Difference Between Regiochemistry and Stereochemistry

Main Difference – Regiochemistry vs Stereochemistry

Regiochemistry and stereochemistry are two specific branches of chemistry. Regiochemistry is the chemistry of regioselective reactions. It is a term that describes how a chemical reaction takes place. Regioselectivity is the preference of one direction of chemical bond making or breaking over all other possible directions. Stereochemistry, on the other hand, is a branch of chemistry that involves the study of spatial arrangements of organic molecules. Stereochemistry describes the arrangement of stereoisomers. The main difference between regiochemistry and stereochemistry is that regiochemistry describes the atomic arrangement of the final product of a chemical reaction whereas stereochemistry describes the atomic arrangement of molecules and their manipulation.

Key Areas Covered

1. What is Regiochemistry
      – Definition, Markovnikov’s Rule and Anti Markovnikov’s Rule
2. What is Stereochemistry
     – Definition, Stereoisomers, Geometric Isomers, Optical Isomers, Chirality
3. What is the Difference Between Regiochemistry and Stereochemistry
     – Comparison of Key Differences

Key Terms: Cis Isomers, Geometric Isomers, Isomers, Regiochemistry, Regioselectivity, Stereochemistry, Stereoisomers, Trans Isomers

What is Regiochemistry

Regiochemistry is the branch of chemistry which explains the regioselectivity of chemical reactions. Regioselectivity is the preference for one orientation over all other possible orientations in the arrangement of a reaction product.

Regiochemistry indicates which product is the major product and which one is the minor product in a chemical reaction that gives multiple products. This depends on the possible positions of the target molecule to which the reagent molecules are going to be added. For example, in a substituted benzene ring, the reagent molecule can get attached to one of the three possible positions, ortho, para and meta positions, depending on the substituent that is already present in the benzene ring.

Figure 1: Toluene Chlorination is Regioselective

The above reaction shows the chlorination of toluene. There are several possible positions for the chlorine atom to get attached to the toluene molecule. But the para substitution is the most stable among all. Therefore it is the major product given by this reaction.

Several rules have been introduced in order to determine what would be the major product of a certain chemical reaction. The very first rule is Markovnikov’s rule. According to Markovnikov Rule, the proton is added to the carbon atom that has the highest number of hydrogen atoms attached to it in addition reactions of alkenes or alkynes. This rule helps to predict the end product of a certain chemical reaction.

However, according to Anti Markovnikov Rule, which was later introduced, in addition reactions of alkenes or alkynes, the proton is added to the carbon atom that has the least number of hydrogen atoms attached to it. The end product obtained from this reaction is called Anti Markovnikov product. This mechanism does not involve the formation of a carbocation intermediate. Chemical reactions can be made into reactions that give the Anti Markovnikov product by adding a peroxide such as HOOH to the reaction mixture.

Some other rules regarding Regiochemistry include Fürst-Plattner rule for the addition reactions of nucleophiles, Baldwin’s rule for the Regioselectivity of ring-closure reactions, etc.

What is Stereochemistry

Stereochemistry is a branch of chemistry that involves the study of spatial arrangements of organic molecules and their manipulation. It involves the study of stereoisomers. Stereoisomers are molecules having the same molecular formula and the atomic arrangement, but different spatial arrangements. The two main groups of stereoisomers are:

• Geometric isomers
• Optical isomers

Geometric isomers are also known as cis-trans isomers. These isomers always occur in pairs. The two isomers are the cis-isomer and the trans-isomer. These isomers occur in molecules having double bonds. The attachment of a functional group to the vinylic carbon atom is the difference between these two isomers. (Vinylic carbon is the carbon atom having a double bond with another carbon atom.)

Figure 2: Geometric Isomerism

In addition, stereochemistry also describes the concept of chirality. Chirality is the property of a molecule that says its mirror image is non-superimposable with the molecule. A chiral carbon is an asymmetric carbon. A carbon atom can have a maximum of four bonds. The chiral carbon is bonded to four different groups and is asymmetric. The carbon atom should always be sp³ hybridized in order to be a chiral carbon. A chiral molecule normally contains at least one chiral carbon. sp or sp2 hybridized carbon atoms cannot be chiral because they cannot have four different groups around them due to the presence of π-bonds. Optical isomers occur in molecules with a chiral carbon. This chiral carbon causes the occurrence of a stereoisomer, which is the non-superimposable mirror image of that molecule.

Difference Between Regiochemistry and Stereochemistry

Definition

Regiochemistry: Regiochemistry is the branch of chemistry which explains the regioselectivity of chemical reactions.

Stereochemistry: Stereochemistry is a branch of chemistry that involve the study of spatial arrangements of organic molecules and their manipulation.

Focus

Regiochemistry: Regiochemistry explains the rules used to determine the end products of certain chemical reactions.

Stereochemistry: Stereochemistry explains the atomic arrangements of different stereoisomers.

Specifications

Regiochemistry: Regiochemistry includes important rules such as Markovnikov rule, Anti Markovnikov rule, Fürst-Plattner rule, Baldwin’s rule, etc.

Stereochemistry: Stereochemistry includes geometric isomers, optical isomers and chirality of molecules.

Conclusion

Regiochemistry and stereochemistry are two important subcategories of chemistry. The main difference between regiochemistry and stereochemistry is that regiochemistry describes the atomic arrangement of the final product of a chemical reaction whereas stereochemistry describes the atomic arrangement of molecules and their manipulation.

Reference:

1. “Regioselectivity.” Wikipedia, Wikimedia Foundation, 13 Jan. 2018, Available here.

Image Courtesy:

1. “Regioselectivity toluene chlorination” By Mfomich – Own work (CC0) via Commons Wikimedia
2. “Cis-trans example” By JaGa – Self-made using BKChem and Inkscape (CC BY-SA 3.0) via Commons Wikimedia