Butanol or butyl alcohol (sometimes also called biobutanol when produced biologically), is a primary alcohol with a 4 carbon structure and the molecular formula of C₄H₁₀O. It belongs to the higher alcohols and branched-chain alcohols.

It is primarily used as a solvent, as an intermediate in chemical synthesis, and as a fuel.

There are four isomeric structures for butanol.

Isomers

The unmodified term butanol usually refers to the straight chain isomer with the alcohol functional group at the terminal carbon, which is also known as n-butanol or 1-butanol. The straight chain isomer with the alcohol at an internal carbon is sec-butanol or 2-butanol. The branched isomer with the alcohol at a terminal carbon is isobutanol; 2-methyl-1-propanol, and the branched isomer with the alcohol at the internal carbon is tert-butanol; 2-methyl-2-propanol.

Butanol isomers, due to their different structures, have somewhat different melting and boiling points. All are moderately miscible in water, less so than ethanol, and more so than the higher (longer carbon chain) alcohols. This is because all alcohols have a hydroxyl group which makes them polar which in turn tends to promote solubility in water. At the same time the carbon chain of the alcohol resists solubility in water. Methanol, ethanol and propanol, are fully miscible in water because the hydroxyl group predominates while butanol is moderately miscible because of the balance between the two opposing solubility trends.

Toxicity

Like many alcohols, butanol is considered toxic. It has shown low order of toxicity in single dose experiments to laboratory animals. and is considered safe enough for use in cosmetics. Brief, repeated overexposure with the skin can result in depression of the central nervous system, as with other short-chain alcohols. Exposure may also cause severe eye irritation and moderate skin irritation. The main dangers are from prolonged exposure to fumes. In extreme cases this includes suppression of the central nervous system and even death. Under most circumstances, butanol is quickly metabolized to carbon dioxide. It has not been shown to damage DNA or cause cancer.

Biobutanol

Main: Butanol fuel

Butanol is considered as a potential biofuel (butanol fuel). Butanol at 85 percent strength can be used in cars designed for gasoline (petrol) without any change to the engine (unlike 85% ethanol), and it contains more energy for a given volume than ethanol and almost as much as gasoline, so a vehicle using butanol would return fuel consumption more comparable to gasoline than ethanol. Butanol can also be used as a blended additive to diesel fuel to reduce soot emissions. Fact: date=June 2008

Other uses

Butanol sees use as a solvent for a wide variety of chemical and textile processes, in organic synthesis and as a chemical intermediate. It is also used as a paint thinner and a solvent in other coating applications where it is used as a relatively slow evaporating latent solvent in lacquers and ambient-cured enamels. It finds other uses such as a component of hydraulic and brake fluids.