Cycloheptene is a 7-membered
cycloalkeneA 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....
with a flash point of -6 C°. It is a raw material in
organic chemistryOrganic chemistry is a discipline within chemistry which involves the scientific study of the structure, properties, composition, reactions, and preparation of chemical compounds that contain carbon...
and a
monomerA monomer is a small molecule that may become chemically bonded to other monomers to form a polymer...
in polymer synthesis. Cycloheptene can exist as either the
cis- or the
trans-isomer.
With cycloheptene, the
cis-isomer is always assumed but the
trans-isomer does also exist. One procedure for the
organic synthesisOrganic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic molecules can often contain a higher level of complexity compared to purely inorganic compounds, so the synthesis of organic compounds has...
of
trans-cycloheptene is by singlet photosensitization of cis-cycloheptene with
methyl benzoateMethyl benzoate is an ester with the chemical formula C6H5COOCH3. It is formed by the condensation of methanol and benzoic acid...
and ultraviolet light at -35°C..
Cycloheptene is a 7-membered
cycloalkeneA 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....
with a flash point of -6 C°. It is a raw material in
organic chemistryOrganic chemistry is a discipline within chemistry which involves the scientific study of the structure, properties, composition, reactions, and preparation of chemical compounds that contain carbon...
and a
monomerA monomer is a small molecule that may become chemically bonded to other monomers to form a polymer...
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 synthesisOrganic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic molecules can often contain a higher level of complexity compared to purely inorganic compounds, so the synthesis of organic compounds has...
of
trans-cycloheptene is by singlet photosensitization of cis-cycloheptene with
methyl benzoateMethyl benzoate is an ester with the chemical formula C6H5COOCH3. It is formed by the condensation of methanol and benzoic acid...
and ultraviolet light at -35°C.. The
double bondA double bond in chemistry is a chemical bond between two chemical elements involving four bonding electrons instead of the usual two. The most common double bond, that between two carbon atoms, can be found in alkenes. Many types of double bonds between two different elements exist, for example in...
in the
trans isomer is very
strainedIn a molecule, strain energy is released when the constituent atoms are allowed to rearrange themselves in a chemical reaction or a change of chemical conformation in a way that:* angle strain,* torsional strain,* ring strain and/or steric strain,...
. Part of the steric strain is relieved by pyramidalization of the alkene carbons, the pyramidalization angle is estimated at 37° (compared to a zero angle in an unstrained alkene) and the p-orbital misalignment is 30.1°.. Because the barrier for rotation of the double bond in
ethyleneEthylene is the chemical compound with the formula C
2H
4. It is the simplest alkene. Because it contains a carbon-carbon double bond, ethylene is called an unsaturated hydrocarbon or an olefin. It is extremely important in industry and also has a role in biology as a hormone...
is approximately 65 kcal/
molThe mole is a unit of amount of substance: it is an SI base unit, and one of the few units used to measure this physical quantity. The name "mole" was coined in German by Wilhelm Ostwald in 1893, although the related concept of equivalent mass had been in use at least a century earlier...
(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-cycloocteneCyclooctene is a cycloalkene with an eight-membered ring. It can exist as either the cis- or trans-isomer with the cis-isomer normally being the predominant configuration. Cyclooctene is a smaller cycloalkene in which the cis-isomer is stable at room temperature...
. In fact, it is not; and unless the temperature is kept very low, quick isomerization to the
cis-isomer takes place. In a 2005 publication, it is argued that
trans-cycloheptene isomerization occurs by 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
diradicalIn chemistry, radicals are atoms, molecules, or ions with unpaired electrons on an otherwise open shell configuration. These unpaired electrons are usually highly reactive, so radicals are likely to take part in chemical reactions...
dimerA dimer is a chemical or biological entity consisting of two structurally similar subunits called monomers, which are joined by bonds, which can be strong or weak.- Organic chemistry :...
. The two
heptanen-Heptane is the straight-chain alkane with the chemical formula H
3C
5CH
3 or C
7H
16. It is the zero point of the octane rating scale .-Uses:...
radical rings then untwist to an unstrained conformation and in the final step the dimer is cleaved again into two
cis-cycloheptene molecules. Note that the photoisomerization of
maleic acidMaleic acid or -butenedioic acid or cis-butenedioic acid. is an organic compound that is a dicarboxylic acid...
to
fumaric acidFumaric acid or trans-butenedioic acid, is the chemical compound with the formula HO2CCH=CHCO2H.
This white crystalline compound is one of two isomeric unsaturated dicarboxylic acids, the other being maleic acid, wherein the carboxylic acid groups are cis. It has a...
with
bromineBromine , is a chemical element with the symbol Br and atomic number 35. A halogen element, bromine is a reddish-brown volatile liquid at standard room temperature that is intermediate in reactivity between chlorine and iodine. Bromine vapors are corrosive and toxic. Approximately 556,000 metric...
is also bimolecular.