Hexane (C₆H₁₄)

Hexane is an alkane hydrocarbon with the chemical formula CH₃(CH₂)₄CH₃ or C₆H₁₄. The "hex" prefix refers to its six carbons, while the "ane" ending indicates that its carbons are connected by single bonds. Hexane isomers are largely unreactive, and are frequently used as an inert solvent in organic reactions because they are very non-polar. They are also common constituents of gasoline and glues used for shoes, leather products, and roofing. Additionally, it is used in solvents to extract oils for cooking and as a cleansing agent for shoe, furniture and textile manufacturing. In laboratories, hexane is used to extract oil and grease from water and soil before determination by gravimetric analysis or gas chromatography.

Isomers

Hexane has five isomers:

• Hexane, CH₃CH₂CH₂CH₂CH₂CH₃, a straight chain of six carbon atoms.
• 2-Methylpentane (Isohexane), CH₃CH(CH₃)CH₂CH₂CH₃, a five-carbon chain with one methyl branch on the second.
• 3-Methylpentane, CH₃CH₂CH(CH₃)CH₂CH₃, a five-carbon chain with one methyl branch on the third.
• 2,3-Dimethylbutane, CH₃CH(CH₃)CH(CH₃)CH₃, a four-carbon chain with one methyl branch on the second and third.
• 2,2-Dimethylbutane (neohexane), CH₃C(CH₃)₂CH₂CH₃, a four-carbon chain with two methyl branches on the second.

Production

Hexane is produced by the refining of crude oil. The exact composition of the fraction depends largely on the source of the oil (crude or reformed) and the constraints of the refining. The industrial product (usually around 50% by weight of the straight-chain isomer) is the fraction boiling at 65–70 °C.

Toxicity

The acute toxicity of hexane is relatively low, although it is a mild anesthetic. Inhalation of high concentrations produces first a state of mild euphoria, followed by somnolence with headaches and nausea.

Chronic intoxication from hexane has been observed in recreational solvent abusers and in workers in the shoe manufacturing, furniture restoration and automobile construction industries. The initial symptoms are tingling and cramps in the arms and legs, followed by general muscular weakness. In severe cases, atrophy of the skeletal muscles is observed, along with a loss of coordination and problems of vision.

The neuropathic toxicity of n-hexane in humans is well known; cases of polyneuropathy have typically occurred in humans chronically exposed to levels of n-hexane ranging from 400 to 600 ppm, with occasional exposures up to 2,500 ppm. The unusual toxicity of n-hexane (compared with other alkanes) has resulted in the chemical industry switching away from n-hexane in favour of n-heptane where possible.

Similar symptoms are observed in animal models. They are associated with a degeneration of the peripheral nervous system (and eventually the central nervous system), starting with the distal portions of the longer and wider nerve axons. The toxicity is not due to hexane itself but to one of its metabolites, hexane-2,5-dione. It is believed that this reacts with the amino group of the side chain of lysine residues in proteins, causing cross-linking and a loss of protein function.

The effects of hexane poisoning in humans are uncertain. In 1994, n-hexane was included in the list of chemicals on the Toxic Release Inventory. In the latter part of the 20th and early part of the 21st centuries, a number of explosions have been attributed to the combustion of hexane gas. In 2001, the U.S. Environmental Protection Agency issued regulations on the control of emissions of hexane gas due to its potential carcinogenic properties and environmental concerns.