The propyl group is a functional group derived from propane (C₃H₈) by the removal of a hydrogen atom. It has the chemical formula C₃H₇-. The propyl group exists in two distinct isomeric forms. The first is normal propyl (n propyl or 1 propyl) which has the chemical formula CH₃CH₂CH₂-. The second is isopropyl (2 propyl) which has the formula (CH₃)₂CH-. Like all isomers the properties of the two propyl forms are slightly different. For example, propan-1-ol has a boiling point of 206°F(96.6°C), whereas propan-2-ol has a boiling point of 180°F(82°C). In this example, the first molecule is a primary alcohol and the second is a secondary alcohol. Each has slightly different chemical as well as physical properties.

In common with other alkyl groups, both forms of the propyl group modify the behavior of a functional group. As molecular mass increases, for example, the boiling point of a liquid will also increase. As can be seen in the previous example, the arrangement of the propyl group can also have an effect. This is due to the fact that with propan-2-ol the hydroxyl group of the alcohol is protected and consequently unavailable for hydrogen bonding. As a result, there are less intermolecular forces to overcome so less energy has to be added to the system to overcome these internal forces.