Butanol (also called butyl alcohol) is a four-carbon alcohol with a formula of C₄H₉OH. There are four possible isomeric structures for butanol, from a straight-chain primary alcohol to a branched-chain tertiary alcohol; all are a butyl or isobutyl group linked to a hydroxyl group (sometimes represented as BuOH, n-BuOH, and i-BuOH). These are n-butanol, 2-butanol, tert-butanol, and isobutanol. It is primarily used as a solvent, as an intermediate in chemical synthesis, and as a fuel. It is sometimes also called biobutanol when produced biologically.