Chapter 3: Stereoisomerism and Chirality
Table of contents of main topics:
How to identify chiral centers?
Chiral center (also called stereogenic center, chiral carbon and stereo center) is a carbon that is attached to four different groups. In order to identify whether there are four different groups, we must go atom by atom until we find a difference. If no difference is found, the groups are identical.
Let's take a look at some examples:
In molecule A let's look at the carbon with an asterisk. This carbon is directly attached to a bromine, a hydrogen, carbon on the left and another carbon on the right. Bromine and hydrogen are clearly different but what about the carbons? The carbon on the left is attached to hydrogens only (not shown), while the carbon on the right is attached to another carbon. Therefore all four groups are different. In molecule B the carbon with an asterisk is attached to a hydrogen, a methyl group, a carbon on the left and another carbon on the right. On the left the carbon is attached to another carbon while on the right, the carbon is attached to another carbon through a double bond. That's a difference! Therefore, there are four different groups here as well. Naming Chiral Centers—The R,S System (Cahn, Ingold, Prelog Priorities)
Whenever we have a chiral carbon, there is a configuration that can be assigned to it, describing the spatial orientation of the groups attached to this carbon.
Identify a chiral center (carbon with four different groups) and groups attached to it. If the carbon has three bonds only, show a hydrogen.
Assign priority from 1 to 4 to the atoms bonded to the chiral center where 1 is the biggest atomic number and 4 is the smallest atomic number.
If you can't assign priority right away because atoms are the same, you have to continue going atom by atom until the first point of difference.
Atoms that have a double of triple bond are considered to be bonded to 2 or 3 other atoms. For example C=O is considered as O-C-O
Connect 1,2,3. If the arrow is going clockwise, it is R (right) and counterclockwise is S. However, group number 4 must be dashed (going into the plane for this to happen).
If group number 4 is not dashed, switch 4 with the group that is dashed. Then, determine R/S configuration and say the OPPOSITE! (This trick was taken from David Klein book Orgo As A Second Language).
How to determine if a molecule is chiral?
A chiral molecule is that it is a molecule whose mirror image is not superimposable. However, for those of us who have a hard time rotating 3D molecules in their mind and trying to superimpose them on one another, I came up with an easy step by step chart to help with this task.
The chart below shows how to determine whether a molecule is chiral or achiral. Please keep in mind that there are exceptions to the chart since there are molecules that do not have stereogenic centers/carbons and are still chiral but these are much more rare.
1. First we need to determine if the molecule has any stereogenic carbons (chiral carbons). These are carbons that are connected to four different groups. If no, the molecule is achiral. If yes, we go to the next question.
2. Are there 2 or more stereogenic carbons? If no, the molecule is chiral. If yes, another question.
3. Is there a line of symmetry that cuts the molecule in half with two identical halfs. The line of symmetry can be vertical, diagonal, horizontal, etc. If yes, the molecule is called MESO and is achiral. If no, the molecule is chiral
Enantiomers, diastereomers and meso compounds
Enantiomers are nonsuperimposable mirror images.
Diastereomers are nonsuperimposable non mirror images.
Meso compounds are molecules with two or more chiral centers that have a plane of symmetry are are achiral.
These definitions are hard to put into actual use and I am going to give you easy steps to identify each one.
How To Identify Enantiomers
Before we begin this discussion, please note that there are chiral molecules that do not have any chiral carbons (carbons attached to four different groups). Therefore, my tricks will apply to most but not all molecules.
Identify chiral carbons in both molecules and assign R and S absolute configuration to them.
Compare the chiral centers in the two molecules. If all of the chiral centers have opposite configuration going from one molecule to another, then those molecules are enantiomers. For example RR on one molecule and SS in another molecule. Make sure to compare the same chiral carbons to one another. For example, let's say there are two chiral carbons in molecule A, one with a bromine and another with a chlorine. Compare the stereochemistry of the carbon with bromine in molecule A to the stereochemistry of carbon with bromine in molecule B.
Watch out for MESO molecules. MESO molecule are achiral and therefore, their mirror images will be the SAME molecule! Forgot how to identify a meso compound? Go HERE
IF a molecule is MESO, it's mirror image will be the same molecule, not enantiomer because meso molecules are achiral.
DIASTEREOMERS
Diastereomers are molecules that are nonsuperimposable non mirror images of one another. Again there are always exceptions, but these tips and tricks should help you deal with most of the cases.
How To Identify Diastereomers?
Identify chiral carbons in both molecules and assign R and S absolute configuration to them.
Compare the chiral centers in the two molecules. If at least one chiral carbon switches it's configuration (R/S) and at least one stays the same, then the two molecules are diastereomers.
A meso molecule is a molecule that has two or more chiral centers (carbons with four different groups) but yet is achiral due to having a plane of symmetry.
The plane of symmetry has to cut the molecule into two identical halves. The plane of symmetry can be horizontal, vertical, diagonal etc.
Let's take a look at some examples.
Both molecules above have 2 chiral carbons each. In the first molecule the plane of symmetry is vertical, cutting it into two identical halves (note that the stereochemistry of the groups is the same as well). For the molecule on the right, there is a horizontal plane of symmetry.
Optical Activity
Chiral molecules are said to be optically active (rotate the plane of polarized line in a specific direction. Achiral molecules in solution are optically inactive. Racemic mixture ( a mixture that consists of both enantiomers) is optically inactive because the two enantiomers will rotate the light equally in opposite direction which leads to them cancelling each other and zero specific rotation.