How To Determine If The Molecule Is Chiral?
A chiral molecule is a molecule whose mirror image is non-superimposable. An example that is usually given in textbooks is that of our hands. Our hands are mirror images of one another. Yet, if you try to superimpose them on top of one another, it will be impossible since the thumb and pinky will not match. Our hands are therefore, chiral. If you can imagine a 3D molecule in space, then imagine its’ mirror image and try to superimpose them, you have an amazing 3D understanding. For most people, like myself, this is hard to do, and that is why I came up with an easier way to determine whether a given molecule is chiral or achiral. But before we do that, we first must learn how to identify chiral centers, also called chirality centers, stereocenters, stereogenic centers, assymetric centers and chiral carbon.
A CHIRAL CENTER refers to a carbon that is attached to four different groups.
PS: Some textbooks define it as any atom with four different groups and some talk about carbon specifically. Since all the exercises I have seen always refer to a carbon, we will define a chiral center as a carbon that is attached to four different groups to avoid confusion.
In order to identify whether there is a chiral center in the molecule, we will look at the carbon and four atoms it is attached to. If the atoms are the same, we will proceed to go atom by atom until we find a difference. If there is no difference, then the carbon is not a chiral center! Examples will make it more clear.
Learning how to assign R/S configuration will help tremendously with identifying chiral centers as well.
Now, let’s look at my easy, step-by-step way to determine if the molecule is chiral or achiral.
First you need to find out if the molecule has any chiral centers? If not, the molecule is most likely achiral, unless it’s a rare exception. Then you have to ask are there two or more chiral centers? If not, the molecule is chiral. If yes, you have to ask the final question. Is there a plane of symmetry cutting the molecule in two identical halves? If no, the molecule is chiral. If yes, the molecule is a Meso molecule which is achiral.
Let’s practice this concept.
Is there a chiral center? YES
Are there two or more chiral centers? YES
Is there a place of symmetry? NO, because one bromine is on a wedge and another is on a dash.
The molecule is CHIRAL
Is there a chiral center? YES
Are there two or more chiral centers? NO
The molecule is CHIRAL
Is there a chiral center? YES
Are there two or more chiral centers? YES
Is there a place of symmetry? YES
The molecule is ACHIRAL because it is a MESO molecule
Is there a chiral center? NO
The molecule is ACHIRAL
Is there a chiral center? NO
The molecule is ACHIRAL
SPECIAL CASE:
Some molecules need to be rotated about a single bond for all the parts to align.
If a molecule could be symmetrical when rotated about a single bond, you need to rotate, draw the new molecule and check for the plane of symmetry. When you rotate about the single bond, you will switch the stereochemistry of the group attached to the carbon being rotated.
Looking at the top molecule, when you rotate about single bond, bringing bromine up, you need to change its’ stereochemistry. This molecule has a plane of symmetry. It is Meso and therefore achiral.
The molecule on the bottom is chiral, because when rotated, one chlorine is on a dash and another is on a wedge. There is no plane of symmetry in this molecule.
LINKS
Online Organic Chemistry Tutoring
References:
Organic Chemistry by Paula Yurkanis Bruice
Organic Chemistry by David Klein
Organic Chemistry by Maitland Jones Jr,
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