see and is discussed in such detail. 1 use the addition of HBr to an alkene as the lead-off to illustrate general principles of organic chemistry for several reasons the reaction is relatively straightforward it involves a common but important functional group no prior knowledge of stereochemistry or kinetics in needed to understand it and, most important, it is a polar reaction. As such, 1 believe that electrophilic addition reactions represent a much more useful and realistic introduction to functional-group chemistry than a lead-off such as radical alkane chlorination. 

Figure 6.1. Summary of the most common electrophilic addition reactions of olefins. In each case, the olefin reacts as a Lewis base. All reactions are regioselective. The overall stereochemistry is (a) stereospecific anti (b) stereospecific syn (c) not stereospecific, in general.

The word mechanism will often be used loosely here. In contrast to the S l reaction of alkyl halides or the electrophilic addition reactions of alkenes, the details of some of the mechanisms presented in Chapter 12 are known with less certainty. For example, although the identity of a particular intermediate might be confirmed by experiment, other details of the mechanism are suggested by the structure or stereochemistry of the final product. 

In Chapter 4 we saw that aUcenes undergo electrophilic addition reactions, and we looked at the different kinds of reagents that add to alkenes. We also examined the step-by-step process by which each reaction occurs (the mechanism of the reaction), and we determined what products are formed. However, we did not consider the stereochemistry of the reactions. 

Before we examine the stereochemistry of electrophilic addition reactions, we need to become familiar with some terms used in describing the stereochemistry of a reaction. 

Section 5.19 Stereochemistry of Electrophilic Addition Reactions of Alkenes... 

Now that you are familiar with electrophilic addition reactions and with stereoisomers, we can combine the two topics and look at the stereochemistry of electrophilic addition reactions. In other words, we will look at the stereoisomers that are formed in the electrophilic addition reactions that were discussed in Chapter 4. 

The reactions of organic compounds can be divided into three main types addition reactions, substitution reactions, and elimination reactions. The particular type of reaction a compound undergoes depends on the functional group in the compound. Part 2 discusses the reactions of compounds whose functional group is a carbon-carbon double bond or a carbon-carbon triple bond. We will see that these compounds undergo addition reactions, or, more precisely, electrophilic addition reactions. Part 2 also examines stereochemistry, thermodynamics and kinetics, and electron delocalization—topics that can be important when trying to determine the outcome of a reaction. 

We will begin our discussion of the stereochemistry of electrophilic addition reactions by looking at reactions that form a product with one asyimnetric center. Then we will look at reactions that form a product with two asyimnetric centers. 

The stereochemistry of the products obtained from electrophilic addition reactions to alkenes is summarized in Table 6.1. 

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