N-Butanol

n-Butanol or n-butyl alcohol or normal butanol is a primary alcohol with a 4-carbon structure and the molecular formula C4H9OH. Its isomers include isobutanol, 2-butanol, and tert-butanol. Butanol is one of the group of "fusel alcohols" (from the German for "bad liquor"), which have more than two carbon atoms and have significant solubility in water.

n-Butanol occurs naturally as a minor product of the fermentation of sugars and other carbohydrates, and is present in many foods and beverages. It is also a permitted artificial flavorant in the United States, used in butter, cream, fruit, rum, whiskey, ice cream and ices, candy, baked goods and cordials. It is also used in a wide range of consumer products.

The largest use of n-butanol is as an industrial intermediate, particularly for the manufacture of butyl acetate (itself an artificial flavorant and industrial solvent). It is a petrochemical, manufactured from propylene and usually used close to the point of manufacture. Estimated production figures for 1997 are: United States 784,000 tonnes; Western Europe 575,000 tonnes; Japan 225,000 tonnes.

Production
n-Butanol is produced industrially from the petrochemical feedstock propylene. Propylene is hydroformylated to butyraldehyde (oxo process) in the presence of a rhodium-based homogeneous catalyst similar to Wilkinson's catalyst. The butyraldehyde is then hydrogenated to produce n-butanol.

Industrial use
n-butanol is an intermediate in the production of butyl acrylate, butyl acetate, dibutyl phthalate, dibutyl sebacate, and other butyl esters, butyl ethers such as ethylene glycol monobutyl ether, di- and triethylene glycol monobutyl ether, and the corresponding butyl ether acetates. Other industrial uses include the manufacture of pharmaceuticals, polymers, pyroxylin plastics, herbicide esters (e.g., 2,4-D, 2,4,5-T) and butyl xanthate. It is also used as a diluent/reactant in the manufacture of urea–formaldehyde and melamine–formaldehyde resins.

Other uses
n-Butanol is used as an ingredient in perfumes and as a solvent for the extraction of essential oils. n-Butanol is also used as an extractant in the manufacture of antibiotics, hormones, and vitamins; a solvent for paints, coatings, natural resins, gums, synthetic resins, dyes, alkaloids, and camphor. Other miscellaneous applications of n-butanol are as a swelling agent in textiles, as a component of brake fluids, cleaning formulations, degreasers, and repellents; and as a component of ore floation agents, and of wood-treating systems.

n-Butanol has been proposed as a substitute for diesel fuel and gasoline. It is produced in small quantities in nearly all fermentations (see fusel oil), but species of Clostridium produce much higher yields of butanol, and research is currently underway to increase the ultimate yield of biobutanol from biomass.

The production or, in some cases, use of the following substances may result in exposure to n-butanol: artificial leather, butyl esters, rubber cement, dyes, fruit essences, lacquers, motion picture, and photographic films, raincoats, perfumes, pyroxylin plastics, rayon, safety glass, shellac varnish, and waterproofed cloth.

Occurrence in Nature
Alarm pheromones emitted by the Koschevnikov gland of honey bees contain n-butanol.

Occurrence in food
n-Butanol occurs naturally as a result of carbohydrate fermentation in a number of alcoholic beverages, including beer, grape brandies, wine, and whisky. It has been detected in the volatiles of hops, jack fruit, heat-treated milks, musk melon, cheese, southern pea seed, and cooked rice. n-Butanol is also formed during deep frying of corn oil, cottonseed oil, trilinolein, and triolein.

n-Butanol is used as an ingredient in processed and artificial flavourings, and for the extraction lipid-free protein from egg yolk, natural flavouring materials and vegetable oils, the manufacture of hop extract for beermaking, and as a solvent in removing pigments from moist curd leaf protein concentrate.

Metabolism and toxicity
n-Butanol is readily absorbed through the intestinal tract and lungs, and also to some extent through the skin. It is metabolized completely in vertebrates in a manner similar to ethanol: alcohol dehydrogenase coverts n-butanol to butyraldehyde; this is then converted to butyric acid by aldehyde dehydrogenase. Butyric acid can be fully metabolized to carbon dioxide and water by the β-oxidation pathway, of which it is naturally the penultimate step. In the rat, only 0.03% of an oral dose of 2000 mg/kg was excreted in the urine.

The acute toxicity of n-butanol is relatively low, with oral LD50 values of 2290–4360 mg/kg (rat; comparable values for ethanol are 7000–15000 mg/kg). No deaths were reported at an inhaled concentration of 8000 ppm (4-hour exposure, rats). At sub-lethal doses, n-butanol acts as a depressant of the central nervous system, similar to ethanol: one study in rats indicated that the intoxicating potency of n-butanol is some six times higher than that of ethanol, possibly because of its slower transformation by alcohol dehydrogenase.

n-Butanol is a natural component of many alcoholic beverages, albeit in low (but variable) concentrations. It (along with similar fusel alcohols) is reputed to be responsible for severe "hangovers", although experiments in animal models show no evidence for this. n-Butanol has occasionally been abused by alcoholics, leading to a range of adverse health effects.

Other hazards
Liquid n-butanol, as is common with most organic solvents, is extremely irritating to the eyes; repeated contact with the skin can also cause irritation. This is believed to be a generic effect of "defatting". No skin sensitization has been observed. Irritation of the respiratory pathways occurs only at very high concentrations (>2400 ppm).

With a flash point of 29 °C (84 °F), n-butanol presents a moderate fire hazard: it is slightly more flammable than kerosene or diesel fuel but less flammable than many other common organic solvents. The depressant effect on the central nervous system (similar to ethanol intoxication) is a potential hazard when working with n-butanol in enclosed spaces, although the odour threshold (0.2–30 ppm) is far below the concentration which would have any neurological effect.

n-Butanol is of low toxicity to aquatic vertebrates and invertebrates. It is rapidly biodegraded in water, although an estimated 83% partitions to air where it is degraded by hydroxyl radicals with a half-life of 1.2–2.3 days. It has low potential to bioaccumulate. A potential hazard of significant discharges to watercourses is the rise in chemical oxygen demand (C.O.D.) associated with its biodegradation.