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What Is a Stereospecific Chemical Reaction?

Stereoisomers are molecules identical in all ways save one: their atoms are found in different 3-D arrangements. For example, 1,2-dichloroethene has two stereoisomers -- one where the two chlorine atoms are on the same side of the double bond and one where they are on opposite sides. When chemists are trying to make only one of two or more possible stereoisomers, they often make use of stereospecific reactions, which yield only one stereoisomer as a product.

Stereospecificity
- In a stereospecific reaction, the stereochemistry of the reactant dictates the stereochemistry of the product. In other words, for a given set of reactants, only one stereoisomer is formed in the reaction. This is different from a stereoselective reaction, where multiple stereoisomers are produced but some of them form in greater quantities than others. To remember this distinction, just think about the last part of each word; a stereospecific reaction specifies what the product will be, whereas a stereoselective reaction merely selects for one of several possible products.
Sn2
- Sn2 reactions are a common example. When an Sn2 reaction takes place, an electron-rich reactant or nucleophile attacks an electron-poor or electrophilic carbon in a second reactant, kicking out an atom or group from the second reactant in the process. The group that gets kicked out is called the leaving group. The nucleophile attacks the electrophilic carbon atom from the side opposite the leaving group, and the arrangement of the atoms around this electrophilic carbon becomes inverted in the process -- a little like blowing an umbrella inside-out.
Mechanisms
- The mechanism of most Sn2 reactions ensures that where more than one stereoisomer is possible as a product, only one will be formed. When the hydrogen sulfide ion reacts with R-2-bromooctane, for example, S-2-octanethiol is the only product. There are other kinds of stereospecific reactions, but all of them share this feature in common. The mechanism or sequence of events that takes place during the reaction leads directly to a specific stereoisomer as the product.
Uses
- Chemists often confront situations where they need to synthesize one of several possible stereoisomers. Many amino acids, for example, have two possible stereoisomers, but nature makes use of only one. To make amino acids synthetically, then, chemists usually need to either make a mixture of the two different stereoisomeric forms and separate them, which reduces the yield, or use stereospecific/stereoselective reactions. Stereospecific reactions are also important in the manufacture of various drugs.

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