''trans''-Cyclooctene is a cyclic

hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and Hydrophobe, hydrophobic; their odor is usually fain ...

with the formula ��(CH₂)₆CH=CH– where the two C–C single bonds adjacent to the double bond are on opposite sides of the latter's plane. It is a colorless liquid with a disagreeable odor. Cyclooctene is notable as the smallest cycloalkene that is readily isolated as its ''trans''-

isomer In chemistry, isomers are molecules or polyatomic ions with identical molecular formula – that is, the same number of atoms of each element (chemistry), element – but distinct arrangements of atoms in space. ''Isomerism'' refers to the exi ...

. The ''cis''-isomer is much more stable; the ring-strain energies being 16.7 and 7.4 kcal/mol, respectively. A planar arrangement of the ring carbons would be too strained, and therefore the stable conformations of the ''trans'' form have a bent (non-planar) ring. Computations indicate that the most stable "crown" conformation has the carbon atoms alternately above and below the plane of the ring. A "half-chair" conformation, with about 6 kcal/mol higher energy, has carbons 2,3,5,6, and 8 on the same side of the plane of carbons 1,4, and 7. All conformations of ''trans''-cyclooctene are

chiral Chirality () is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek language, Greek (''kheir''), "hand", a familiar chiral object. An object or a system is ''chiral'' if it is dist ...

(specifically, what some call planar-chiral) and the

enantiomer In chemistry, an enantiomer (Help:IPA/English, /ɪˈnænti.əmər, ɛ-, -oʊ-/ Help:Pronunciation respelling key, ''ih-NAN-tee-ə-mər''), also known as an optical isomer, antipode, or optical antipode, is one of a pair of molecular entities whi ...

s can be separated. In theory, conversion of between the enantiomers can be done, without breaking any bonds, by twisting the whole –CH=CH– group, rigidly, by 180 degrees. However, that entails passing one of its hydrogens through the crowded ring.

Preparation

''trans''-Cyclooctene was first synthesized on a preparatory scale by Arthur C. Cope with a Hofmann elimination reaction of ''N,N,N''-trimethylcyclooctylammonium iodide. The reaction gives a mixture of ''cis'' and ''trans'' isomers, and the ''trans'' isomer is selectively trapped as a

complex Complex commonly refers to: * Complexity, the behaviour of a system whose components interact in multiple ways so possible interactions are difficult to describe ** Complex system, a system composed of many components which may interact with each ...

with

silver nitrate Silver nitrate is an inorganic compound with chemical formula . It is a versatile precursor to many other silver compounds, such as those used in photography. It is far less sensitive to light than the halides. It was once called ''lunar causti ...

. Other methods exist where the ''trans'' isomer is synthesized from the ''cis'' isomer in several synthetic steps. For instance, it can be prepared in almost 100% yield by converting the ''cis'' isomer to 1,2-epoxycyclooctane ("cyclooctene oxide") followed by reactions with lithium diphenylphosphide () and with

methyl iodide Iodomethane, also called methyl iodide, and commonly abbreviated "MeI", is the chemical compound with the formula CH3I. It is a dense, colorless, volatile liquid. In terms of chemical structure, it is related to methane by replacement of one h ...

. (Similar procedures can give ' isomers of 1,4-cyclooctadiene and

1,5-cyclooctadiene 1,5-Cyclooctadiene (also known as cycloocta-1,5-diene) is a cyclic compound, cyclic hydrocarbon with the chemical formula , specifically . There are three configurational isomers with this structure, that differ by the arrangement of the four C� ...

). In addition, a photochemical method exists for the direct ''cis''–''trans'' isomerisation. Although this equilibrium strongly favours the more stable ''cis'' form, the reaction can be driven towards the ''trans'' form by trapping with silver ions.

Reactions

Because of the higher internal strain on the double bond, the ''trans'' isomer is more reactive than the ''cis'' isomer and of typical unsaturated hydrocarbons. For instance, its double bond will rapidly add tetrazine and its derivatives. The compound also readily polymerizes with a

ruthenium Ruthenium is a chemical element; it has symbol Ru and atomic number 44. It is a rare transition metal belonging to the platinum group of the periodic table. Like the other metals of the platinum group, ruthenium is unreactive to most chem ...

-based initiator.

References

{{Authority control Cycloalkenes Foul-smelling chemicals