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Cycloalkenes: chemical structure, properties, nomenclature

They are part of the unsaturated organic compounds of straight chain called alkenes or olefins, because they have an oily (oily) appearance, but the difference is that cycloalkenes have closed chains, forming cycles or rings.

Cycloalkenes
Cyclopropene, a type of cycloalkene

As in alkenes , the double bond corresponds to a σ bond (high energy sigma) and a π bond (lower energy pi). It is this last link that allows reactivity to occur, due to its ease in breaking and forming free radicals.

Cycloalkenes

They have a general formula given  n H 2n-2. In this formula  n indicates the number of carbon atoms that the structure has. The smallest cycloalkene is cyclopropene, meaning it has only 3 carbon atoms and a single double bond.

Cycloalkenes

If you want to obtain a structure with a number of carbon atoms = 3 applying the formula C n H n-2,  simply replace  n with 3, obtaining the following molecular formula:

3 H 2 (3) -2 = C 3 H 6-2 = C 3 H 4 .

Then, there is a cycle with 3 carbon atoms and 4 hydrogens, the same as it appears in the figure.

These chemical compounds are very useful at an industrial level due to their ease in forming new substances such as polymers (due to the presence of the double bond), or for obtaining cycloalkanes with the same number of carbon atoms, which are precursors for the formation of other compounds.

Chemical structure of cycloalkenes

Cycloalkenes can have one or more double bonds in their structure that must be separated by a single bond; this is known as a conjugate structure. Otherwise, repulsive forces are created between them causing the molecule to break down.

The most energetically stable structures do not have many double bonds in their cycle, because the molecular structure is distorted due to the large amount of energy caused by the moving electrons in it.

Cycloalkenes

One of the most important trienes is cyclohexatriene, a compound that has six carbon atoms and three double bonds. This compound belongs to a group of elements called arenes or aromatics. Such is the case of naphthalene, phenanthrene and anthracene, among others.

Nomenclature

To name cycloalkenes, the standards according to the International Union of Pure and Applied Chemistry (IUPAC) must be taken into account:

Cycloalkene with a single double bond and no alkyl or radical substituents

– The number of carbons in the cycle is counted.

– The word “cycle” is written, followed by the root that corresponds to the number of carbon atoms (met, et, prop, but, pent, among others), and is given the ending “eno” because it corresponds to an alkene .

Cycloalkenes

Cycloalkenes with two or more double bonds and without alkyl or radical substituents

The carbon chain is numbered in such a way that the double bonds are located between two consecutive numbers with the smallest possible value.

The numbers are written separated by commas. Once the numbering is complete, a hyphen is written to separate the numbers from the letters.

The word “cycle  is then written , followed by the root that corresponds to the number of carbon atoms that the structure has. Write the letter “a” and then write the number of double bonds, using the prefixes di (two), tri (three), tetra (four), penta (five) and so on. It ends with the suffix “eno”.

The following example shows two enumerations: one circled in red and one circled in blue. 

The numbering in the red circle shows the correct form according to IUPAC standards, while the one in the blue circle is not correct because the double bond is not included between consecutive numbers of lesser value.

Cycloalkenes

Cycloalkenes with alkyl substituents

With an alkyl substituent

The cycle is listed as already mentioned. The propyl radical is written with the corresponding numbering and separated by a hyphen from the name of the main chain:

Cycloalkenes

The numbering begins with the carbon that has one of the radicals. This gives the smallest possible value, both to the radical and to the double bond. The radicals are named in alphabetical order.

Cycloalkenes

Properties of cycloalkenes

Physical properties

The physical properties of cycloalkenes depend on their molecular weight , the amount of double bonds they have and the radicals that are attached to them.

The physical state of the first two cycles (C 3  and C 4 ) are gases, up to C 10 they are liquid and from then on they are solid.

The boiling point of cycloalkenes is higher compared to that of cycloalkanes of the same number of carbon atoms. This is due to the electron density caused by the π (pi) bonds of the double bond, while the melting point decreases.

The density increases as the molar weight of the cyclic compound increases, and can increase more as the double bonds increase.

For cyclopentene the density is 0.744 g / mL, while for cyclopentadiene the density is 0.789g / mL.

The same number of carbon atoms is found in both compounds; however, cyclopentadiene has a lower molar weight due to the absence of hydrogens, but it has two double bonds. This makes its density a bit higher.

Cycloalkenes are poorly soluble in water, especially since their polarity is very low compared to that of water.

They are soluble in organic compounds and thus the principle that “similar dissolves similar” is fulfilled. This generally means that substances that have similar structure and intermolecular forces are more mutually miscible than those that are not.

Chemical properties

Like straight chain alkenes, cycloalkenes present addition reactions on the double bond because they have higher energy and shorter length than a single bond or sigma (σ).

The main reactions of cycloalkenes are of addition, but they also exhibit oxidation, reduction and allylic halogenation reactions.

The following table illustrates the most important reactions of cycloalkenes:

If one of the carbons that has the double bond is replaced by a radical, the hydrogen from the reactant joins the carbon that has a greater number of hydrogen atoms. This is called the Markovnicov Rule.

Examples of cycloalkenes

Cyclohexene
  • Cyclohexene: C6H10.
  • Cyclobutene: C4H6.
  • Cyclopentene: C5H8.
  • 1,5-Cyclooctadiene: C8H12.
  • 1,3-cyclobutadiene: C4H4.
  • 1,3-cyclopentadiene: C5H6.
  • 1,3,5,7-cyclooctatetraene: C8H8.
  • Cyclopropene
  • Cycloheptene

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