Regioselectivity is the preferential formation or breaking of chemical bonds in one direction over all possible directions. It results in the formation of regioisomers and one of these regioisomers is formed in excess. Note that, regioselective reactions take place at a specific reaction site. This reaction site is preferred over other reaction sites.
The word regio is derived from the Latin word “regionem” which means direction. Additionally, in a regioselective reaction, only active functional groups take part in a chemical reaction and the reactivity of other functional groups is suppressed.
The products formed in a reaction due to regioselectivity are positional isomers (regioisomers) a type of constitutional isomers. They have the same functional groups attached to different positions on the carbon chain. Regioselective reactions take place without a change in the stereogenic center. They depend upon the reaction site and do not form new chiral centers.
Example of regioselectivity
There are numerous chemical reactions that show regioselectivity. Some important examples are given below:
Hydrobromination of unsymmetrical alkene(HBr regioselectivity)
When an unsymmetrical alkene is treated with HBr in the presence of peroxides we get the anti-markonikov product. According to markonikov’s rule, the electron-rich part goes to carbon having less number of hydrogens but in this example, due to regioselectivity, the predominant product is opposite to makonikov’s rule. Here electron rich part moves to carbon with a higher number of hydrogen.
Elimination reaction also shows regioselectivity. We get one product in excess. For instance, when alkyl halide is treated with sodium ethoxide in an alcoholic medium, it results in the elimination of hydrogen from beta carbon and halogen from alpha carbons. The 2-alkene is the major product whereas 1-alkene is a minor product. This is because according to Zatsiev’s rule, more substituted alkenes are more stable than least substituted alkenes thus, formed in excess.
Oxidation of alkene
In this reaction, we get alcohol as a result of the hydroboration oxidation reaction. In this reaction product is anti markonikov. This is due to regioselectivity. in this reaction, the hydroxyl (OH) group goes to carbon with more hydrogen atoms.
Nucleophilic Addition to enones
Nucleophiles can be added to enones through conjugate addition or direct addition. This addition results in the formation of positional isomers. Here conjugate addition reaction is thermodynamically favored whereas direct addition is kinetically favored. Another advantage of direct addition is these reactions are reversible reactions.
Aerobic oxidative cyclization
This reaction also shows regioselectivity. It is the aerobic oxidative cyclization of aromatic, heteroatomic, and alkenyl acids with alkynes. We get two regioisomers in different ratios as a result of this reaction.
Define regioselective reactions.
A chemical reaction in which one of the reaction sites is preferred over all possible sites is called a regioselective reaction. Hence, we get one isomer in excess as a result of a chemical reaction.
Are elimination reactions are regioselective?
Elimination reactions given by alcohols and alkyl halides are regioselective reactions.
What is the Zaitsev rule (Saytzeff rule)?
According to this rule, the major product of the elimination reaction is more substituted alkenes because these are more substituted alkenes compared to the least substituted alkenes.
How do we identify whether a reaction is regioselective or not?
We can identify regioselective reactions in two ways:
- In regioselective reactions, one reaction site is preferred over all other reaction sites.
- This reaction results in the formation of an unequal mixture of constitutional isomers.
What is Markonikov’s Addition?
According to Markonikov while in addition to unsymmetrical alkenes negative parts go to carbon with less number of hydrogen atoms and positive parts go to the carbon with more number of hydrogen atoms.
What are regioisoemers?
Regioisomers are constitutional isomers with the same molecular formula but different positions of the same functional groups. These are also known as positional isomers.
Why does a regioselective reaction take place?
Because we get a more stable product as a result of regioselectivity. We get one of the products in excess.
What is meant by anti-Markonikov addition?
During the addition of unsymmetrical species to the unsymmetrical alkene, in the presence of peroxide, the electron-rich part goes to carbon with more hydrogen atoms whereas the positive part goes to carbon with less number of hydrogen atoms.
What is the Hofmann Rule?
According to this rule, the major product of the elimination reaction is the least substituted alkene. It means terminal alkenes are more stable than substituted alkenes by using Hoffman’s reagents,
What is the main difference between regioselectivity and stereoselectivity?
The key difference between regioselectivity and stereoselectivity is that the regioselective reaction results in the formation of constitutional isomers, on the other hand, the stereoselective reaction results in the formation of stereoisomers.
- 10th edition of Organic Chemistry by T.W Graham Solomons and Craig B. Fryhle.
- Organic stereochemistry experimental and computational method by Hua-Jie Zhu.