The term sec-butyl refers to a secondary butyl, which is also known as 1-methylpropyl or butan-2-yl. It is part of the butyl group, each of which contains four atoms of carbon and nine atoms hydrogen, with a molecular structure expressed as CH3–CH2–CH(CH3)–.
Although other types of butyls contain the same numbers of atoms, they are differentiated based on the structure and where the different carbon atoms can form connections with other substances. Each different structure is known as an isomer, which refers to compounds with the same chemical formula but a different arrangement of atoms. Regular butyl has a structure expressed as CH3–CH2–CH2–CH2–, while isobutyl is shown by the structure (CH3)2CH–CH2– and tertiary butyl by (CH3)3C–.