The key difference between partial and absolute asymmetric synthesis is that partial asymmetric synthesis is the creation of less favorable chirality in symmetrical molecules, whereas absolute asymmetric synthesis is the creation of preferential chirality in a symmetrical environment from symmetric reagent.
Asymmetric synthesis is a type of chemical synthesis where a chemical reaction takes place, forming one or more new elements of chirality form in a substrate molecule. We can find this term in two forms as partial asymmetric synthesis and absolute asymmetric synthesis. However, the term partial asymmetric synthesis is rarely used in chemistry.
CONTENTS
1. Overview and Key Difference
2. What is Asymmetric Synthesis
3. What is Partial Asymmetric Synthesis
4. What is Absolute Asymmetric Synthesis
5. Side by Side Comparison – Partial vs Absolute Asymmetric Synthesis in Tabular Form
6. Summary
What is Asymmetric Synthesis?
Asymmetric synthesis, also known as stereoselective synthesis, is a chemical reaction or a reaction sequence where one or more new elements of chirality formation are observable in a specific substrate molecule. This forms stereoisomeric (specifically enantiomeric or diastereoisomeric) products that are unequal in amounts. This synthesis method is important in producing a specific enantiomer from achiral compounds or racemic mixtures. There are three forms of asymmetric synthesis: absolute synthesis, partial asymmetric synthesis and enantio specific synthesis.
This type of chemical reaction is a result of the influence that comes from some dissymmetry in the reacting system, e.g. the presence of a dissymmetry center in the molecule, presence of a dissymmetric solvent or a catalyst, presence of circularly polarized light, etc. Often, we can classify the asymmetric synthesis reactions as stereoselective reactions. Here, if one of the products form exclusively, then we call the reaction a stereospecific reaction.
What is Partial Asymmetric Synthesis?
Partial asymmetric synthesis is a chemical reaction that involves the creation of less favorable chirality in symmetrical molecules. This term is rarely used in chemistry because this type of chemical reaction has less significance compared to absolute asymmetric synthesis.
Figure 01: Chirality in a Simple Diagram
For example, for partial asymmetric synthesis, we can give the reaction of optically active styrene oxide with triethyl alpha-phosphonopropionate, which gives 2-phenyl-1-methylcyclopropanecarboxylate, which is chiral. This resultant product has one asymmetric center which is originating from styrene oxide.
What is Absolute Asymmetric Synthesis?
Absolute asymmetric synthesis is a chemical reaction that includes the creation of preferential chirality in a symmetrical environment from a symmetric reagent. For example, if we can prepare 2-hydroxypropanenitrile from ethanal and hydrogen cyanide in the absence of other chiral reagents, it gives an excess of one enantiomer over the other.
What is the Difference Between Partial and Absolute Asymmetric Synthesis?
The terms absolute and partial synthesis are mainly used in the asymmetric synthesis, where chirality is created in substrate molecules of a chemical reaction. The key difference between partial and absolute asymmetric synthesis is that partial asymmetric synthesis is the creation of less favorable chirality in symmetrical molecules, whereas absolute asymmetric synthesis is the creation of preferential chirality in a symmetrical environment from symmetric reagent.
The below infographic lists the differences between partial and absolute asymmetric synthesis in tabular form.
Summary – Partial vs Absolute Asymmetric Synthesis
Asymmetric synthesis is a type of chemical synthesis where a chemical reaction takes place forming one or more new elements of chirality form in a substrate molecule. There are two forms as absolute and partial asymmetric synthesis. The key difference between partial and absolute asymmetric synthesis is that partial asymmetric synthesis is the creation of less favorable chirality in symmetrical molecules whereas absolute asymmetric synthesis is the creation of preferential chirality in a symmetrical environment from symmetric reagent.
Reference:
1. “Asymmetric Synthesis.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., Available here.
2. “19.10: Asymmetric Synthesis.” Chemistry LibreTexts, Libretexts, 5 Mar. 2021, Available here.
3. “Asymmetric Synthesis.” SlideShare, Available here.
Image Courtesy:
1. “Chirality with hands” By Original:UnknownVector: — πϵρήλιο – Chirality with hands.jpg (Public Domain) via Commons Wikimedia