Nissa has a masters degree in chemistry and has taught high school science and college level chemistry.
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.
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.
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.
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.
This meso compound has two chiral centers. Another characteristic of meso compounds is that they need to have at least two chiral centers.
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.
Let's imagine that light passes through an optical filter that polarizes light, or a polarizer, so that the light waves become parallel to one another. If this polarized light interacts with, say, the left side of 2,3-dichlorobutane, it will turn one way. Then, once it interacts with the other half (the right side), which is the mirror image of the left side, it will turn the polarized light back to its original position. The result is no net rotation of the polarized light, which makes the achiral compound optically inactive. Now we can also say that a meso compound is achiral and optically inactive.
Let's go over a few examples of compounds and determine if they can be classified as meso compounds or not. Examples 1 and 2 are classified as meso compounds because they have at least two chiral centers and internal mirror planes, as indicated by the broken lines.
In Example 3, hydrogen (H) and chlorine (Cl) are on the internal mirror plane, so Example 3 is still classified as a meso compound. In Example 4, the mirror plane is horizontal. The same goes with Example 5, where hydrogen (H) and bromine (Br) are on the internal mirror plane, so the area at the top of the mirror plane and at the bottom are mirror images of each other.
We have to be careful of compounds that appear to be meso compounds but are not. If we look carefully at Example 1, we see that this compound has two chiral centers. However, there is no internal mirror plane.
At first glance, Example 2 looks like a meso compound, but when we try to locate the chiral centers, we find there are none. This is because the carbon atoms in the middle do not have four unique substituents - there are two hydrogen atoms attached to these carbons, so the substituents are not unique.
Meso compounds are compounds that have an internal mirror plane, or a plane of symmetry, and at least two chiral centers. The chiral centers should have four unique substituents. Meso compounds are symmetric, so one side and the other of the mirror plane are mirror images of each other. Meso compounds have superimposable mirror images, so this makes them achiral compounds, and therefore, optically inactive. A compound is optically inactive when polarized light interacts with it, and there is no resulting net rotation.
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