Cyclohexane
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| Cyclohexane | |
|---|---|
    |
|
| General | |
| Molecular formula | C6H12 |
| SMILES | C1CCCCC1 |
| Molar mass | 84.16 g/mol |
| Appearance and smell | Colourless liquid, greasy smell or petroleum |
| CAS number | [110-82-7] |
| Properties | |
| Density and phase | 0.779 g/ml, liquid |
| Solubility in water | Immiscible |
| Solubility in ethanol | Miscible |
| Melting point | 6.55 °C |
| Boiling point | 80.74 °C |
| Viscosity | 1.02 cP at 17 °C |
| Index of refraction, nD | 1.423 |
| Thermodynamic data | |
| Standard enthalpy of formation ΔfH |
-156 kJ/mol |
| Standard enthalpy of combustion ΔcH |
-3920 kJ/mol |
| Standard molar entropy S |
? J.K−1.mol−1 |
| Hazards | |
| MSDS | External MSDS |
| EU classification | Flammable (F) Harmful (Xn) Dangerous for the environment (N) Severe eye irritant, may cause corneal clouding |
| Flash Point | -20 C |
| NFPA 704 |
3
1
0
|
| R-phrases | R11, R38, R65, R67, R50/53 |
| S-phrases | S2, S9, S16, S25, S33, S60, S61, S62 |
| Supplementary data page | |
| Structure and properties |
n, εr, etc. |
| Thermodynamic data |
Phase behaviour Solid, liquid, gas |
| Spectral data | UV, IR, NMR, MS |
| Regulatory data | Flash point, RTECS number, etc. |
| Related compounds | |
| Related cycloalkanes | Cyclopentane Cycloheptane |
| Related compounds | Cyclohexene |
| Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references |
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Cyclohexane is a cycloalkane with the molecular formula C6H12. Cyclohexane is used as a nonpolar solvent for the chemical industry, and also as a raw material for the industrial production of adipic acid and caprolactam, both of which are intermediates used in the production of nylon. Due to its unique chemical and conformational properties, cyclohexane is also used in labs in analysis and as a standard.
Contents |
[edit] Chemical conformation
Contrary to popular belief, the 6 vertexed ring does not conform to the shape of a perfect hexagon. Although the conformation of a flat 2D planed hexagon would reduce angle strain to 0, the torsional strain would be considerable. Therefore to reduce torsional strain, cyclohexane adopts a three-dimensional structure known as the chair conformation. The new conformation puts the carbons at an angle of 109.5°. Half of the substituents are in the plane of the ring (equatorial) while the other half are perpendicular to the plane (axial). This conformation of substituents allows for the most stable structure of cyclohexane and its substituents. If cyclohexane is saturated with hydrogens with only one larger substituent attached the ring (called mono-substituted), then the larger substituent will most likely be found in the equatorial form, once again for the sake of stability.
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 when burned compared to the other cycloalkanes.
However 0 ring strain only occurs when cyclohexane is in the chair formation. Cyclohexane can take other confomational forms, however these forms are less stable and as a result, cyclohexane will be found in the chair formation 99.99% at 25 °C (i.e. 99.99% of all molecules in a solution sample will be in the chair formation). Other conformations, starting with the most stable to the least are: the twisted boat, boat, half-chair and plane (flat) formation. Rarely are the half-chair and, even less, the flat formation, found in solution.
Substituents found on cyclohexane adopt cis and trans formations and cannot be easily switched by simple single sigma bond rotation as with linear molecules. Cis formation means that both substituents are found on the upper side of the 2 substituent placements on the carbon, while trans would mean that they were on opposing sides. Despite the fact that carbons on cyclohexane are linked by a single bond, the ring remains rigid, in that switching from cis to trans would require breaking the ring. The nomenclature for cis is dubbed (Z) while the name for trans is (Z) to be placed in front of the IUPAC name.
For di-substituted cyclohexane rings (i.e. two groups on the ring), the relative orientation of the two substituents affect the energy of the possible conformations. For 1,2- and 1,4-di-substituted cyclohexane, a cis configuration leads to one axial and one equatorial group. This configuration can undergo chair flipping. For 1,2- and 1,4-di-substituted cyclohexane, a trans configuration leads to either both groups axial or both equatorial. In this case, the diaxial conformation is effectively prevented by its high steric strain (four gauche interactions more than the diequatorial). For 1,3-di-substituted cyclohexanes, the cis form is diequatorial and the flipped conformation suffers additional steric interaction between the two axial groups. Trans-1,3-di-substituted cyclohexanes are like cis-1,2- and cis-1,4- and can flip between the two equivalent axial/equatorial forms.
[edit] Reactions with cyclohexane
Pure cyclohexane in itself is rather unreactive, being a non-polar, hydrophobic, hydrocarbon. It can react with very strong acids such as superacids like the famous superacid system HF + SbF5 which will cause forced protonation and "hydrocarbon cracking". Substituted cyclohexane molecules are reactive to many different reactions, however, many which are important to organic chemistry. Some reactions and their required conditions are as follows:
Reactions of cyclohexane halides
Possible substituents:
Alcohols, ethers, thiols, alkyl nitriles, thioethers, alkyl azides, alkynes, esters, alkyl iodides, tetraalkyl ammonium salts
Reaction type:
SN2 substitution
Favorable conditions:
- strong nucleophile
- good leaving group (weak base)
- polar, non-ionizing solvent
- small, less sterically hindered Nu
Reaction type:
SN1 substitution
Favorable conditions:
- greater carbocation stability
- good leaving group (weak base)
- polar ionizing solvent
Reaction type:
E2 elimination
Favorable conditions:
- H and leaving group must be axial in cyclohexyl systems
- high nucleophile concentration
- aprotic solvent
- sterically hindered base
- increased temperature
- increased entropy in products
Reaction type:
E1 elimination
Favorable conditions:
- greater carbocation stability
- good leaving group (weak base)
- polar ionizing solvent
Reaction type:
Dehydrohalogenation
Favorable conditions:
- more substituted cycloalkene (Zaitsev product) favoured if base is small
- less substituted cycloalkene (Hofmann product) favoured if base is bulky
- strong, bulky base
- high temperature
- these favour the cycloalkene as opposed to SN2 substitution*
Reaction type:
Dehydrohalo-genation of vicinal (1,2)-dihalides
Favorable conditions:
- requires a strong base (i.e. NaNH2) in NH3(l) or mineral oil
- requires high temperature
Reactions of alcohols
Reaction type:
Dehydration
Favorable conditions:
- must be acidic environment
- 1° requires concentrated H2SO4
- 2° requires 85% H3PO4
- 3° requires 10-20% H2SO4 and 60-80 °C
Reactions of cycloalkenes
Reaction type:
Addition of alkyl halides
Favorable conditions:
-Markovnikov addition of the hydrogen atom to the more substituted carbon
Reaction type:
Acid-catalyzed addition of water
Favorable conditions:
-high H2O concentration
-low temperature
Reaction type:
Bromination
Favorable conditions:
- a bridged carbocation is formed
Reaction type:
Oxymercuration-demercuration
Favorable conditions:
- requires Hg(OAc)2 in THF-H2O, then NaBH4/OH−
Reaction type:
Oxidation
Favorable conditions:
- KMnO4 is less easy to work with, even at lower temperatures, because it is strong enough to make the diol into a ketone and a carboxylic acid
- requires cold KMnO4 in OH−/H2O or OsO4/pyridine followed by NaHSO3/H2O
Reaction type:
Oxidative cleavage
Favorable conditions:
- if an aldehyde is desired, ozonolysis and a reductive workup is required (described in next reaction)
- requires hot KMnO4 in OH−/H2O
Reaction type:
Ozonolysis and a reductive workup
Favorable conditions:
- requires ozone in CH2Cl2 at −78°C
- second step requires Zn in HOAc or Me2S
Reaction type:
Hydroboration-oxidation
Favorable conditions:
- requires BH3·THF and H2O2/OH−
[edit] Mechanisms for reactions
Arrangement of groups in cyclohexane, and indeed in most cycloalkane molecules, is extremely important in chemical reactions, especially reactions involving nucleophiles. Substituents on the ring must be in the axial formation to react with other molecules. For example, the reaction of bromocyclohexane and a common nucleophile, a hydroxide anion (OH−), would result in cyclohexene:
C6H11Br + OH− → C6H10 + H2O
This reaction, commonly known as an elimination reaction or dehalogenation (specifically E2), requires that the Br substituent 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. 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 Br for an 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).
[edit] Cyclohexane in research
The usefulness of cyclohexane in research is not to be underestimated. Although much is already known about this cyclic hydrocarbon, research is still being done on cyclohexane and benzene mixtures and solid phase cyclohexane to determine hydrogen yields of the mix when irradiated at −195 °C.
[edit] See also
- The Flixborough disaster, a major industrial accident caused by an explosion of cyclohexane.
- Hexane
[edit] External links
