tert-Butyl peroxyisobutyrate, with more than 77 percent in solution

In organic chemistry, butyl is a four-carbon alkyl radical or substituent group with general chemical formula −C4H9, derived from either of the two isomers (n-butane and isobutane) of butane. The isomer n-butane can connect in two ways, giving rise to two "-butyl" groups: If it connects at one of the two terminal carbon atoms, it is normal butyl or n-butyl: −CH2−CH2−CH2−CH3 (preferred IUPAC name: butyl) If it connects at one of the non-terminal (internal) carbon atoms, it is secondary butyl or sec-butyl: −CH(CH3)−CH2−CH3 (preferred IUPAC name: butan-2-yl) The second isomer of butane, isobutane, can also connect in two ways, giving rise to two additional groups: If it connects at one of the three terminal carbons, it is isobutyl: −CH2−CH(CH3)2 (preferred IUPAC name: 2-methylpropyl) If it connects at the central carbon, it is tertiary butyl, tert-butyl or t-butyl: −C(CH3)3 (preferred IUPAC name: tert-butyl) Wikipedia

Hazardous Materials Table