Cyclohexane

Cyclohexane is a cycloalkane with the molecular formula C 6 H 12 . Cyclohexane is non-polar . Cyclohexane is a colorless, flammable liquid with a characteristic detergent -like odor, reminiscent of cleaning products (in which it is sometimes used). Cyclohexane is primarily used for industrial production of adipic acid and caprolactam , which are precursors to nylon .

Cyclohexyl ( C6H11 ) is the alkyl substituent of cyclohexane and is abbreviated as Cy .

Production

modern production

On an industrial scale, cyclohexane is produced by the hydrogenation of benzene in the presence of a Raney nickel catalyst. [7] Producers of cyclohexane account for about 11.4% of the global demand for benzene. [8] The reaction is highly exothermic, with H(500 K) = 216.37 kJ/mol). Dehydrogenation began above 300 °C, indicating a favorable entropy for dehydrogenation.

Cyclohexane
Cyclohexane

historical ways

Unlike benzene, cyclohexane is not found in natural resources such as coal. For this reason, early investigators synthesized their own cyclohexane samples. [10]

initial failures

  • In 1867, Marcellin Berthelot reduced benzene with hydroiodic acid at elevated temperatures. [11] [12]
  • In 1870, Adolf von Bayer repeated the reaction [13] and pronounced the same reaction product “hexahydrobenzene”.
  • In 1890 Vladimir Markovnikov believed he was able to distill the same compound from Caucasus petroleum, calling his concoction “hexanaftene”.

Surprisingly, their cyclohexanes boiled over 10 °C from either hexahydrobenzene or hexanaphtene, but this puzzle was solved in 1895 by Markovnikov, NM Kishner, and Nikolay Zelinsky when they used “hexahydrobenzene” and “hexanaphtene”. “Reassigned as methylcyclopentane, the result of an unexpected rearrangement reaction.

Cyclohexane
Cyclohexane

success

In 1894, Bayer synthesized cyclohexane, beginning with the ketonization of pimelic acid and followed by several reductions:

Cyclohexane
Cyclohexane

In the same year, E. Howarth and WH Perkin Jr. (1860–1929) prepared it through the Wurtz reaction of 1,6-dibromohexane.

Reactions and Uses

Although unreactive, cyclohexane undergoes catalytic oxidation to produce cyclohexanone and cyclohexanol. The cyclohexanone-cyclohexanol mixture, referred to as ” KA oil “, is a raw material for adipic acid and caprolactam, precursors of nylon. Several million kilograms of cyclohexanone and cyclohexanol are produced annually.

Laboratory solvent and other specific uses

It is used as a solvent in some brands of correction fluid. Cyclohexane is sometimes used as a non-polar organic solvent, although n-hexane is more widely used for this purpose. It is often used as a recrystallization solvent, as many organic compounds exhibit good solubility in hot cyclohexane and poor solubility at low temperatures.

Due to a convenient crystal–crystal transition at -87.1 °C, cyclohexane is also used for the calibration of differential scanning calorimetry (DSC) devices.

Cyclohexane vapor is used in vacuum carburizing furnaces, in the manufacture of heat treatment equipment.

structure

6-The top edge ring does not correspond to the shape of a perfect hexagon. There is considerable angle stress in the construction of a plane 2D planar hexagon because its bonds are not 109.5°; The torsion stress will also be considerable because all of the bonds will assume the bonds. Therefore, in order to reduce torsional stresses, cyclohexane adopts a three-dimensional structure known as the chair structure, which rapidly expands at room temperature through a process known as a chair flip. intertwines. During the chair flip, three other intermediate structures come to the fore: the half-chair, which is the most unstable conformation, the more stable boat structure, and the turn-boat, which is more stable than the boat but still much lower than the chair. steady. Chair and twist-boat are energy minima and therefore conformers, Whereas half-chair and boat are transition states and represent energy maxima. The idea that the chair structure is the most stable structure for cyclohexane was first proposed by Hermann Sachs in 1890, but later gained widespread acceptance. The new structure places the carbon at an angle of 109.5°. half hydrogen ring (equatorial ) while the other half are perpendicular to the plane ( axial ). This structure allows the most stable structure of cyclohexane. Another structure of cyclohexane exists, known as the boat structure, but it converts to a slightly more stable chair formation. If cyclohexane is mono-substituted with a larger substituent, then the substituent will most likely be found attached to an equatorial position, as this is slightly more stable than the conformation.

Cyclohexane has the lowest angle and torsional strain of all cycloalkanes; The resulting cyclohexane is assumed to have 0 in total ring strain.

solid phase

Cyclohexane has two crystalline phases. The high-temperature phase I, stable between 186 K and melting point 280 K, is a plastic crystal, meaning that the molecules maintain some rotational degrees of freedom. The low-temperature (below 186 K) phase II is ordered. Two other low-temperature (metastable) phases III and IV have been achieved by application of medium pressures above 30 MPa, where phase IV appears especially in deuterated cyclohexane (application of pressure can reduce all transition temperature values). extends).

Nosymmetryspace groupA (A)b 0 a)CA)jadet (k)P (MPa)
ISolidfm3m8.614१९५0.1
Secondmonoclinicc2/c11.236.448.2041150.1
thirdorthorhombicPMN6.547.955.29223530
IVmonoclinicP12(1)/n16.507.645.51416037

Here Z is the number of structural units per unit cell; The unit cell constants A, B and C were measured at a given temperature T and pressure P.

Chair structure

The structure of the chair is the most stable confirmer. At 25 °C, 99.99% of all molecules of cyclohexane solution adopt this conformation.

The symmetry is D3D . All carbon centers are the same. Six hydrogen centers are located in the axial position, approximately parallel to the C3 axis, and six hydrogen atoms are located near the equator. These H atoms are called axial and equatorial, respectively.

Each carbon has one “up” and one “bottom” hydrogen. The C-H bonds in successive carbons are staggered such that there is little torsional strain . The chair geometry is often preserved when hydrogen atoms are replaced by halogens or other simpler groups .

If we think of a carbon atom with four half-bonds sticking out towards the vertices of a tetrahedron , we can imagine that they are standing on a surface with one half-bond pointing straight up. does. Viewed from above, the other three appear to move outwards towards the vertices of an equilateral triangle , so the bondsAn angle of 120° will appear between Now consider six atoms that are standing on the surface so that their non-vertical half-bonds meet and form a perfect hexagon. If we reflect three atoms below the surface, we have something similar to a chair-conforming cyclohexane. In this model, the six vertical semi-bonds are exactly perpendicular, and the ends of the six non-vertical semi-bonds that protrude from the ring are exactly at the equator (that is, at the surface). Since C-H bonds are actually longer than half C-C bonds, the “equatorial” hydrogen atom of chair cyclohexane will actually be below the equator when attached to a carbon that is above the equator, and vice versa. This is true for other substances as well. dihedral angleExactly for a chain of four carbon atoms going around the ring in this model options between +60° and -60° (called Gauche ).

The chair structure cannot deform without changing the binding angle or length. We can think of it as two chains, mirror images of each other, with atoms 1-2-3-4 and 1-6-5-4 with opposite dihedral angles. The distance from atom 1 to atom 4 depends on the absolute value of the dihedral angle. If these two dihedral angles change (still opposite each other), it is not possible to maintain the correct bond angles on both carbon 1 and carbon 4.

Boat and turn-boat conformance

Boat conformation has more energy than chair conformation. The interaction between two flagpole hydrogens, in particular, generates steric strain . Torsional strain also exists between the C2–C3 and C5–C6 bonds (carbon number 1 is one of the two on the mirror plane), which are assumed —that is, these two bonds are parallel to each other in a mirror plane. Due to this stress, the boat configuration is unstable (ie there is no minimum local energy).

The molecular symmetry is C 2 V.

Boat conformation spontaneously distorts into twist-boat conformation. Here the symmetry is D 2 , a purely rotational point group with three two-fold axes. This conformation can be obtained from the boat structure by adding a slight twist to the molecule so as to remove the assumption of two pairs of methylene groups. The turn-boat structure is chiral, existing in right-handed and left-handed versions.

The concentration of the twist-boat structure at room temperature is less than 0.1%, but at 1073 Kelvin it can reach up to 30%. Rapid cooling of a sample of cyclohexane from 1073 K to 40 K will result in a large concentration of twist-bottom conformation, which then gradually converges to the chair structure upon heating.