Cycloheptene is a 7-membered

cycloalkene A cycloalkene or cycloolefin is a type of alkene hydrocarbon which contains a closed ring of carbon atoms and either one or more double bonds, but has no aromatic character. Some cycloalkenes, such as cyclobutene and cyclopentene, can be used as ...

with a flash point of −6.7 °C. It is a raw material in

organic chemistry Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, J ...

and a

monomer In chemistry, a monomer ( ; '' mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or three-dimensional network in a process called polymerization. Classification ...

in polymer synthesis. Cycloheptene can exist as either the ''cis''- or the ''trans''-isomer. :

''trans''-Cycloheptene

With cycloheptene, the ''cis''-isomer is always assumed but the ''trans''-isomer does also exist. One procedure for the

organic synthesis Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...

of ''trans''-cycloheptene is by singlet

photosensitization Photosensitizers produce a physicochemical change in a neighboring molecule by either donating an electron to the substrate or by abstracting a hydrogen atom from the substrate. At the end of this process, the photosensitizer eventually returns to ...

of cis-cycloheptene with

methyl benzoate Methyl benzoate is an organic compound. It is an ester with the chemical formula C6H5CO2CH3. It is a colorless liquid that is poorly soluble in water, but miscible with organic solvents. Methyl benzoate has a pleasant smell, strongly reminiscen ...

and

ultraviolet light Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiatio ...

at −35 °C. The

double bond In chemistry, a double bond is a covalent bond between two atoms involving four bonding electrons as opposed to two in a single bond. Double bonds occur most commonly between two carbon atoms, for example in alkenes. Many double bonds exist betwee ...

in the ''trans'' isomer is very strained. The directly attached atoms on a simple alkene are all coplanar. In ''trans''-cycloheptene, however, the size of the ring makes it impossible for the alkene and its two attached carbons to have this geometry because the remaining three carbons could not reach far enough to close the ring (see also

Bredt's rule Bredt's rule is an empirical observation in organic chemistry that states that a double bond cannot be placed at the bridgehead of a bridged ring system, unless the rings are large enough. The rule is named after Julius Bredt, who first discussed ...

). There would have to be unusually large angles (

angle strain In organic chemistry, ring strain is a type of instability that exists when bonds in a molecule form angles that are abnormal. Strain is most commonly discussed for small rings such as cyclopropanes and cyclobutanes, whose internal angles are ...

), unusually long bond-lengths, or the atoms of the alkane-like loop would collide with the alkene part ( steric strain). Part of the strain is relieved by pyramidalization of each alkene carbon and their rotation relative to each other. The pyramidalization angle is estimated at 37° (compared to an angle of 0° for an atom with normal trigonal–planar geometry) and the

p-orbital In atomic theory and quantum mechanics, an atomic orbital is a function describing the location and wave-like behavior of an electron in an atom. This function can be used to calculate the probability of finding any electron of an atom in any ...

misalignment is 30.1°. Because the barrier for rotation of the double bond in

ethylene Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon-carbon double bonds). Ethylene ...

is approximately 65

kcal The calorie is a unit of energy. For historical reasons, two main definitions of "calorie" are in wide use. The large calorie, food calorie, or kilogram calorie was originally defined as the amount of heat needed to raise the temperature of o ...

/ mol (270 kJ/mol) and can only be lowered by the estimated strain energy of 30 kcal/mol (125 kJ/mol) present in the ''trans''-isomer, ''trans''-cycloheptene should be a stable molecule just as its homologue ''trans''-cyclooctene. In fact, it is not: unless the temperature is kept very low, rapid isomerization to the ''cis''-isomer takes place. The ''trans''-cycloheptene isomerization mechanism is not simple alkene-bond rotation, but rather an alternative lower energy pathway. Based on the experimentally observed second order reaction kinetics for isomerization, two ''trans''-cycloheptene molecules in the proposed pathway first form a

diradical In chemistry, a diradical is a molecular species with two electrons occupying molecular orbitals (MOs) which are degenerate. The term "diradical" is mainly used to describe organic compounds, where most diradicals are extremely reactive and i ...

dimer Dimer may refer to: * Dimer (chemistry), a chemical structure formed from two similar sub-units ** Protein dimer, a protein quaternary structure ** d-dimer * Dimer model, an item in statistical mechanics, based on ''domino tiling'' * Julius Dimer ( ...

. The two

heptane Heptane or ''n''-heptane is the straight-chain alkane with the chemical formula H3C(CH2)5CH3 or C7H16. When used as a test fuel component in anti-knock test engines, a 100% heptane fuel is the zero point of the octane rating scale (the 100 poin ...

radical rings then untwist to an unstrained conformation, and finally the dimer fragments back into two ''cis''-cycloheptene molecules. Note that the photoisomerization of

maleic acid Maleic acid or ''cis''-butenedioic acid is an organic compound that is a dicarboxylic acid, a molecule with two carboxyl groups. Its chemical formula is HO2CCH=CHCO2H. Maleic acid is the ''cis''-isomer of butenedioic acid, whereas fumaric ac ...

fumaric acid Fumaric acid is an organic compound with the formula HO2CCH=CHCO2H. A white solid, fumaric acid occurs widely in nature. It has a fruit-like taste and has been used as a food additive. Its E number is E297. The salts and esters are known as fu ...

with

bromine Bromine is a chemical element with the symbol Br and atomic number 35. It is the third-lightest element in group 17 of the periodic table ( halogens) and is a volatile red-brown liquid at room temperature that evaporates readily to form a simi ...

is also bimolecular.

References

External links

MSDS
{{cycloalkenes Cycloalkenes Monomers Seven-membered rings