Difference Between Stereospecific and Stereoselective Reactions

Main Difference – Stereospecific vs Stereoselective Reactions

The terms Stereospecific and Stereoselective are used to name chemical reactions involving organic compounds that form products with different atomic arrangements. A stereospecific reaction is a reaction in which the stereochemistry of the reactant completely determines the stereochemistry of the product without any other option. A stereoselective reaction is a reaction in which there is a choice of pathway, but the product stereoisomer is formed due to its reaction pathway being more favorable than the others available. The main difference between stereospecific and stereoselective reactions is that a stereospecific reaction gives one specific product whereas stereoselective reaction gives multiple products.

Key Areas Covered

1. What are Stereospecific Reactions
     – Definition, Explanation with Examples
2. What are Stereoselective Reactions
     – Definition, Factors Affecting the Final Product Formation
3. What is the Difference Between Stereospecific and Stereoselective Reactions
     – Comparison of Key Differences

Key Terms: Diastereoselective Reaction, Enantioselective Reaction, Stereochemistry, Stereoisomer, Stereoselective Reactions, Stereospecific Reactions, Steric Effect

What are Stereospecific Reactions

A stereospecific reaction is a reaction where the stereochemistry of the reactant completely determines the stereochemistry of the product without any other option. Therefore, a stereospecific reaction specifies the final product given by a particular reactant. From a stereoisomerically pure reactant, a stereospecific reaction gives 100% of a specific isomer. Let us consider an example in order to understand this concept.

Figure 1: Stereospecific Carbene Addition to an Olefin

Above image shows stereospecific dibromocarbene addition to an olefin. Olefins are alkenes. This reaction is stereospecific because the geometry of the alkene is preserved in the product. Cis reactant gives the cis-2,3-dimethyl-1,1-dibromocyclopropane whereas trans reactant gives trans cyclopropane. These are stereospecific addition reactions.

What are Stereoselective Reactions

A stereoselective reaction is a reaction where there is a choice of pathway, but the product stereoisomer is formed due to its reaction pathway being more favorable than the others available. The reactants in these reactions give unequal mixtures of stereoisomers as final products.

The selectivity of the reaction pathway depends on differences in steric effects (presence of bulky groups cause steric hindrance) and electronic effects. These effects lead to the formation of different products. There are two types of stereoselective reactions: enantioselective reaction and diastereoselective reaction. Enantioselective reactions are reactions that form enantiomers. Diastereoselective reactions are reactions that form diastereomers. In order to understand the concept of stereoselective, let us consider an example.

Figure 2: Stereoselective Dehalogenation

Dehalogenation is the removal of a halogen atom. The above image shows a reaction of dehalogenation that takes place as a stereoselective reaction. It gives multiple products. But the trans isomer is given about 60% since it has the least steric effects and electronic effects.

Difference Between Stereospecific and Stereoselective Reactions

Definition

Stereospecific Reactions: A stereospecific reaction is a reaction in which the stereochemistry of the reactant completely determines the stereochemistry of the product without any other option.

Stereoselective Reactions: A stereoselective reaction is a reaction in which there is a choice of pathway, but the product stereoisomer is formed due to its reaction pathway being more favorable than the others available.

Number of Products

Stereospecific Reactions: A stereospecific reaction gives a specific product from a certain reactant.

Stereoselective Reactions: A stereoselective reaction can result in multiple products.

Effects

Stereospecific Reactions: The final product of a stereospecific reaction depends on the stereochemistry of the reactant.

Stereoselective Reactions: The selectivity of the reaction pathway depends on differences in steric effects (presence of bulky groups cause steric hindrance) and electronic effects.

Conclusion

Stereospecific and stereoselective reactions are two types of reaction that can be found in organic chemistry. The main difference between stereospecific and stereoselective reactions is that a stereospecific reaction gives one specific product whereas stereoselective reaction gives multiple products.

Reference:

1. “Stereospecificity.” Wikipedia, Wikimedia Foundation, 13 Jan. 2018, Available here.
2. “Stereoselectivity.” Wikipedia, Wikimedia Foundation, 14 Jan. 2018, Available here.
3. “Stereoselectivity.” OChemPal, Available here.
4. “Virtual Chemistry .” Stereospecific and Stereoselective Reactions, University of Oxford, Available here.

Image Courtesy:

1. “Stereospecific carbene reaction” By V8rik – en:Image:Stereospecific_carbene_reaction.gif (CC BY-SA 3.0) via Commons Wikimedia
2. “Stereoselectivedehalogenation” By Dissolution – Own work (CC BY-SA 3.0) via Commons Wikimedia