A cycloalkene or cycloolefin is a type of
alkene
In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond.
Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...
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 hydrophobic, and their odors are usually weak or ex ...
which contains a closed ring of
carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent
In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...
atoms
Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons.
Every solid, liquid, gas, an ...
and either one or more double bonds, but has no
aromatic
In chemistry, aromaticity is a chemical property of cyclic ( ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to satur ...
character. Some cycloalkenes, such as
cyclobutene
Cyclobutene is a cycloalkene. It is of interest in research but currently has no practical applications. It is a colorless easily condensed gas. A modern synthesis involves the 2-step dehydration of cyclobutanol. The compound was first prepared ...
and
cyclopentene
Cyclopentene is a chemical compound with the formula . It is a colorless liquid with a petrol-like odor. It has few applications, and thus is mainly used as a minor component of gasoline, present in concentrations of less than 1%. It is one of t ...
, can be used as
monomers
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
M ...
to produce
polymer
A polymer (; Greek '' poly-'', "many" + ''-mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
chains. Due to geometrical considerations, smaller cycloalkenes are almost always the
''cis'' isomers, and the term ''cis'' tends to be omitted from the names. Cycloalkenes require considerable p-orbital overlap in the form of a bridge between the carbon-carbon double bond, however, this is not feasible in smaller molecules due to the increase of strain that could break the molecule apart. In greater carbon number cycloalkenes, the addition of CH
2 substituents decreases strain. trans-Cycloalkenes with 7 or fewer carbons in the ring will not occur under normal conditions because of the large amount of ring strain needed. In larger rings (8 or more atoms),
''cis''–''trans'' isomerism of the double bond may occur. This stability pattern forms part of the origin of
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 i ...
, the observation that alkenes do not form at the bridgehead of many types of
bridged ring systems because the alkene would necessarily be ''trans'' in one of the rings.
Examples
File:Cyclopropene 2D skeletal.svg, Cyclopropene
Cyclopropene is an organic compound with the formula . It is the simplest cycloalkene. Because the ring is highly strained, cyclopropene is difficult to prepare and highly reactive. This colorless gas has been the subject for many fundamental st ...
File:Cyclobutene.svg, Cyclobutene
Cyclobutene is a cycloalkene. It is of interest in research but currently has no practical applications. It is a colorless easily condensed gas. A modern synthesis involves the 2-step dehydration of cyclobutanol. The compound was first prepared ...
File:Cyclopentene.svg, Cyclopentene
Cyclopentene is a chemical compound with the formula . It is a colorless liquid with a petrol-like odor. It has few applications, and thus is mainly used as a minor component of gasoline, present in concentrations of less than 1%. It is one of t ...
File:Cyclohexene.svg, 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. Cyclohexene is not very stable upon long term storage with exposure to light a ...
File:Cycloheptene.svg, Cycloheptene
Cycloheptene is a 7-membered cycloalkene with a flash point of −6.7 °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''-Cy ...
File:1,3-cyclohexadiene.svg, 1,3-Cyclohexadiene
File:1,4-cyclohexadiene.svg, 1,4-Cyclohexadiene
File:1,5-Cyclooctadiene.svg, 1,5-Cyclooctadiene
Cycloocta-1,5-diene is a cyclic hydrocarbon with the chemical formula , specifically .
There are three configurational isomers with this structure, that differ by the arrangement of the four C–C single bonds adjacent to the double bonds. Each ...
File:Cis-cyclooctene.png, cis-cyclooctene
''cis''-Cyclooctene is a cycloalkene with the formula (CH2)6(CH)2. It is a colorless liquid that is used industrially to produce a polymer. It is also a ligand in organometallic chemistry.
Cyclooctene is the smallest cycloalkene that can be iso ...
File:(S)-(+)-trans-Cyclooctene Structural Formula V.1.svg, trans-cyclooctene
''trans''-Cyclooctene is a cyclic hydrocarbon 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.
Cyclo ...
Nomenclature
Cycloalkenes follow a similar
nomenclature
Nomenclature (, ) is a system of names or terms, or the rules for forming these terms in a particular field of arts or sciences. The principles of naming vary from the relatively informal naming conventions, conventions of everyday speech to the i ...
system to
alkenes
In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond.
Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...
, but the carbons are numbered starting at a carbon on the double bond and then through the double bond and around the ring. This method is used to keep the index numbers small.
File:Cycloalkene nomenclature.svg, 1-methylcyclohexene
File:3-methylcyclohexene nomenclature.svg, 3-methylcyclohexene
Properties
Cycloalkenes with a small ring have about 20° more bond angle strain than a cycloalkane of the same size.
This is because the bond angle for an alkene, C-C=C, is 122°, while the bond angle for an alkane, C-C-C, is 112°. When these carbons form a small ring, the alkene which has a larger bond angle will have to compress more than the alkane causing more bond angle strain.
Cycloalkenes have a lower melting point than cycloalkanes of the same size. The lowered melting point is due to the double bond preventing the compound from compact packing.
Cycloalkenes generally reflect physical properties of their cycloalkane. In physical states, only the smaller cycloalkenes are gases while the others are mostly liquid. These molecules are also more reactive than cycloalkanes due to increased electron density shifts of the double bond.
Trans Isomers
As previously mentioed, cis-isomers of cycloalkenes exhibit more stability than trans-isomers; however, on an experimental and computational level, this property is only applicable to cycloalkenes with 10 carbons or less. As the number of carbons increase, the possibility of a trans-isomer occurring also increase. The geometrical considerations as analyzed by computational analysis are as follows.
The most stable trans-isomers of 10 ring or greater cycloalkenes exhibit 4 irregularities from standard geometric norms. The first irregularity is twisted planes of substituents along the C=C. Using C=C as the stable axis, 2 substituents of 1 carbon can be visualized on the same plane, equally applied to the other carbon. These planes are not planar and instead one carbon substituent plane twists along the axis away or toward the other carbon’s plane. This twisting leads to pyramidalization forming a
pyramidal alkene Pyramidal alkenes are alkenes in which the two carbon atoms making up the double bond are not coplanar with their four substituents. This deformation results from geometric constraints. Pyramidal alkenes only are of interest because much can be lear ...
which is the second irregularity. A greater angle of twisting, usually results in lower carbon number rings and decreases as the carbon number increases. Pyramidalization is important in highered number rings, because it increases p-orbital overlap for stability, and reduces torisional strain.
Bond length between the C=C and corresponding vinyllic carbons also vary. In smaller cycloalkenes, it is expected for the bonds to be greater in length uniformly to account for increased strain, but for example, trans-cycloheptane has varying bond lengths. Also, the vinyllic carbons on trans cyclohexanes exhibit longer bond lengths than their respective cis isomer for trans-cycloheptane through trans-cyclononene (7 carbon and 9 carbon cycloalkenes).
Synthesis Reactions
Ring Closing Metathesis
Ring-Closing Metathesis switches out functional groups from one or multiple terminal alkenes to form a cycloalkene. This process can be used to form cycloalkenes of either E or Z configurations, depending on the stereochemistry of the second ring strain.
Birch Reduction
Birch reduction is a possible method to reduce reduces aromatic compounds into cycloalkenes, specifically cyclohexadiene.
Diels-Alder Reaction
The Diels-Alder reaction, also known as cycloaddition, combines a conjugated diene and an alkene to form cycloalkene. This is a concerted process, with bonds forming and breaking simultaneously.
Cyclization reactions
Cyclization reactions, or intramolecular addition reactions, can be used to form cycloalkenes. These reactions primarily form cyclopentenones, a cycloalkene that contains two functional groups: the cyclopentene and a ketone group. However, other cycloalkenes, such as Cyclooctatetraene, can be formed as a result of this reaction.
Electrocyclic Reactions
Reactions of conjugated double-bond systems can be synthesized into cycloalkenes through electrocyclic reactions.
Addition of heat or photolysis causes a reversible reaction that causes one pi bond to become a sigma bond, which closes the ring and creates a cycloalkene.
Intramolecular McMurry Reactions
When two carbonyl groups are coupled and undergo a McMurry reaction, there is a possibility of the formation of cycloalkenes under specific conditions.
When both carbonyls are within the same molecule and not sufficiently separated from each other, a cycloalkene can be formed through an intramolecular McMurry reaction.
See also
*
Perfluorocycloalkene A perfluorocycloalkene (PFCA) fluorocarbon structure with a cycloalkene core. PFCAs have shown reactivity with a wide variety of nucleophiles including phenoxides, alkoxides, organometallic, amines, thiols, and azoles. They or their derivatives are ...
(PFCA)
*
Alicyclic compound
*
Olefin
*
Cycloalkyne
In organic chemistry, a cycloalkyne is the cyclic analog of an alkyne (). A cycloalkyne consists of a closed ring of carbon atoms containing one or more triple bonds. Cycloalkynes have a general formula Because of the linear nature of the alkyn ...
*
Cycloalkane
In organic chemistry, the cycloalkanes (also called naphthenes, but distinct from naphthalene) are the monocyclic saturated hydrocarbons. In other words, a cycloalkane consists only of hydrogen and carbon atoms arranged in a structure containing ...
References
{{Orgchem-stub