The key difference between regioselectivity and stereoselectivity is that the regioselectivity refers to the formation of one positional isomer over another. Meanwhile, stereoselectivity refers to the formation of one stereoisomer over another.
The terms regioselectivity and stereoselectivity are very important in organic synthesis. These terms describe the structure of the final product of chemical reactions. The branch of chemistry that studies regioselectivity is known as regiochemistry while the branch of chemistry that studies stereoselectivity is known as stereochemistry.
CONTENTS
1. Overview and Key Difference
2. What is Regioselectivity
3. What is Stereoselectivity
4. Side by Side Comparison – Regioselectivity vs Stereoselectivity in Tabular Form
5. Summary
What is Regioselectivity?
Regioselectivity refers to the making or breaking of chemical bonds in one direction over all other directions that are possible. This concept can be applied for determining the position of a chemical compound to which a reagent can affect, i.e. protons affected by a strong base.
Figure 01: Halohydrin Formation Reaction Shows Regioselectivity
The term regioselectivity arises from the combination of two chemical concepts that take place in the same chemical reaction; “regio + selective” means the formation of the positional isomer (or constitutional isomer) is preferred (or selective). All reactions involving regioselectivity give a mixture of constitutional isomers with a major component and a minor component. However, sometimes the minor product is not detectable because it is formed in a very small amount.
What is Stereoselectivity?
Stereoselectivity refers to the formation of an unequal mixture of stereoisomers during a chemical reaction. It simply means that the reaction gives one stereoisomer over another as the final product of the reaction. Also, this type of reactions occurs on two different occasions. One is during the non-stereospecific creation of a new stereocenter, and the other is during the non-stereospecific transformation of an existing stereocenter. This selectivity happens due to steric effects and electronic effects.
Stereoselectivity may vary in different reactions but none of the reactions form one total stereoisomer; all these reactions give a mixture of stereoisomers with a major component and a minor component. However, sometimes the minor product is not detectable because it is formed in a very little amount.
There are different types of stereoselectivity.
Enantioselective – one enantiomer forms over other isomers; a chiral molecule forms from an achiral molecule, degree of selectivity is measured from enantiomeric excess
Diastereoselective – one diastereomer forms over the other isomer; one or more chiral centers forms from either an achiral center or a preexisting chiral center, the degree of selectivity can be measured from diastereomeric excess
Stereoconvergence – opposite of the stereoselectivity in which two different stereoisomers form a single stereoisomer
What is the Difference Between Regioselectivity and Stereoselectivity?
The terms regioselectivity and stereoselectivity are very important in organic synthesis. The key difference between regioselectivity and stereoselectivity is that regioselectivity refers to the formation of one positional isomer over another, whereas stereoselectivity refers to the formation of one stereoisomer over another. Therefore, regioselectivity forms a positional or a constitutional isomer while stereoselectivity forms a stereoisomer. However, both pathways form the other isomer as a minor product along with the preferred isomer, which forms as the major product. Besides, the study of regioselectivity comes under regiochemistry, whereas the study of stereoselectivity comes under stereochemistry.
Below infographic summarizes the difference between regioselectivity and stereoselectivity.
Summary – Regioselectivity vs Stereoselectivity
The terms regioselectivity and stereoselectivity are very important in organic synthesis. The key difference between Regioselectivity and Stereoselectivity is that regioselectivity refers to the formation of one positional isomer over another, whereas stereoselectivity refers to the formation of one stereoisomer over another. The study of regioselectivity comes under regiochemistry, whereas the study of stereoselectivity comes under stereochemistry.
Reference:
1. Hunt, Ian R. “Ch 6 : Regio- and Stereoselectivity.” Ucalgary, Available here.
2. “Stereoselective.” Chemistry LibreTexts, Libretexts, 5 June 2019, Available here.
3. “Stereoselectivity.” Wikipedia, Wikimedia Foundation, 17 Sept. 2019, Available here.
Image Courtesy:
1. “RegioselectivityInhalohydrinformation” By V8rik at English Wikipedia (CC BY-SA 3.0) via Commons Wikimedia
2. “Stereoselectivedehalogenation” By Dissolution – Own work (CC BY-SA 3.0) via Commons Wikimedia