4.5 : Overview of Functional Groups

Functional groups are a group of atoms with characteristic properties, which when linked to the carbon skeleton of a molecule, alter the properties of that molecule. For example, certain functional groups will make a molecule hydrophilic, whereas others will make them hydrophobic. These functional groups are an indispensable part of organic chemistry and important components of biological molecules, such as carbohydrates, proteins, lipids, and nucleic acids. Each functional group is a unique arrangement of atoms and is assigned a name for the combination of atoms that make up the group.

Hydroxyl Group

The compounds containing hydroxyl groups are generally called alcohols. The hydroxyl group consists of oxygen and hydrogen, and is present in carbohydrates, proteins, and lipids. It imparts hydrophilic properties to these organic compounds, making them highly water-soluble. The hydroxyl group is involved in the hydrolysis and dehydration synthesis reaction. During hydrolysis, a larger molecule, such as polysaccharide, is split into its monomers by adding a hydroxyl group from a water molecule. In contrast, during dehydration synthesis reactions, the hydroxyl group of one monomer can combine with the hydrogen of another to synthesize a polysaccharide.

Carbonyl Group

The carbonyl group is a carbon double–bonded with oxygen found in different classes of organic compounds, such as ketones, amides, esters, and aldehydes. It imparts polarity to compounds and therefore, such compounds have higher boiling points than non-polar compounds. They are also more reactive and undergo nucleophilic addition and substitution reactions.

Carboxylic Acid Group

The carboxyl group can ionize to release hydrogen atoms from the hydroxyl group as free protons (H⁺), and due to this, the carbonyl-containing compounds are acidic. This group also adds polar properties to the compounds, and such compounds have higher melting and boiling points. In a human body, lactic acid— a short-chain fatty acid— is an example of a carboxyl group containing substance. It is a metabolite formed after glucose catabolism.

Amino Group

The compounds containing amino groups are called amines. In this group, nitrogen is more electronegative than hydrogen, making it polar. As a result, amines are polar compounds. This group is found in protein, vitamins, and hormones.