What is an Isomer?

Let’s start by talking about “what is an isomer?” When two molecules have the exact same molecular formula but are different from one another, they are called isomers. In the isomer definition, there are two general kinds: constitutional isomers and stereoisomers.

Constitutional isomers are molecules that have the exact same molecular formula but have atoms that are connected differently from one another. They tend to be very easy to spot. Here are a few examples:

2 examples of what is an isomer.

Stereoisomers have atoms that are connected the exact same way as one another, but they differ in the spatial arrangement of atoms. A common example of stereoisomers is your 2 hands, as they are not exactly the same. You can’t make your left hand look exactly like your right. Therefore, it is often said that your two hands are not superimposable. To show this, hold your 2 hands out in front of you with your palms facing downward. Even if you flip your left hand over so your left thumb is in the same direction as your right thumb, your palm is facing up on your left hand, while the back of your hand faces up on your right hand. Although your two hands have the exact same “structure,” their spatial arrangement is different.

When discussing the isomer definition, there are subtypes that must be understood.

Stereoisomers are hugely important in many applications of life. For example, almost all proteins are “left-handed” (we’ll see what this means later), while your DNA is almost entirely “right-handed.” The left-handed version of a right-handed molecule often does NOT have the same functionality as its counterpart and vice versa, making stereochemistry critically important to biology.

As we just learned, the question “what is an isomer?” covers a wide range of subtypes in organic chemistry that can be difficult for students to understand in a single isomer definition. This chapter will go more in-depth into stereoisomers such as enantiomers, diastereomers, and meso compounds. We’ll also introduce Fischer projections, a different way to visualize organic chemistry molecules.