Difference Between Stereospecific and Stereoselective Reactions

Key Difference – Stereospecific vs Stereoselective Reactions
 

The key difference between stereospecific and stereoselective reactions is that, in stereospecific reactions, different stereospecific reactants give different stereoisomer of the product under ideal conditions (product is specific to the stereoisomer of the reactant), whereas in stereoselective reactions, a single reactant may give different types of stereoisomers.

Stereochemistry is the part of chemistry that deals with the three-dimensional structures of the molecules. Stereochemical reactions are classified into two groups as stereospecific and stereoselective, based on the stereochemistry of the product. These products are called stereoisomers.

CONTENTS

1. Overview and Key Difference
2. What are Stereospecific Reactions
3. What are Stereoselective Reactions
4. Side by Side Comparison – Stereospecific vs Stereoselective Reactions in Tabular Form
5. Summary

What are Stereospecific Reactions?

In a stereospecific reaction, each stereoisomeric reactant produces a different stereoisomeric product or a different set of stereoisomeric products. All stereospecific reactions are essentially stereoselective, but stereoselective reactions are not certainly stereospecific. Examples of stereospecific reactions include trans-addition of bromine to (E)- and (Z) alkenes, electrocyclic reactions like disrotatory ring closures, cheletropic syn-addition of singlet carbenes to alkenes, and the sigmatropic Claisen rearrangement of the cis- and trans- isomers of (4S)-vinyloxypent-2-enes.

Figure 1: Stereospecificity Electrocyclic Ring Opening

In all these reactions, stereoisomeric substrates are converted to stereoisomeric products. It is not mandatory for a reaction to be 100% stereospecific. If a reaction produces a mixture of two different stereoisomers in the ratio of 80:20, the reaction is then called 80% stereospecific.

What are Stereoselective Reactions?

In stereoselective reactions, a single reactant gives two or more steroisomeric products, and one product is more prominent than the other product or products. The stereoselective reactions can be described as being moderately stereoselective, highly stereoselective, or completely stereoselective based on the degree of preference for a specific stereoisomer.

Figure 02: D-A stereoselectivity

Stereoselective reactions take place during the addition of formic acid to norbornene, diastereoselective reduction of 4-tert-butylcyclohexanone with lithum aluminum hydride, and enantioselective alkylation of benzaldehyde with organozinc reagents in the presence of (1R, 2S)-N,N-dibutylnorephedrine as the catalyst.

What is the Difference Between Stereospecific and Stereoselective Reactions?

Summary – Stereospecific vs Stereoselective Reactions

The terms of stereoselective and stereospecific reactions are assigned by observing the 3D structure of the stereoisomers in stereochemical reactions. In stereospecific reactions, each stereoisomeric reactant produces a different stereoisomeric product, whereas, in stereoselective reactions, a single reactant can produce two or more different stereoisomeric products. This is the difference between stereospecific and stereoselective reactions.

Reference:

1. Wolf, Christian. Dynamic Stereochemistry of Chiral Compounds: Principles and Applications. RSC Publishing, 2008.
2. Anslyn, Eric V., and Dennis A. Dougherty. Modern Physical Organic Chemistry. University Science, 2006.
3. Bruice, Paula Yurkanis. Essential Organic Chemistry. Pearson Education, 2006.
4. Carey, Francis A., and Richard J. Sundberg. Advanced Organic Chemistry Part A: Structure and Mechanisms. Springer, 2007.

Image Courtesy:

1. “Stereospecificity electrocyclic ring opening1” By Ivogt – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “D-A stereoselectivity” By arnomba – self-prepared (CC BY-SA 3.0) via Commons Wikimedia

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