AROMATIC HYDROCARBONS: CHARACTERISTICS AND PROPERTIES - CHEMISTRY - 2025

The aromatic hydrocarbons are organic compounds that are based mostly on their structure to the benzene cyclic compound six carbon atoms combined with hydrogen, so that arise conjugated bonds by delocalisation having electrons among molecular orbitals.

In other words, this means that this compound possesses sigma (σ) carbon-hydrogen bonds and pi (π) carbon-carbon bonds, which allow the electrons freedom of movement to show the resonance phenomenon and other unique manifestations that are their own. of these substances.

Benzene

The term "aromatics" was designated to these compounds long before their reaction mechanisms were known, due to the simple fact that a large number of these hydrocarbons give off certain sweet or pleasant odors.

Characteristics and properties of aromatic hydrocarbons

Taking as a premise the enormous amount of aromatic hydrocarbons based on benzene, it is important to know that this is presented as a colorless, liquid and flammable substance that is obtained from some processes related to oil.

So this compound, whose formula is C ₆ H ₆, has a low reactivity; This means that the benzene molecule is quite stable and is due to electronic delocalization between its carbon atoms.

Heteroarenes

Likewise, there are many aromatic molecules that are not based on benzene and are called heteroarenes, since in their structure at least one carbon atom is replaced by another element such as sulfur, nitrogen or oxygen, which are heteroatoms.

That said, it is important to know that the C: H ratio is large in aromatic hydrocarbons and, because of this, when they are incinerated a strong yellow flame is produced that gives off soot.

As mentioned above, a large part of these organic substances give off an odor while being handled. Furthermore, these types of hydrocarbons are subjected to electrophilic and nucleophilic substitutions to obtain new compounds.

Structure

In the case of benzene, each carbon atom shares an electron with a hydrogen atom and an electron with each neighboring carbon atom. It is then a free electron to migrate within the structure and generate resonance systems that provide this molecule with the great stability that is characteristic of it.

For a molecule to have aromaticity it has to obey certain rules, among which the following stand out:

- Be cyclical (allows the existence of resonant structures).

- Be flat (each of the atoms belonging to the ring structure have sp ² hybridization).

- Being able to delocalize its electrons (as it has alternating single and double bonds, it is represented by a circle within the ring).

In the same way, aromatic compounds must also comply with Hückel's rule, which consists of counting the π electrons present in the ring; only if this number is equal to 4n + 2 are they considered aromatic (n being an integer equal to or greater than zero).

As well as the benzene molecule, many derivatives of it are also aromatic (as long as they comply with the aforementioned premises and the ring structure is preserved), like some polycyclic compounds such as naphthalene, anthracene, phenanthrene and naphtacene.

This also applies to other hydrocarbons that do not have benzene as a base but that are considered aromatic, such as pyridine, pyrrole, furan, thiophene, among others.

Nomenclature

For benzene molecules with a single substituent (monosubstituted), which are those benzenes in which a hydrogen atom has been supplanted by a different atom or group of atoms, the name of the substituent is designated in a single word followed by the word benzene.

An example is the representation of ethylbenzene, shown below:

Similarly, when there are two substituents in a benzene, the location of substituent number two in relation to number one should be noted.

To achieve this, you start by numbering the carbon atoms from one to six. Then it can be seen that there are three types of possible compounds that are named according to the atoms or molecules that are as substituents, as follows:

The prefix o- (ortho-) is used to indicate the substituents at positions 1 and 2, the term m- (meta-) to indicate the substituents at atoms 1 and 3, and the expression p- (for -) to name the substituents at positions 1 and 4 of the compound.

Similarly, when there are more than two substituents, they must be named indicating their positions with numbers, so that they can have the smallest possible number; and when these have the same priority in their nomenclature, they must be mentioned alphabetically.

You can also have the benzene molecule as a substituent, and in these cases it is called phenyl. However, when polycyclic aromatic hydrocarbons are the case, they must be named indicating the position of the substituents, then the name of the substituent and finally the name of the compound.

Applications

- One of the main uses is in the oil industry or in organic synthesis in laboratories.

- Vitamins and hormones stand out (almost entirely), as do the vast majority of condiments used in the kitchen.

- Organic tinctures and perfumes, whether of natural or synthetic origin.

- Other aromatic hydrocarbons considered important are non-alicyclic alkaloids, as well as compounds with explosive properties such as trinitrotoluene (commonly known as TNT) and components of tear gas.

- In medical applications, certain analgesic substances that have the benzene molecule in their structure can be named, including acetylsalicylic acid (known as aspirin) and others such as acetaminophen.

- Some aromatic hydrocarbons are highly toxic to living beings. For example, benzene, ethylbenzene, toluene, and xylene are known to be carcinogenic.

References

1. Aromatic hydrocarbons. (2017). Wikipedia. Recovered from en.wikipedia.org
2. Chang, R. (2007). Chemistry. (9th ed). McGraw-Hill.
3. Calvert, JG, Atkinson, R., and Becker, KH (2002). The Mechanisms of Atmospheric Oxidation of the Aromatic Hydrocarbons. Recovered from books.google.co.ve
4. ASTM Committee D-2 on Petroleum Products and Lubricants. (1977). Manual on Hydrocarbon Analysis. Recovered from books.google.co.ve
5. Harvey, RG (1991). Polycyclic Aromatic Hydrocarbons: Chemistry and Carcinogenicity. Recovered from books.google.co.ve