Pentyl is a five-carbon alkyl group or substituent with chemical formula -C5H11.

In older literature, the common non-systematic name amyl was often used for the pentyl group.

Conversely, the name pentyl was used for several five-carbon branched alkyl groups, distinguished by various prefixes.

The nomenclature has now reversed, with "amyl" being more often used to refer to the terminally branched group also called isopentyl, as in amobarbital.

Radicals like pentyl are reactive, they react with neighboring atoms or molecules (like oxygen, water, etc.)