Regioselectivity, Stereoselectivity & Chemoselectivity: Definitions & Examples Video

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  • 0:03 Details
  • 0:38 Regioselectivity
  • 1:43 Stereoselectivity
  • 2:50 Chemoselectivity
  • 3:43 Lesson Summary
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Lesson Transcript
Instructor: Korry Barnes

Korry has a Ph.D. in organic chemistry and teaches college chemistry courses.

What is the what, where, when, why and how of a chemical reaction? Learn about regioselectivity, stereoselectivity, and chemoselectivity by discussing their definitions and how they apply to organic reactions by looking at specific examples.

Details

Do you know anyone who always wants to know all the little details that go along with any story? This could be a great trait for someone like a lawyer or forensic scientist. You might refer to them as a who, what, where, when, why person.

Sometimes, details can be tedious and tiresome to explain, but they can be helpful or needed, too. Did you know that questions like where, how, and why are also important in the context of organic chemical reactions? Let's learn about some concepts by likening them to the where, how, and why of chemical reactions. Let's get started!

Regioselectivity

Let's get started by talking about the 'where' in a chemical reaction. If there are multiple places a chemical reaction could happen, why one region or atom of the compound over another? Regioselectivity in organic chemistry refers to the favoring of a reagent to bond to one atom over another (think 'region' when you see the prefix 'regio-').

It turns out that chemists can predict with a high degree of accuracy how a reagent will react simply by looking at the nature of the reagent itself and the substrate it is going to ultimately react with.

Example of Regioselectivity

Let's take a look at an example of regioselectivity. In the reaction of propene with hydrochloric acid (HCl), two possible products are possible, both of which are called regioisomers (isomers of a compound with the same atom-makeup but different connectivity of the atoms). It turns out, however, that only one of the products is formed as the major product and only trace amounts of the minor is observed.

We can say that the reaction is thus regioselective in the sense that even though two products are possible, only one of them is actually formed.


Example of a regioselective reaction of propene with hydrochloric acid
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Stereoselectivity

When a reaction is said to be stereoselective it means that a single stereoisomer is formed even though more than one may be possible. Stereoisomers are compounds that have the same chemical formula and the same atom connectivity, but different three-dimensional orientations.

Essentially, a stereoselective reaction always forms a compound with a specific shape and three-dimensional configuration.

Example of Stereoselectivity

Let's now take a look at an example of stereoselectivity. A good example of two reactions that are stereoselective are those of butadiene with two alkenes that contain ester functional groups:


An example of a stereoselective reaction
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Notice that in the top reaction of this grouping, when the ester groups are on the same side of the double bond of the alkene, the sole product is the six-membered ring, in which the ester groups are cis (same side) to one another.

When the ester groups are on opposite sides of the double bond however, note that the six-membered ring product places the esters trans (opposite sides) relative to one another. Both these reactions are stereoselective because they each result in a specific stereoisomer as the products.

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