Relative Configuration in Organic Chemistry: Definition & Examples

Instructor: Laura Foist

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

Relative configuration refers to the relationship between molecules or atoms. In this lesson, we will learn what relative configuration is and look at a few examples.

What is Relative Configuration?

In a family, you are related to others in the family in different ways. You are related to your mom in a parent-child relationship. You are related to your brother in a sibling relationship. Each relationship between members of a family has a specific name to define the relationship. In organic chemistry, the relationship between different atoms in a molecule is also defined, this is called relative configuration. Relative configuration refers to the configuration of a molecule in relation to other atoms on the same molecule, in relation to other molecules, or in relation to another form of the same molecule.

There are several different types of relative configurations possible, these include:

  • Alpha and beta
  • Cis and trans

Alpha and Beta Configuration

Alpha and beta are used specifically for describing the relative configuration of carbohydrate molecules. They are used to describe the relationship between the orientation of one of the OH molecules on the cyclic compound vs. the Fischer projection.

When carbohydrates go from the linear form (shown in the Fischer projection) to the cyclic form, one of the carbon atoms (C-5 in the linear form and C-1 in the cyclic form) can change the orientation of the OH group during the transition. If the orientation stays on the same side in the cyclic form as in the linear form, this is an alpha relationship. If the orientation switches sides in the cyclic form from the linear form, this is a beta relationship.

Let's look at a glucose molecule to determine the relative configuration of the molecule. First, let's start with the Fischer, linear, form:

Glucose in the Fischer projection
Glucose fisher

In the linear form, there is only one way that D-glucose can present itself. Now, notice carbon 5, circled in red. In this carbon, the OH group is on the right, which is the same as though it is pointing up or out of the page. Now, let's turn this glucose molecule into the cyclic form. Notice that what was carbon 5 in now carbon 1, circled again in red:

The squiggly line indicates it can be pointing up or down
Glucose general cyclic

The OH group on C-1 doesn't actually show if it is up or down, it is simply shown with a squiggly line. This is because when this molecule is formed it allows for either the OH to end up pointing up or pointing down:

Alpha-D-glucose has the OH on C1 pointing down
Alpha glucose

When the OH is pointing down, this is the same direction that the OH was pointing in the linear form, so it is the alpha. When it is pointing up, this is the opposite direction, so it is the beta.

Beta-D-glucose has the OH on C1 pointing up
Beta glucose

Cis and Trans

Perhaps you are more familiar with the terminology cis and trans. This is often used to describe fat molecules. Have you ever heard of trans fats? The description of this type of fat relies on the description of the relative configuration around a carbon double bond.

In general, trans and cis configuration refers to what side of the bond the largest atoms are located in relationship to each other and the bond. If the largest atom connected to C1 is above the bond while the largest atom connected to C2 is below the bond, it is a trans configuration. If the largest atom connected to C1 and the largest atom connected to C2 are both on the same side, it is a cis configuration.

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