R-Groups

In your chemistry journey, you might have seen the letter "R" in a molecule. Looking at the periodic table, you'll notice that there is no element's symbol is just the letter "R". Is this a mistake? Did the person drawing the molecule forget a letter and is actually referring to radon (Rn) or maybe radium (Ra)?

The truth is, R isn't an element at all, it's actually an abbreviation for a molecule. Keep reading to find out more!

• This article is about R-groups
• First, we will define what an R-group is and why we would use this notation
• Next, we will look at some examples of an R-group
• Then, we will learn about the importance of R-groups for Amino Acids and learn about the different types

R-Group Definition

Let's start by looking at the definition of an R-group.

You can think of "R" as standing for "Rest of molecule". It is essentially an abbreviation.

R-group chemistry

We use R-groups for two reasons:

1. To refer to a general functional group without referring to a specific molecule

2. To focus on another part of the molecule during a reaction

For example, below is what the general formula for a ketone looks like:

Figure 1. General form of a ketone.

Functional groups are classes of molecules with the same reactivity and basic structure. When I mention ketones, I might not want to refer to a specific ketone, which is why you would write down an R group instead. The superscript is to denote that these are different groups. R-groups can also be written like R' or R" to mark that they are different groups.

As another example, take a look at this reaction:

In this reaction, the identity of the R-group doesn't matter, which is why it is abbreviated. We instead want to focus on the parts of the molecule that affect its reactivity.

Basically, R-groups are used when the general chemistry/reactivity does not matter, so we want to focus on the "stars" of the molecule.

Example of an R-group

Now that we have a general idea of what an R-group is, let's look at an example of what one looks like. Remember that ketone from earlier? Well, here are some examples of what those R-groups could be:

• Circled in red is our R¹ group, while the R² group is circled in yellow.

Figure 3. Examples of R-groups for a Ketone.

When we write molecules like this, each "point" represents a Carbon with however many hydrogens it needs to be balanced (i.e., have a total of four bonds). So in our first example, both R¹ and R² are CH₃. Even those these are the same, the molecules still count as a ketone. Having R¹ and R² means that these groups can be different.

R-groups and Amino Acids

While R-groups aren't the "star" in many organic molecules, they are incredibly important for identifying Amino Acids.

Each of the 20 standard amino acids has a specific side chain called an R-group that is also attached to the carbon.

There are many shapes, sizes, charges, and reactivities in the R groups. This lets amino acids be put into groups based on how their side chains behave chemically. Some amino acids, like serine, threonine, and asparagine, have side chains that are polar and dissolve in water. Other amino acids, like isoleucine, phenylalanine, and valine, don't like water and are called hydrophobic.

The side chain of the amino acid cysteine is chemically active and can bond with another cysteine. Some amino acids, like lysine, are basic, while others, like glutamic acid, are acidic. Each protein can fold into a certain three-dimensional shape and do a biological job because of the order and interactions between the side chains of the different amino acids.

Structure of Amino Acids and R groups

Below is the general structure of an amino acid:

Figure 4. General Structure of an Amino Acid.

The general structure shows an alpha-carbon bonded to an amino group, a carboxyl group, and an R-group.

Types of R-groups

Amino acids can be classified in a few different ways. For example, we can divide them based on polarity, types of functional groups, and acidity.

In this case, we are going to divide these R-groups like this:

1. Positively charged side chain

2. Negatively charged side chain

3. Polar, uncharged side chain

4. Hydrophobic side chain

5. Special cases

Below is a chart showing the amino acids in these different groups:

Figure 5.The amino acids' side chains classified into different groups

One thing you might notice is the three letter "code" written below the names of each amino acid. These codes are shorthand for the specific side chain/R-group that each amino acid possesses. Instead of writing just "R", we can instead write the three-letter code to denote the chain.

Below is a table with these codes: