Difference Between Chiral & Enantiomers

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

Since we can even use the word chiral in the very definitions of enantiomers, it can get confusing to differentiate the two terms. Let's learn what the difference between chiral and enantiomers is, and how to calculate the number of stereoisomers.

What are Enantiomers and Chiral?

When we compare two things to each other, we may find that they are completely the opposite, such as black and white. Yet, in order to determine that these two are opposites, we need criteria to compare them by.

In comparing colors perhaps we are determining how many of the colors are absorbed. Black and white end up being opposites because black has absorbed all of the colors while white has none. Colors are being compared, and absorption is the criteria by which we compare them.

For our lesson today, enantiomers are the comparison of two things, and chiral is the criteria by which they are compared.

  • Enantiomers is describing a comparison between two molecules; it is telling us how two different molecules are related.
  • Chiral is only describing what a single atom looks like; that it has four distinct groups attached.

So, the main difference between chiral and enantiomers is what they are describing, chiral describes the atom, while enantiomers describes the molecules' comparison.

Notice that we could define enantiomers as two chiral stereoisomers. In this definition we are even putting chiral into the definition of enantiomers, but they are still distinct terms.

Calculating Number of Stereoisomers

Since chirality can be used to define enantiomers, we can use the chiral centers to discover whether a molecule has any enantiomers. Let's take a look at the molecule 5-chloro-2-hexanol:

5 chloro 2 hexanol

We can see that there are 2 chiral centers:

Circled in red are the chiral centers in this molecule
ID chiral centers

In the first one the carbon is bonded to a methyl group, a chlorine, a hydrogen, and a 2-butanol group. Now, remember we need to look at whether or not the entire group is different, not just what it is directly attached to.

Technically this carbon is directly attached to two carbon atoms, which at first glance would make it appear to not be chiral. But, since one of the carbon atoms is part of a methyl group and the other carbon atom is part of a 2-butanol group, these two groups are different, making this carbon chiral:

Chiral center A

In the second chiral center, the carbon is bonded to a methyl group, an alcohol group, a hydrogen, and a 2-chlorobutane group:

Chiral center B

Since there are two chiral centers we know that there are four stereoisomers of this molecule. How do we determine this? We take 2 to the power of 'n', where 'n' is the number of chiral centers in the molecule. 22 = 4, so we have four stereoisomers of this molecule.

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