CYCLOALKENES: CHEMICAL STRUCTURE, PROPERTIES, NOMENCLATURE - CHEMISTRY - 2025

The cycloalkenes belong to the group of binary organic compounds; that is, they are only made up of carbon and hydrogen. The ending "ene" indicates that they have a double bond in their structure, called unsaturation or hydrogen deficiency (indicates that there is a lack of hydrogens in the formula).

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

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.

They have a general formula that is C _n H _2n-2. In this formula n indicates the number of carbon atoms that the structure has. The smallest cycloalkene is cyclopropene, which means it has only 3 carbon atoms and a single double bond.

If you want to obtain a structure with a number of carbon atoms = 3 applying the formula C _n H _{n-2, it is} enough to replace n by 3, obtaining the following molecular formula:

C 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

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 breakdown of the molecule.

If in the chemical structure a cycloalkene has two double bonds, it is said to be a "diene". If it has three double bonds, it is a "triene." And if there are four double bonds, we speak of a "tetraene", and so on.

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.

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 the ending "eno" is given because it corresponds to an alkene.

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 lowest possible value.

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

Then the word "cycle " is written, followed by the root that corresponds to the number of carbon atoms that the structure has. The letter “a” is written and then the number of double bonds is written, 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.

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

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

References

1. Tierney, J, (1988, 12), Markownikoff's rule: What did he say and when did he say it ?. J.Chem.Educ. 65, pp. 1053-1054.
2. Hart, H; Craine, L; Hart, D. Organic Chemistry: A Short Course, (Ninth Edition), Mexico, McGraw-Hill.
3. García, A., Aubad, A., Zapata, R., (1985), Hacia la Química 2, Bogotá: Temis
4. Pine, S., Hammond, G., Hendrickson, J., Cram, D., (1980), Organic Chemistry (4th ed.), Mexico: McGraw-Hill.
5. Morrison, r., Boyd, R., (1998), Organic Chemistry, (5th edition), Spain, Addison Wesley Iberoamericana