Regiochemistry

How would we describe Regiochemistry?

The term “regiochemistry” refers to the area of a reaction where changes occur. It could also be a clue as to how the reaction occurred.

The solution is simple in aliphatic nucleophilic substitution: the reaction occurs at the electrophilic carbon, which is linked to the electronegative halogen. That is where the departing group is located. The nucleophile will be there when the departing group is replaced. However, this isn’t always the case.

• The nucleophile is usually present on the carbon where the leaving group used to be in an S_N2 reaction.
• The nucleophile in an S_N1 reaction is frequently found on the carbon where the leaving group was previously located. It moves from time to time.

Table of Contents

• Regiochemistry vs. Stereochemistry: What’s the Difference?
• Regioselectivity
• Markovnikov’s Rule
• The Diels-Alder Reaction Regiochemistry
• Frequently Asked Questions

Regiochemistry vs. Stereochemistry: What’s the Difference?

The areas of chemistry known as regiochemistry and stereochemistry are two distinct branches of chemistry. The chemistry of regioselective processes is known as regiochemistry. It’s a word that describes the process of a chemical reaction. Regioselectivity refers to a preference for one path of chemical bond formation or breakage over all others.

Stereochemistry, on the other hand, is a discipline of chemistry concerned with the study of organic molecules’ spatial configurations. Stereochemistry is the study of how stereoisomers are arranged.

The major distinction between regiochemistry and stereochemistry is that regiochemistry refers to the atomic organisation of a chemical reaction’s end product, whereas stereochemistry refers to the atomic arrangement of molecules and their manipulation.

What is Regiochemistry?

Regiochemistry is the discipline of chemistry that explains chemical processes’ regioselectivity. In a chemical reaction with many products, it identifies which one is the major product and which one is the minor product. This is dependent on the target molecule’s probable sites to which the reagent molecules will be attached. In a substituted benzene ring, for example, the reagent molecule can attach to one of three potential places, ortho, para, or meta, depending on the substituent already present in the benzene ring.

The chlorination of toluene is seen in the above reaction. The chlorine atom can bind to the toluene molecule in a number of different places. The para replacement, on the other hand, is the most reliable. As a result, it is the main product of this reaction.

Several rules have been established to determine what the principal product of a chemical reaction will be. Markovnikov’s rule is the very first rule.

What is Stereochemistry?

Stereochemistry is a discipline of chemistry that studies and manipulates the spatial configurations of organic molecules. It entails the investigation of stereoisomers. Stereoisomers are molecules that have the same chemical formula and atomic arrangement but differ spatially. Stereoisomers are divided into two categories:

1. Geometric isomers
2. Optical isomers

Cis-trans isomers are another name for geometric isomers. These isomers are always found together. The cis-isomer and the trans-isomer are the two isomers. The distinction between these two isomers is the attachment of a functional group to the vinylic carbon atom. (A carbon atom with a double bond to another carbon atom is called vinyl carbon.)

In molecules with chiral carbon, optical isomers occur. This chiral carbon results in the formation of a stereoisomer, which is the molecule’s non-superimposable mirror image.

Difference Between Regiochemistry and Stereochemistry

Further Reading:

Stereochemistry

Regioselectivity

The preference of chemical bonding or breaking in one direction over all other potential orientations is referred to as regioselectivity in chemistry. It can refer to any of the numerous possible positions that a reagent can affect, such as which proton a strong base will extract from an organic molecule or where an additional substituent will be applied to a substituted benzene ring.

• In the presence of water, the synthesis of halohydrin from an alkene is regioselective.
• This reaction follows the Markovnikov pathway and proceeds with anti stereospecificity.
• The nucleophile is connected to the more substituted carbon and the electrophile to the less substituted carbon.

A halohydrin production from an alkene is an example:

Bromine works as an electrophile in the given case, whereas the OH group from water acts as a nucleophile.

Further Reading:

Synthesis of Halohydrin

Markovnikov’s Rule

The result of some chemical addition reactions can be described using Markovnikov’s Rule, also known as Markownikoff’s Rule. This rule was initially proposed in 1865 by Russian scientist Vladimir Vasilyevich Markovnikov. Markovnikov’s rule is an empirical rule for predicting the regioselectivity of alkene and alkyne electrophilic addition processes.

When a protic acid (HX) is introduced to an asymmetric alkene, the acidic hydrogen binds to the carbon with the most hydrogen substituents, while the halide group binds to the carbon with the most alkyl substituents.

“Hydrogen is added to the carbon with the most hydrogens, and halide is added to the carbon with the least hydrogens,” as a simplified version of the rule goes.

The addition of hydrobromic acid (HBr) to propene, as shown below, is an example of a reaction that follows Markovnikov’s rule.

The reaction depicted above shows that the bulk of the products generated follow Markovnikov’s rule, while the minority do not.

It’s worth noting that Markovnikov’s rule was designed especially for use in the addition reaction of hydrogen halides to alkenes. Based on the regioselectivity of the reaction, Anti-Markovnikov addition reactions are the opposite of ‘Markovnikov’ addition reactions.

Further Reading:

Examples of Markovnikov and Anti-Markovnikov Addition Reactions

The Diels-Alder Reaction Regiochemistry

Two regioisomers are possible when non-symmetrical dienes combine with non-symmetrical dienophiles. Dienes with substituents on the terminal (“1-substituted dienes”) tend to produce “1,2” (ortho) products. Dienes having substituents in the second position (“2-substituted dienes”) are more likely to produce the “1,4” product (“para”). “1,3” products (“meta”) are often minor byproducts.

The German chemists Otto Diels and Kurt Alder discovered this reaction in 1928, for which they were awarded the Nobel Prize in Chemistry in 1950. Because two new carbon-carbon bonds are formed simultaneously, the Diels-Alder process can be employed to produce six-membered rings.

Two pi bonds have been changed into two sigma bonds, as can be seen in the image above. The coordinated bonding of two distinct pi-electron systems causes this. The diene’s four pi electrons and the dienophile’s two pi electrons are shifted in the Diels-Alder process.

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Conformational Stereoisomerism

Further Reading:

Mechanism of Diels-Alder Reaction