Cyclohexane
Overview
 
Cyclohexane is a cycloalkane
Cycloalkane
Cycloalkanes are types of alkanes that have one or more rings of carbon atoms in the chemical structure of their molecules. Alkanes are types of organic hydrocarbon compounds that have only single chemical bonds in their chemical structure...

 with the molecular formula C
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

6H
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

12. Cyclohexane is used as a nonpolar solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...

 for the chemical industry, and also as a raw material for the industrial production of adipic acid
Adipic acid
Adipic acid is the organic compound with the formula 42. From the industrial perspective, it is the most important dicarboxylic acid: About 2.5 billion kilograms of this white crystalline powder are produced annually, mainly as a precursor for the production of nylon...

 and caprolactam
Caprolactam
Caprolactam is an organic compound with the formula 5CNH. This colourless solid is a lactam or a cyclic amide of caproic acid. Approximately 2 billion kilograms are produced annually...

, both of which being intermediates used in the production of nylon
Nylon
Nylon is a generic designation for a family of synthetic polymers known generically as polyamides, first produced on February 28, 1935, by Wallace Carothers at DuPont's research facility at the DuPont Experimental Station...

. On an industrial scale, cyclohexane is produced by reacting benzene
Benzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....

 with hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

.
Because of its unique chemical and conformational properties, cyclohexane is also used in labs in analysis and as a standard.
Encyclopedia
Cyclohexane is a cycloalkane
Cycloalkane
Cycloalkanes are types of alkanes that have one or more rings of carbon atoms in the chemical structure of their molecules. Alkanes are types of organic hydrocarbon compounds that have only single chemical bonds in their chemical structure...

 with the molecular formula C
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

6H
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

12. Cyclohexane is used as a nonpolar solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...

 for the chemical industry, and also as a raw material for the industrial production of adipic acid
Adipic acid
Adipic acid is the organic compound with the formula 42. From the industrial perspective, it is the most important dicarboxylic acid: About 2.5 billion kilograms of this white crystalline powder are produced annually, mainly as a precursor for the production of nylon...

 and caprolactam
Caprolactam
Caprolactam is an organic compound with the formula 5CNH. This colourless solid is a lactam or a cyclic amide of caproic acid. Approximately 2 billion kilograms are produced annually...

, both of which being intermediates used in the production of nylon
Nylon
Nylon is a generic designation for a family of synthetic polymers known generically as polyamides, first produced on February 28, 1935, by Wallace Carothers at DuPont's research facility at the DuPont Experimental Station...

. On an industrial scale, cyclohexane is produced by reacting benzene
Benzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....

 with hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

.
Because of its unique chemical and conformational properties, cyclohexane is also used in labs in analysis and as a standard. Cyclohexane has a distinctive detergent-like odor, reminiscent of cleaning product (in which it is sometimes used).

Conformation

The 6-vertexed ring does not conform to the shape of a perfect hexagon. The conformation of a flat 2D planar hexagon has considerable angle strain because its bonds are not 109.5 degrees; the torsional
Torsion (mechanics)
In solid mechanics, torsion is the twisting of an object due to an applied torque. In sections perpendicular to the torque axis, the resultant shear stress in this section is perpendicular to the radius....

 strain would also be considerable because all of the bonds would be eclipsed bonds. Therefore, to reduce torsional strain, cyclohexane adopts a three-dimensional structure known as the chair conformation. This was first proposed as early as 1890 by Hermann Sachse, but only gained widespread acceptance much later. The new conformation puts the carbons at an angle of 109.5°. Half of the hydrogens are in the plane of the ring (equatorial) while the other half are perpendicular to the plane (axial). This conformation allows for the most stable structure of cyclohexane. Another conformation of cyclohexane exists, known as boat conformation, but it interconverts to the slightly more stable chair formation. If cyclohexane is mono-substituted with a large substituent
Substituent
In organic chemistry and biochemistry, a substituent is an atom or group of atoms substituted in place of a hydrogen atom on the parent chain of a hydrocarbon...

, then the substituent will most likely be found attached in an equatorial position, as this is the slightly more stable conformation
Chemical structure
A chemical structure includes molecular geometry, electronic structure and crystal structure of molecules. Molecular geometry refers to the spatial arrangement of atoms in a molecule and the chemical bonds that hold the atoms together. Molecular geometry can range from the very simple, such as...

.

Cyclohexane has the lowest angle and torsional strain of all the cycloalkanes, as a result cyclohexane has been deemed a 0 in total ring strain, a combination of angle and torsional strain. This also makes cyclohexane the most stable of the cycloalkanes and therefore will produce the least amount of heat (per CH2 unit) when burned compared to the other cycloalkanes.


Solid phases

Cyclohexane has two crystalline phases. The high-temperature phase I, stable between 186 K and the melting point 280 K, is a plastic crystal, which means the molecules retain some rotational degree of freedom. The low-temperature phase II is ordered. A metastable phase III has been obtained by rapid quenching.

Reactions

Pure cyclohexane in itself is rather unreactive, being a non-polar, hydrophobic hydrocarbon. It can react with very strong acids such as the superacid
Superacid
According to the classical definition superacid is an acid with an acidity greater than that of 100% pure sulfuric acid, which has a Hammett acidity function of −12. According to the modern definition, superacid is a medium, in which the chemical potential of the proton is higher than in pure...

 system HF + SbF5
Fluoroantimonic acid
Fluoroantimonic acid is a mixture of hydrogen fluoride and antimony pentafluoride in various ratios. The 1:1 combination forms the strongest known superacid, which has been demonstrated to protonate even hydrocarbons to afford carbocations and H2....

, which will cause forced protonation and "hydrocarbon cracking". Substituted cyclohexanes, however, may be reactive under a variety of conditions, many of which being important to organic chemistry. Cyclohexane is highly flammable.

Derivatives

The specific arrangement of functional group
Functional group
In organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction regardless of the size of the molecule it is a part of...

s in cyclohexane derivatives, and indeed in most cycloalkane molecules, is extremely important in chemical reactions, especially reactions involving nucleophile
Nucleophile
A nucleophile is a species that donates an electron-pair to an electrophile to form a chemical bond in a reaction. All molecules or ions with a free pair of electrons can act as nucleophiles. Because nucleophiles donate electrons, they are by definition Lewis bases.Nucleophilic describes the...

s. Substituents on the ring must be in the axial formation to react with other molecules. For example, the reaction of bromocyclohexane
Bromocyclohexane
Bromocyclohexane is an organic compound with the chemical formula C6H11Br....

 and a common nucleophile, a hydroxide
Hydroxide
Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and a hydrogen atom held together by a covalent bond, and carrying a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, as a ligand, a nucleophile, and a...

 anion (OH), would result in cyclohexene
Cyclohexene
Cyclohexene is a hydrocarbon with the formula C6H10. This cycloalkene is a colorless liquid with a sharp smell. It is an intermediate in various industrial processes...

:
C6H11Br + OH → C6H10 + H2O + Br


This reaction, commonly known as an elimination reaction
Elimination reaction
An elimination reaction is a type of organic reaction in which two substituents are removed from a molecule in either a one or two-step mechanism...

 or dehalogenation (specifically E2), requires that the bromine
Bromine
Bromine ") is a chemical element with the symbol Br, an atomic number of 35, and an atomic mass of 79.904. It is in the halogen element group. The element was isolated independently by two chemists, Carl Jacob Löwig and Antoine Jerome Balard, in 1825–1826...

 substituent
Substituent
In organic chemistry and biochemistry, a substituent is an atom or group of atoms substituted in place of a hydrogen atom on the parent chain of a hydrocarbon...

 be in the axial formation, opposing another axial H atom to react. Assuming that the bromocyclohexane was in the appropriate formation to react, the E2 reaction would commence as such:
  1. The electron pair bond between the C-Br moves to the Br, forming Br and setting it free from cyclohexane
  2. The nucleophile (-OH) gives an electron pair to the adjacent axial H, setting H free and bonding to it to create H2O
  3. The electron pair bond between the adjacent axial H moves to the bond between the two C-C making it C=C

Note: All three steps happen simultaneously, characteristic of all E2 reactions.

The reaction above will generate mostly E2 reactions and as a result the product will be mostly (~70%) cyclohexene
Cyclohexene
Cyclohexene is a hydrocarbon with the formula C6H10. This cycloalkene is a colorless liquid with a sharp smell. It is an intermediate in various industrial processes...

. However, the percentage varies with conditions, and generally, two different reactions (E2 and SN2) compete. In the above reaction, an SN2 reaction would substitute the bromine for a hydroxyl
Hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...

 (OH-) group instead, but once again, the Br must be in axial to react. Once the SN2 substitution is complete, the newly substituted OH group would flip back to the more stable equatorial position quickly (~1 millisecond).

Uses

Commercially most of cyclohexane produced is converted into cyclohexanone
Cyclohexanone
Cyclohexanone is the organic compound with the formula 5CO. The molecule consists of six-carbon cyclic molecule with a ketone functional group. This colorless oil has an odor reminiscent of peardrop sweets as well as acetone. Over time, samples assume a yellow color due to oxidation...

cyclohexanol
Cyclohexanol
Cyclohexanol is the organic compound with the formula 5CHOH. The molecule is related to cyclohexane ring by replacement of one hydrogen atom by a hydroxyl group. This compound exists as a deliquescent colorless solid, which, when very pure, melts near room temperature...

 mixture (or "KA oil") by catalytic oxidation. KA oil is then used as a raw material for adipic acid
Adipic acid
Adipic acid is the organic compound with the formula 42. From the industrial perspective, it is the most important dicarboxylic acid: About 2.5 billion kilograms of this white crystalline powder are produced annually, mainly as a precursor for the production of nylon...

 and caprolactam
Caprolactam
Caprolactam is an organic compound with the formula 5CNH. This colourless solid is a lactam or a cyclic amide of caproic acid. Approximately 2 billion kilograms are produced annually...

. Practically, if the cyclohexanol content of KA oil is higher than cyclohexanone, it is more likely (economical) to be converted into adipic acid, and the reverse case, caprolactam production is more likely. Such ratio in KA oil can be controlled by selecting suitable oxidation catalyst. Some of cyclohexane is used as an organic solvent.

Although much is already known about this cyclic hydrocarbon, research is still being done on cyclohexane and benzene
Benzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....

 mixtures and solid phase cyclohexane to determine hydrogen yields of the mix when irradiated at −195 °C.

Cyclohexane is also used for calibration of differential scanning calorimetry
Differential scanning calorimetry
Differential scanning calorimetry or DSC is a thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference is measured as a function of temperature. Both the sample and reference are maintained at nearly the same temperature...

 (DSC) instruments, because of a convenient crystal-crystal transition at −87.1 C.

Heat treating equipment manufacturer Surface Combustion
Surface Combustion
Surface Combustion, Inc. is a North American manufacturer of thermal processing and heat treating equipment. Surface Combustion was founded in 1915 and is headquartered in Maumee, Ohio...

 uses cyclohexane as a carbon carrying gas in their high purity vacuum carburizing furnaces.

History

Unlike compounds like benzene
Benzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....

, cyclohexane cannot easily be obtained from natural resources such as coal. Toward the end of the nineteenth century early chemical investigators had to depend on organic synthesis. It took them 30 years to flesh out the details. In 1867 Marcellin Berthelot
Marcellin Berthelot
Marcelin Pierre Eugène Berthelot was a French chemist and politician noted for the Thomsen-Berthelot principle of thermochemistry. He synthesized many organic compounds from inorganic substances and disproved the theory of vitalism. He is considered as one of the greatest chemists of all time.He...

 reduced benzene
Benzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....

 with hydroiodic acid at elevated temperatures. He incorrectly identified the reaction product as n-hexane not only because of the convenient match in boiling point
Boiling point
The boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid....

 (69 °C) but also because he did not believe benzene was a cyclic molecule (like his contemporary August Kekulé) but rather some sort of association of acetylene
Acetylene
Acetylene is the chemical compound with the formula C2H2. It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in pure form and thus is usually handled as a solution.As an alkyne, acetylene is unsaturated because...

. In 1870 one of his sceptics Adolf von Baeyer
Adolf von Baeyer
Johann Friedrich Wilhelm Adolf von Baeyer was a German chemist who synthesized indigo, and was the 1905 recipient of the Nobel Prize in Chemistry. Born in Berlin, he initially studied mathematics and physics at Berlin University before moving to Heidelberg to study chemistry with Robert Bunsen...

 repeated the reaction and pronounced the same reaction product hexahydrobenzene and in 1890 Vladimir Markovnikov believed he was able to distill the same compound from Caucasus petroleum
Petroleum
Petroleum or crude oil is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface. Petroleum is recovered mostly through oil drilling...

 calling his concoction hexanaphtene.

In 1894 Baeyer synthesized cyclohexane starting with a Dieckmann condensation
Dieckmann condensation
The Dieckmann condensation is the intramolecular chemical reaction of diesters with base to give β-ketoesters. It is named after the German chemist Walter Dieckmann . The equivalent intermolecular reaction is the Claisen condensation....

 of pimelic acid
Pimelic acid
Pimelic acid is the organic compound with the formula HO2C5CO2H. Derivatives of pimelic acid are involved in the biosynthesis of the amino acid called lysine. Pimelic acid is one methylene longer than a related dicarboxylic acid, adipic acid, a precursor to many polyesters and polyamides...

 followed by multiple reductions:


and in the same year E. Haworth and W.H. Perkin Jr. (1860–1929) did the same in a Wurtz reaction
Wurtz reaction
The Wurtz reaction, named after Charles-Adolphe Wurtz, is a coupling reaction in organic chemistry, organometallic chemistry and recently inorganic main group polymers, whereby two alkyl halides are reacted with sodium to form a new carbon-carbon bond:...

 of 1,6-dibromohexane.


Surprisingly their cyclohexanes boiled higher by 10°C than either hexahydrobenzene or hexanaphtene but this riddle was solved in 1895 by Markovnikov, N.M. Kishner and Nikolay Zelinsky when they re-diagnosed hexahydrobenzene and hexanaphtene as methylcyclopentane
Alkyl cycloalkane
Alkyl cycloalkanes are chemical compounds with an alkyl group with a single ring of carbons to which hydrogens are attached according to the formula...

, the result of an unexpected rearrangement reaction
Rearrangement reaction
A rearrangement reaction is a broad class of organic reactions where the carbon skeleton of a molecule is rearranged to give a structural isomer of the original molecule. Often a substituent moves from one atom to another atom in the same molecule...

.

See also

  • The Flixborough disaster
    Flixborough disaster
    The Flixborough disaster was an explosion at a chemical plant close to the village of Flixborough, England, on 1 June 1974. It killed 28 people and seriously injured 36.-Background:...

    , a major industrial accident caused by an explosion of cyclohexane.
  • Hexane
    Hexane
    Hexane is a hydrocarbon with the chemical formula C6H14; that is, an alkane with six carbon atoms.The term may refer to any of four other structural isomers with that formula, or to a mixture of them. In the IUPAC nomenclature, however, hexane is the unbranched isomer ; the other four structures...

  • cyclohexane (data page)
    Cyclohexane (data page)
    - Material Safety Data Sheet : The handling of this chemical may incur notable safety precautions. It is highly recommend that you seek the Material Safety Datasheet for this chemical from a reliable source and follow its directions.***...


External links

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