Cyclohexane

• Hexahydrobenzene
• Hexamethylene
• Naphthene

Characteristic smelling, colorless liquid

Liquid

0.78 g -cm -3

6.72 ° C

81 ° C.

104 hPa ( 20 ° C)

Insoluble in water (50 mg · l-1 at 20 ° C)

1.42623

Risk

200 ml · m -3, 700 mg · m-3

12705 mg · kg -1 ( LD50, rat, oral)

Template: Infobox chemical / molecular formula search available

Cyclohexane ( also hexahydrobenzene, hexamethylene, naphthene ) is a colorless liquid. It is a cycloalkane with molecular formula C6H12, that occurs in the oil and is used as the solvent and raw material in the synthesis.

Production and representation

Cyclohexane is mainly obtained by the hydrogenation of benzene, may also be isolated from petroleum. The U.S. is the main producer of cyclohexane, about a million tons of cyclohexane are produced annually in the U.S., representing about 34 percent of global production. The total production of cyclohexane in Europe is 835000-925000 tons annually. Approximately 11.4 % of the global benzene market in 2010 were in the production of cyclohexane.

Properties

Cyclohexane melts at 6.7 ° C and boiling at atmospheric pressure at 80.7 ° C. It smells like gasoline, the odor threshold is 1.4 to 88 mg · m -3, the liquid is volatile. Cyclohexane insoluble in water, in organic substances, such as alcohols, hydrocarbons, ethers or chlorinated hydrocarbons, it is well soluble, cyclohexane itself is a solvent for many organic materials. The octane number of the cyclohexane is 77 Cyclohexane is highly flammable and hazardous for water.

Conformations

Cyclohexane played a central role in the development of organic stereochemistry. The molecule can exist in two conformations, the chair and the twist conformation. At room temperature, about 99 percent of Cyclohexanmoleküle are in the chair conformation. A flat ( planar ) cyclohexane molecule would have a CCC bond angle of 120 ° and ecliptic numerous interactions of the C -H bonds, ie a high angle strain ( Baeyer strain ) and torsional stress. In the chair conformation (English chair conformation ), however, all C -H bonds form a torsion angle of 60 °, and the bond angle approaches the tetrahedral angle ( 109.4 °). Therefore, this form is energetically more favorable and has long been regarded as a stress-free ( see Article strain energy ). Thus, the chair conformation is also free of angle strain.

From the chair conformation, the twist conformation is achieved at the input of thermal energy; to have distorted bond angles, and an energy barrier must be overcome. Your maximum is associated with a " half-chair " arrangement. This requires a relatively large amount of energy ( approximately 45 kJ mol -1; 10-11 kcal mol -1) to be applied.

The twist conformation is more flexible than the chair form. The previously discussed boat conformation - is listed first in the German boat conformation - is also free of angle strain; with her but come eclipsed interactions of the C -H bonds at the " sides of the boat " on what generates torsional stress. The close proximity of two hydrogen atoms in the " nose " and " rear " of the boat also leads to substantial van der Waals voltage. The twist conformation can boot arrangement (maximum of the potential energy ) in a slightly new twist on the form go ( energy barrier about 6.3 kJ mol - 1, 1.5 kcal mol -1). The twist forms go through pseudorotation into one another. According to current knowledge, ie half-chair and boat - order maxima of the potential energy, in other words, no "stable" conformations. About the twist conformation can finally chairlike cyclohexane ring " snap over " into another chair conformation ( " freak "); this is called " inversion " of the ring denotes ( ring inversion ).

The interaction of the C -H bonds and hydrogen is thus of decisive importance for the energy of the different conformations. As for the most stable chair conformation, are present here two sets of C -H bonds: six are parallel to the sixfold axis of rotating mirror (S6 ) of the molecule, three up and three down; the other six are based on an axis perpendicular to the ground on this imaginary " equatorial plane " from. According to a proposal by Derek HR Barton, these bonds ( " eq" or "e" german equatorial, abbreviated, blue in the picture) as equatorial, the former as axial ( ax or a, red in the picture ) called. In the ring inversion of the two sets swap places: For axial C -H bonds are equatorial and vice versa.

The structure determination by electron diffraction showed, however, that this model is somewhat simplified. Compared to the ideal chair conformation the molecule is somewhat flattened (English flattened ). The torsion angle is not 60 °, but 55 °, whereby the axial C -H bonds do not extend entirely parallel, but are rotated by about 7 ° "outwardly". The average C -C bond length is 153.6 pm, the CCC bond angles of 111.4 °, and the C -H bond length of 112.1 pm Central.

Thermodynamic properties

The vapor pressure function is given by Antoine corresponding log10 (P) = A- ( B / ( T C )) ( P in bar, T in K) with A = 3.99200, B = 1216.930 and C = -48 621 in the temperature range from 303 K to 343 K.

The temperature dependence of the enthalpy of vaporization can be determined according to equation ΔVH0 = A exp ( - αTr ) (1- Tr) β ( ΔVH0 in kJ / mol, Tr = (T / Tc) reduced temperature ) with A = 43.42 kJ / mol, α = -0.1437, β = 0.4512 and Tc = 553.4 K describe the temperature range between 298 K and 380 K.

Temperature dependence of the heat of vaporization of cyclohexane

Safety characteristics

Cyclohexane is highly flammable vapor - air mixtures. The compound has a flash point of -18 ° C. The explosion range is between 1.0 vol % (35 g/m3) as the lower explosive limit ( LEL) and 9.3 % by volume (326 g/m3 ) and upper explosive limit (UEL ). The explosion limits are pressure dependent. A decrease in the pressure leads to a reduction of the explosion range. The lower explosive limit changes up to a pressure of 100 mbar little and increases only at pressures less than 100 mbar. The upper explosive limit reduced analogously with decreasing pressure.

The maximum explosion pressure and the limiting oxygen concentration are pressure and temperature dependent. The marginal gap width was determined to be 0.94 mm. The result is thus a mapping in the explosion group IIA. With a minimum of 1.38 mJ vapor - air mixtures are extremely flammable. The ignition temperature is 260 ° C. The fabric falls within the temperature class T3. The electrical conductivity is very low at 4.0 × 10-15 S · m -1.

(measured at 20 ° C and 100 ° C)

Use

Cyclohexane is used for the production of synthetic fibers such as nylon. It is also used as the solvent. Cyclohexane occurs in the oil, it is important starting material for organic synthesis. Approximately 39 percent of the cyclohexane produced are consumed in Europe, Europe is the biggest buyer of cyclohexane.

Health Hazards

Inhalation and ingestion of the liquid cause headaches, nausea and dizziness. The eyes and skin become reddened and there is an irritation of the respiratory tract. A long-term exposure to cyclohexane can lead to dermatitis, an inflammatory reaction of the skin.