The cyclic hydrocarbons are organic compounds consisting of carbon and hydrogen atoms that are bound to cause rings or cyclic structures. There are mainly of three types: alicyclic, aromatic and polycyclic.
These types of hydrocarbons can then consist of the closed versions of alkanes, alkenes and alkynes; have rings with aromatic systems, such as benzene and its derivatives; or present intricate and fascinating structures. Of all of them, the alicyclics are the simplest, and are usually represented by using polygons.
Alicyclic hydrocarbons. Source: Gabriel Bolívar.
Above for example you have several alicyclic hydrocarbons. They look like simple polygons: a square, triangle, pentagon, hexagon, etc. If one of its CC bonds could be cut with scissors, the result would be an alkane, alkene (if it has double bonds) or alkyne (if it has triple bonds).
Their properties do not differ much from the open chain hydrocarbons from which they originate; although, chemically, the larger they are, the more stable they will be, and their interaction with a molecular medium becomes more pronounced (due to their greater contact area).
Structure
Going directly to the matter that concerns its structures, it should be clarified that these are not flat, even when in their representation of polygons they seem so. The only exception to this statement is found in cyclopropane (the triangle), since its three points must necessarily rest on the same plane.
It is also necessary to clarify that the terms 'cycles' and 'rings' are often interchangeable; a ring does not have to be circular, and therefore it can acquire endless geometries as long as it is a closed structure. Cyclopropane is then said to have a triangular ring or cycle.
All carbons have sp 3 hybridizations, so their geometries are tetrahedral and their bonds should ideally be separated by an angle of 109.5 °; however, this is not possible for structures as closed as cyclopropane, cyclobutane (square), or cyclopentane (pentagon).
We speak then of a tension that destabilizes the molecule, and is determined experimentally by measuring the average heats of combustion of each CH 2 group.
Conformations
And what happens when there are double or triple bonds? The tension increases, because where there is one of them, the structure will be forced to "shrink" and stay flat; which, consecutively, would force one conformation above others, probably eclipsing neighboring hydrogen atoms.
Conformers for cyclohexane. Source: Sponk
Two conformers for cyclohexane are shown in the upper image to try to explain the above. The atoms that are in the a or e positions are said to be axial or equatorial, respectively. Note that instead of a flat hexagon, you have a chair (left) and a boat (right).
These structures are dynamic and establish a balance between them. If the atoms in a are very bulky, the ring will "snap" to position them at the equatorial positions; since there they are oriented towards the sides of the ring (which would reinforce or impair intermolecular interactions).
If we look at each carbon separately, it will be seen that they are tetrahedral. This would not be the case if there were a double bond: its sp 2 hybridizations would force the structure to flatten; and if there is a triple bond, to align. The planar structure of benzene is the maximum representation of this.
Properties
Simplified process in which a cyclic hydrocarbon is formed. Source: Gabriel Bolívar.
Suppose you have an open chain hydrocarbon (with or without unsaturations or branches). If we could join its ends, it would create a ring or cycle (as in the image above).
From the point of view of organic synthesis, this does not happen just like that; there must be good leaving groups linked at the ends of the chain, which when exiting, promote chain closure (if the solution is very dilute).
With this in mind, it can be seen that the green trace hydrocarbon only undergoes a transformation with respect to its structure; without breaking or adding new bonds with other atoms. It means that chemically it continues to be the same before and after closing or girdling.
Therefore, the chemical or physical properties of these alicyclic hydrocarbons do not differ too much from their open chain counterparts. Both are reactive against the same species (ie halogens under ultraviolet light), and can undergo strong oxidation or burn by releasing heat.
Intermolecular forces
There is an undeniable fact: the ring has a greater contact area than the open chain, and therefore its intermolecular interactions are stronger.
The result is that their boiling and melting points tend to be higher, as do their densities. As many differences can be noted in their refractive indices or vapor pressures.
Nomenclature
Returning to the example of the hydrocarbon with a green stroke, its nomenclature remains unchanged once it has closed in on itself (like a snake biting its own tail). Therefore, the naming rules remain the same; unless polycyclic or aromatic hydrocarbons are addressed.
Three compounds are shown below, which will be given their proper names:
Three examples of cyclic hydrocarbons to assign their nomenclatures. Source: Gabriel Bolívar.
To begin with, as they are closed structures, the prefix cycle is used to refer to them (here the word ring is relegated).
From left to right we have: a cyclopentane, a cyclohexane, and another cyclopentane. The carbons are numbered in such a way that the substituents are assigned the lowest numbers, and they are further mentioned in alphabetical order.
Thus, A is: 1,1-Dimethylcyclopentane. In B, we begin by mentioning the ethyl substituent before fluorine, so its name is: 1-ethyl-2-fluorocyclohexane. And then for C, the double bonds are taken as substituents, indicating the number of carbons that form it: 1,3-cyclopentadiene.
Examples
Throughout the article, mention has been made of several cyclic hydrocarbons. In the first image can be found: cyclopropane, cyclobutane, cyclopentane, cyclohexane, cycloheptane and cyclooctane.
A wide range of derivatives can be obtained from them, and it is enough to place double or triple bonds on them to have cycloalkenes or cycloalkines, respectively. And with regard to aromatic hydrocarbons, it is enough to keep the benzene ring in mind and replace or reproduce it in two dimensions.
However, the most extraordinary (and problematic when it comes to understanding their nomenclature) are polycyclic hydrocarbons; that is, simple polygons are not enough even to represent them in a simple way. Three of them worth mentioning are: Cuban, Canastano and Pagoda (images below).
Skeleton of the Cuban. Source: NEUROtiker.
Canastan skeleton. Source: Benjah-bmm27.
Skeleton of the pagodano. Source: Puppy8800
Each of them has its complex synthesis methodology, its history, art, and a latent fascination for the infinite structural possibilities that simple hydrocarbons can reach.
References
- Morrison, RT and Boyd, R, N. (1987). Organic Chemistry. 5th Edition. Editorial Addison-Wesley Interamericana.
- Carey F. (2008). Organic Chemistry. (Sixth edition). Mc Graw Hill.
- Graham Solomons TW, Craig B. Fryhle. (2011). Organic Chemistry. Amines. (10th edition.). Wiley Plus.
- Reid Danielle. (2019). Cyclic Hydrocarbons: Definition & Example. Study. Recovered from: study.com
- CK-12 Foundation. (June 05, 2019). Cyclic Hydrocarbons. Chemistry LibreTexts. Recovered from: chem.libretexts.org
- Wikipedia. (2019). Cyclic compound. Recovered from: en.wikipedia.org
- Cowboy Miguel. (2019). Cyclic hydrocarbons. Recovered from: deciencias.net