Stereochemistry of Reactions: Regioselective, Stereoselective, and Stereospecific Reactions
In Chapter 4 we saw that alkenes 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.
Stereochemistry is the field of chemistry that deals with the structures of molecules in three dimensions. When we study the stereochemistry of a reaction, we are concerned with the following questions:
- If a reaction product can exist as two or more stereoisomers, does the reaction produce a single stereoisomer, a set of particular stereoisomers, or all possible stereoisomers?
- If stereoisomers are possible for the reactant, do all stereoisomers react to form the same stereoisomeric product, or does each reactant form a different stereoisomer or a different set of stereoisomers?
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.
a regioselective reaction is one in which two constitutional isomers can be obtained as products but more of one is obtained than of the other. In other words, a regioselective reaction selects for a particular constitutional isomer. Recall that a reaction can be moderately regioselective, highly regioselective, or completely regioselective depending on the relative amounts of the constitutional isomers formed in the reaction.
Stereoselective is a similar term, but it refers to the preferential formation of a stereoisomer rather than a constitutional isomer. If a reaction that generates a carbon– carbon double bond or an asymmetric carbon in a product forms one stereoisomer preferentially over another, it is a stereoselective reaction. In other words, it selects for a particular stereoisomer. Depending on the degree of preference for a particular stereoisomer, a reaction can be described as being moderately stereoselective, highly stereoselective, or completely stereoselective.
A reaction is stereospecific if the reactant can exist as stereoisomers and each stereoisomeric reactant leads to a different stereoisomeric product or a different set of stereoisomeric products.
In the preceding reaction, stereoisomer A forms stereoisomer B but does not form D, so the reaction is stereoselective in addition to being stereospecific. All stereospecific reactions, therefore, are also stereoselective. All stereoselective reactions are not stereospecific, however, because there are stereoselective reactions in which the reactant does not have a carbon–carbon double bond or an asymmetric carbon, so it cannot exist as stereoisomers.