CYCLOALKINES: CHARACTERISTICS, APPLICATIONS, EXAMPLES - CHEMISTRY - 2024

The cycloalkynes are organic compounds containing one or more triple bonds and a cyclic unit. Their condensed molecular formulas obey the formula C _n H _2n-4. Thus, if n is equal to 3, then the formula of said cycloalkine will be C ₃ H ₂.

A series of geometric figures is illustrated in the image below, but they actually consist of examples of cycloalkines. Each of them can be considered as a more oxidized version of the respective cycloalkanes (without double or triple bonds). When they lack a heteroatom (O, N, S, F, etc.), they are only "simple" hydrocarbons.

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The chemistry around cycloalkines is very complex, and the mechanisms behind their reactions are even more complex. They represent a starting point for the synthesis of many organic compounds, which in turn are subject to possible applications.

Generally speaking, they are very reactive unless "distorted" or complexed with transition metals. Also, their triple bonds can be conjugated with double bonds, creating cyclic units within molecules.

If this is not the case, in their simplest structures they are capable of adding small molecules to their triple bonds.

Characteristics of cycloalkines

Apolarity and the triple bond

Cycloalkines are characterized by being apolar molecules, and therefore, hydrophobic. This can change if in their structures they have some heteroatom or functional group that gives them a considerable dipole moment; as occurs in triple bonded heterocycles.

But what is a triple bond? They are no more than three simultaneous interactions between two sp hybridized carbon atoms. One bond is simple (σ), and the other two π, perpendicular to each other. Both carbon atoms have a free sp orbital to bond with other atoms (RC≡CR).

These hybrid orbitals have 50% s character and 50% p character. Because the s orbitals are more penetrating than the p orbitals, this fact makes the two carbons of the triple bond more acidic (electron acceptors) than the carbons of alkanes or alkenes.

For this reason the triple bond (≡) represents a specific point for electron donor species to add to it, forming single bonds.

This results in the breaking of one of the π bonds, becoming a double bond (C = C). The addition continues until R ₄ C-CR ₄ is obtained, that is, fully saturated carbons.

The above can also be explained in this way: the triple bond is a double unsaturation.

Intermolecular forces

Cycloalkine molecules interact through scattering forces or London forces and by interactions of the π-π type. These interactions are weak, but as the size of the cycles increases (like the last three on the right side of the image), they manage to form solids at room temperature and pressure.

Angular stress

The bonds in the triple bond are located on the same plane and a line. Hence, -C≡C- has a linear geometry, with the sp orbitals about 180º apart.

This has a serious implication on the stereochemical stability of cycloalkines. It takes a lot of energy to "bend" the sp orbitals since they are not flexible.

The smaller the cycloalkine, the more the sp orbitals must bend to allow for its physical existence. Analyzing the image, it can be seen, from left to right, that in the triangle the angle of the bonds to the sides of the triple bond is very pronounced; while in the decagon they are less abrupt.

The larger the cycloalkine, the closer the bond angle of the sp orbitals approaches the ideal 180º. The opposite occurs when they are smaller, forcing them to bend and creating an angular tension in them, destabilizing the cycloalkine.

Thus, the larger cycloalkines have lower angular stress, which enables their synthesis and storage. With this, the triangle is the most unstable cycloalkine, and the decagon the most stable of them all.

Actually, the cyclooctino (that of the octagon) is the smallest and most stable known; the others only exist as momentary intermediaries in chemical reactions.

Nomenclature

The same IUPAC standards as for cycloalkanes and cycloalkenes must be applied to naming cycloalkines. The only difference lies in the suffix -ico at the end of the name of the organic compound.

The main chain is one that has the triple bond, and begins to be numbered from the end closest to it. If you have for example cyclopropane, then having a triple bond will be called cyclopropino (the triangle in the image). If you have a methyl group attached to the top vertex, then it will be: 2-methylcyclopropino.

The carbons of RC≡CR already have their four bonds, so it lacks hydrogens (as is the case with all cycloalkines in the image). This does not happen only if the triple bond is in a terminal position, that is, at the end of a chain (RC≡CH).

Applications

Cycloalkines are not very common compounds, so neither are their applications. They can serve as binders (groups that coordinate) to transition metals, thus creating an infinity of organometallic compounds that can be used for very rigorous and specific uses.

They are generally solvents in their most saturated and stable forms. When they consist of heterocycles, in addition to having internal C≡CC = CC≡C cyclic units, they find interesting and promising uses as anticancer drugs; such is the case of Dinemycin A. Other compounds with structural analogies have been synthesized from it.

Examples

The image shows seven simple cycloalkines, in which there is hardly one triple bond. From left to right, with their respective names there are: cyclopropino, the triangle; cyclobutino, the square; cyclopentine, the pentagon; cyclohexine, the hexagon; cycloheptin, the heptagon; cyclooctino, the octagon; and cyclodecino, the decagon.

Based on these structures and substituting the hydrogen atoms of the saturated carbons, other compounds derived from them can be obtained. They can also be subjected to oxidative conditions to generate double bonds on other sides of the cycles.

These geometric units can be part of a larger structure, increasing the chances of functionalizing the entire set. Not many examples of cycloalkynes are available, at least not without delving into the deep realms of organic synthesis and pharmacology.

References

1. Francis A. Carey. Organic Chemistry. (Sixth ed., Pp. 372, 375). Mc Graw Hill.
2. Wikipedia. (2018). Cycloalkyne. Taken from: en.wikipedia.org
3. William Reusch. (May 5, 2013). Naming Organic Compounds. Taken from: 2.chemistry.msu.edu
4. Inorganic chemistry. Cycloalkines. Taken from: fullquimica.com
5. Patrizia Diana & Girolamo Cirrincione. (2015). Biosynthesis of heterocycles from Isolation to Gene Cluster. Wiley, page 181.
6. Interesting Organic Chemistry and Natural Products. (2015, April 17). Cycloalkynes. Taken from: quintus.mickel.ch