Cyclooctene
Encyclopedia
Cyclooctene is a cycloalkene
with an eight-membered ring. It is notable because it is the smallest cycloalkene that can exist as either the cis- or trans-isomer
with the cis-isomer more common. Its most stable cis conformer is shaped like a crown ether
with alternating equatorial and axial hydrogens very much like the chair conformation of cyclohexane
; its most stable trans-conformer is shaped like the 8-carbon equivalent chair conformation of cyclohexane.
for cis). This is because trans-cycloalkenes have a longer bridging distance between the two allylic carbons than their respective cis-cycloalkenes. As a result, eight carbons is the minimum ring size required to form a trans-cycloalkene without incurring severe angle strain
which is the cause for the instability of smaller trans-rings. trans-Cycloheptene
and trans-cyclohexene
can exist, but they are very unstable at room temperature. trans-Cyclooctene exists in a helical conformation with the carbon chain lying above the double bond on one side and below it on the other, leading to chirality
(as depicted to the right).
trans-Cyclooctene was first synthesized on a preparatory scale by Arthur C. Cope
beginning from N,N,N-trimethylcyclooctylammonium iodide and using a Hofmann elimination
. Other methods exist where the trans-isomer is synthesized from the cis-isomer in several synthetic steps. In addition, a photochemical method exists for this conversion in just one step:
Although the cis-trans equilibrium is unfavorable, the reaction can be driven to completion by trapping
of the trans-isomer by complexation with silver
.
Cycloalkene
A cycloalkene or cycloolefin is a type of alkene hydrocarbon which contains a closed ring of carbon atoms, but has no aromatic character. Some cycloalkenes, such as cyclobutene and cyclopentene, can be used as monomers to produce polymer chains. Unless the rings are very large, cycloalkenes are...
with an eight-membered ring. It is notable because it is the smallest cycloalkene that can exist as either the cis- or trans-isomer
Isomer
In chemistry, isomers are compounds with the same molecular formula but different structural formulas. Isomers do not necessarily share similar properties, unless they also have the same functional groups. There are many different classes of isomers, like stereoisomers, enantiomers, geometrical...
with the cis-isomer more common. Its most stable cis conformer is shaped like a crown ether
Crown ether
Crown ethers are cyclic chemical compounds that consist of a ring containing several ether groups. The most common crown ethers are oligomers of ethylene oxide, the repeating unit being ethyleneoxy, i.e., -CH2CH2O-. Important members of this series are the tetramer , the pentamer , and the hexamer...
with alternating equatorial and axial hydrogens very much like the chair conformation of cyclohexane
Cyclohexane
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 being intermediates used in the production of nylon...
; its most stable trans-conformer is shaped like the 8-carbon equivalent chair conformation of cyclohexane.
| cis-Cyclooctene | trans-Cyclooctene |
trans-Cyclooctene
trans-Cyclooctene is the smallest cycloalkene in which the trans-isomer is stable at room temperature (compared to cyclopropeneCyclopropene
Cyclopropene is an organic compound with the formula 34. It is the simplest isolable cycloalkene. It has a triangular structure. Because the ring is highly strained, cyclopropene is both difficult to prepare and interesting to study. Like cyclopropane, the carbon ring of cyclopropene is planar...
for cis). This is because trans-cycloalkenes have a longer bridging distance between the two allylic carbons than their respective cis-cycloalkenes. As a result, eight carbons is the minimum ring size required to form a trans-cycloalkene without incurring severe angle strain
Angle strain
Angle strain, also called Baeyer strain in cyclic molecules, is the resistance associated with bond angle compression or bond angle expansion. It occurs when bond angles deviate from the ideal bond angles to achieve maximum bond strength in a specific chemical conformation...
which is the cause for the instability of smaller trans-rings. trans-Cycloheptene
Cycloheptene
Cycloheptene is a 7-membered cycloalkene with a flash point of -6 C°. It is a raw material in organic chemistry and a monomer in polymer synthesis. Cycloheptene can exist as either the cis- or the trans-isomer.- trans-Cycloheptene :...
and trans-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...
can exist, but they are very unstable at room temperature. trans-Cyclooctene exists in a helical conformation with the carbon chain lying above the double bond on one side and below it on the other, leading to chirality
Chirality
Chirality is a property of asymmetry important in several branches of science. It may refer to:* Chirality , a property of molecules having a non-superimposable mirror image...
(as depicted to the right).
trans-Cyclooctene was first synthesized on a preparatory scale by Arthur C. Cope
Arthur C. Cope
Arthur C. Cope was a highly successful and influential organic chemist and member of the National Academy of Sciences. He is credited with the development of several important chemical reactions which bear his name including the Cope elimination and the Cope rearrangement.Cope was born on June...
beginning from N,N,N-trimethylcyclooctylammonium iodide and using a Hofmann elimination
Hofmann elimination
Hofmann elimination is a process where an amine is reacted to create a tertiary amine and an alkene by treatment with excess methyl iodide followed by treatment with silver oxide, water, and heat.After the first step, a quaternary ammonium iodide salt is created...
. Other methods exist where the trans-isomer is synthesized from the cis-isomer in several synthetic steps. In addition, a photochemical method exists for this conversion in just one step:
Although the cis-trans equilibrium is unfavorable, the reaction can be driven to completion by trapping
Chemical trap
In chemistry, a chemical trap is a chemical compound that is used to detect a certain molecule when* The concentration of this molecule is very small and below detection limit...
of the trans-isomer by complexation with silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
.