Meso Compound: Definition & Examples

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  • 0:05 Meso Compounds
  • 1:32 Chiral Centers
  • 2:23 Achiral Compounds
  • 4:07 Examples
  • 5:26 Lesson Summary
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Lesson Transcript
Instructor: Nissa Garcia

Nissa has a masters degree in chemistry and has taught high school science and college level chemistry.

In chemistry, there are compounds that are symmetric when you divide them down the middle. These compounds are called meso compounds. In this lesson, we'll learn about meso compounds and how to identify them.

Meso Compounds

There are many structures and objects around us that are symmetric, like buildings, houses, and even our faces. This smiley face is symmetric, meaning we can draw a line of symmetry that bisects the smiley face in half. When we do that, the left and right sides are mirror images of each other, so the smiley face has an internal mirror plane or a plane of symmetry.

Smiley Face

Just like this smiley face, there are chemical compounds whose structures are symmetric - their left and right sides are mirror images of each other. These compounds are called meso compounds. Let's take a look at the compound 2,3-dichlorobutane, which is classified as a meso compound. Meso compounds are symmetric compounds that have an internal mirror plane, so that the left and right side of the plane are mirror images of each other.


Dichlorobutane Diagram


This diagram includes broken lines and thicker lines that have a wedge shape. What do these lines mean? Well, the broken lines emminating from the hydrogen (H) atoms mean that the hydrogen atoms bonded to the carbon (C) atoms are moving away from you, or into the page. The wedge-shaped line, which shows the chlorine (Cl) atoms bonded to the carbon atoms, indicate that the chlorine atoms bonded to the carbon atoms are moving towards you, or off of the page.

Chiral Centers

Let's look again at 2,3-dichlorobutane, specifically the two carbon (C) atoms labeled 1 and 2. The first carbon (C1) has four different substituents attached to it, namely, a hydrogen (H) atom, a chlorine (Cl) atom, a -CH3 group, and the second carbon (C2) atom. The first carbon atom is a chiral center, which is an atom that has four different substituents attached to it. We can also say that the second carbon (C2) is also a chiral center because it has four different substituents.


Chiral Centers


This meso compound has two chiral centers. Another characteristic of meso compounds is that they need to have at least two chiral centers.

Achiral Compounds

In the mirror images of 2,3-dichlorobutane, when we imagine these two mirror images merging together and placed on top of each other, they look exactly the same, so we can say that 2,3-dichlorobutane is superimposable on its own mirror image.

An achiral compound is a compound that can be superimposed on its own mirror image, so a meso compound is achiral. When we think about plain blue jeans, achiral compounds are just like them: if we lay out two pairs of identical blue jeans and try to put them on top of each other, they will overlap perfectly and look exactly the same.

When polarized light passes through an achiral compound, no net rotation of polarized occurs, so achiral compounds are optically inactive. As shown here, polarized light passes through the achiral compound, 2,3-dichlorobutane.


Optically Inactive Compound


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