When we discuss elements, and how they behave, we often talk about the group they are in on the periodic table. This is because elements from the same group exhibit similar characteristics. In much the same way, when we talk about organic chemistry, rather than talking about specific compounds, we will often discuss functional groups.

Functional groups are groups of molecules that, based on their similar molecular structures, react similarly to one another.

Now, you’ve already encountered a few functional groups. We’ve looked at alkanes, alkenes, and alkynes in previous lessons. In this lesson, we are going to look at more complex functional groups, namely, alcohols, aldehydes, ketones, acids, and esters. We will learn what makes them structurally unique, and how we can identify them.

To begin, let’s look at alcohols. Here, we have a diagram of an alcohol called propanol. As you may have deduced, the name stems from the fact that there are 3 carbons in the central carbon chain, corresponding to the prefix prop-. If we shorten that carbon chain by one, we get an alcohol called ethanol, a common fuel additive, and if we shorten it by one more, we get methanol. These are all alcohols. What makes them an alcohol is not the specific carbon chain, but rather this piece here, called a hydroxyl group. So, when we represent functional groups, we know that there could many different hydrocarbon chains that make this molecule. In the last lesson you saw just how complicated those can get. So, to show the general structure for all alcohols, we replace that hydrocarbon with the letter R, representing the rest of the molecule.

And this here represents the alcohol functional group. It is some hydrocarbon with a hydroxyl group attached to the end. We’re going to use this method of representation going forward.

The next group we want to look at are called aldehydes. Aldehydes look like this. We have our hydrocarbon, represented by the R, but the carbon at the end of the chain is doubled bonded to an oxygen. This carbon-oxygen double bond is called a carbonyl group. We’ll see that in other functional groups as we move forward. Examples of aldehydes include formaldehyde, which you may have used in a biology class to preserve certain tissue samples, and vanillin, which is the chemical that gives vanilla its unique flavor.

Next, let’s look at ketones. Ketones have two different carbon chains, which we mark as R1 and R2. Where they meet, there is a carbonyl group, that carbon oxygen double bond. Examples of ketones include acetone, which is the key ingredient in nail polish remover, and fructose, which is the sugar that makes fruit sweet. Also, you may have heard of the ketogenic, or keto, diet. It gets its name from the fact that when on that diet, the body produces more of these chemicals.

Next, we’ll look at acids. The structure of this functional group looks like this. Now, you may notice that this looks nothing like the Arrhenius acids we’ve seen before in this course, and that’s true. This is a special group of acids called carboxylic acids. That name comes from the fact that connected to the hydrocarbon is a carbonyl group – that carbon oxygen double bond – and a hydroxyl group. Examples of carboxylic acids include amino acids, which are the building blocks of proteins – and by extension life itself, as well as acetic acid, which is the defining component of vinegar.

Alright, the last functional group we are going to look at are esters. Here is the basic structure of an ester. At the center of this molecule we have a carbon that is bonded to two oxygens. One of the bonds is a double bond, signifying a carbonyl group, and the other is a single bond. We have two hydrocarbons: one is bonded to the carbon, and the other is bonded to the single bonded oxygen. It’s like the COO group is in a hydrocarbon sandwich. We see esters all over the place, as they are really important in plastics and oils. Examples include polyester, a common plastic-based fabric, and many essential oils and lipids, which are the fats produced by plants and animals.

And those are the five critical functional groups we need to know: alcohols, aldehydes, ketones, acids, and esters. Each functional group has important structural characteristics that define how they react with other chemicals, and by looking at them in their simplest form, we can better understand why they exhibit their unique set of traits.