Finding chiral carbons is the first step in determining is your molecule is chiral or not.
To make it a little bit more obvious what’s going on in this molecule, let’s consider the condensed structure showing all implicit hydrogens:
To find chiral carbons, we need to find the carbons with four different groups of atoms attached to them. The first branched carbon atom might seem chiral because the stereochemistry is indicated. However, don’t get fooled by this common trick! Upon closer inspection, we can see that it has two the the same groups making it achiral.
This is a really common trick that many instructors use, so always make sure you check if your atom indeed has 4 different group. Just because stereochemistry is shown, doesn’t have to mean that the atom is chiral at all!
Well, what about the other branched carbon, the one next to the -OH group?
This atom is indeed chiral as it is attached to 4 distinctly different groups (indicated in different colors).
Here’s another example:
In this molecule, the potential candidates for chirality would be atome 2, 3, and 4. Let’s analyze them one-by-one.
The carbon number 2 is attached to two methyls, so that automatically makes it achiral. While carbon 3 is attached to 4 distinctly different groups, making it chiral! Similar analysis for carbon number 5 will reveal that it is chiral as well.
So, overall, our 3-ethyl-2,3-dimethylhexane has 2 chiral carbons:
Consider the ENTIRE group attached to a carbon when finding chiral carbons
Never limit your analysis on the first atom or a group of atoms attached to the carbon you’re analyzing. For instance, let’s look at the next molecule:
When we analyze this molecule, it might seem that the carbon “a” is attached to two methylene (-CH2-) groups on each side. This is, however, incorrect. You need to consider the entire group!
Both carbons “a” and “b” in this molecule do have 4 different groups attached to them making both of them chiral.
Finding chiral carbons in cyclic molecules
Dealing with the ring structure when trying to find the chiral carbons can be tricky. Here are two molecules for us to compare:
The left molecule has no chiral carbon. How? It has a plane of symmetry splitting the molecule into two halves.
Right about now you might question if this molecule is indeed symmetrical. All carbons in this molecule are sp3-hybridized. This means that the two groups “hanging” off the cyclic molecule are actually on the plane of symmetry:
The other molecule, however, has no planes of symmetry making it a good candidate for us to look for chiral carbons.
This molecule has altogether 2 chiral carbons. The carbon “a” is attached to ethyl, methyl, and two unequal ring sides making it chiral. The carbon “b” is attached to a methyl group, implicit hydrogen, and two unequal ring sides making it chiral as well.