Hydroxyl Group: Definition & Structure

A hydroxyl group is a pair of atoms that is commonly found in organic compounds, such as sugars and alcohols. Learn more about the importance of this group and quiz yourself at the end.
What Is a Hydroxyl Group?
Many people enjoy a glass of wine on any given day, or use rubbing alcohol to clean a wound. And there probably isn’t a person on Earth who doesn’t consume some form of sugar every day. But it’s highly unlikely that these same people are wondering about the chemical makeup of alcohol and sugar. In this lesson, we will learn about a key component in the chemical makeup of sugars and alcohols. While these substances have many different chemical compositions, sugars and alcohols all have two things in common: they are all carbon-based, and they all contain a pair of atoms called the hydroxyl group.

Before we get into the specifics of hydroxyl groups, it is important to get a better understanding of carbon-based molecules. Organic chemistry is the study of carbon-based molecules. These molecules, otherwise known as organic molecules, are so important that life could not exist without them. DNA, proteins, and carbohydrates are examples of organic molecules that are necessary for every living being. And the hydroxyl group is one of their essential building blocks.

Chemical Formula and Structure of the Hydroxyl Group
A hydroxyl group is composed of one hydrogen atom bonded to one oxygen atom. Its chemical formula is written as either -OH or HO-. The ‘-‘ represents the carbon to which the hydroxyl group is bonded.

The R in the structural formula stands for the carbon backbone of the organic molecule to which the hydroxyl attaches.

Functional Groups in Organic Molecules
To further explore the specifics of a hydroxyl group, let’s go back to our basic understanding of an organic molecule. Organic molecules are carbon-based and also may contain oxygen, hydrogen, nitrogen, sulfur, and/or phosphorus. Structurally, these molecules are composed of two main parts.

The first part is the carbon backbone, in which the carbon atoms are bonded together forming a carbon backbone.

The second are the functional groups, which are small groups of atoms, such as hydrogen and oxygen, that are bonded to the carbon backbone. Functional groups are so named because they function as the chemically reactive area of the molecule.

The hydroxyl group (-OH) is one example of a functional group. When hydroxyl groups are the primary functional group bonded to carbon backbones, the resulting molecules are alcohols. Here we see the structural formula for the organic molecule ethanol (a type of alcohol) with the hydroxyl group on the far right.

Methanol, isopropyl alcohol, and propanol are additional examples of alcohols containing the hydroxyl group.

Carbohydrate molecules, or sugars, have hydroxyl groups, too. However, sugars also contain another important functional group, called the carbonyl group (-CO), that alcohols don’t have. This is what distinguishes sugars from alcohols. Looking at the structure of a sugar called glucose, you can see that there are hydroxyl groups on each side of both examples.