The aromatic hydrocarbons or arenes are a set of organic compounds that are composed only of carbons and hydrogens, and that are characterized by having benzene ring units in their molecular structures.
Some, as their name emphasizes, emit sweet and pleasant smells; which is why the first organic chemists referred to them as aromatics. Methane gas, for example, is an odorless hydrocarbon; while toluene, a volatile liquid, has a rather peculiar and strong odor.
In the image above we have a kind of network or mesh made up of benzene rings. Note its hexagonal geometries and the circle inside. This circle represents what is known as aromaticity, which is an entirely chemical and non-physical property, independent of the odors of these hydrocarbons..
Aromatic hydrocarbons are among the most important substances, extracted or produced, from petroleum and carbon minerals. Benzene is the cornerstone of these hydrocarbons and their applications, since it is derived from endless compounds that are used as raw material for the production of fertilizers, plastics, adhesives, detergents, perfumes, medicines, etc..
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For aromatic hydrocarbons we have the peculiarity that their traditional or common names tend to prevail over those governed by the IUPAC nomenclature..
They all contain the benzene ring, or a ring that fulfills the property of aromaticity. The simplest can be named based on benzene and the relative positions of its substituents.
For example, consider the image above. In the three structures we see the hexagonal ring of benzene, which has two methyl substituents, CH3. Thus, this compound is called dimethylbenzene, as it consists of a benzene with two methyls.
From left to right, we see that the separation between both CH3 gets older, which not only affects the physical properties of the molecule, but also modifies their respective names. In order to differentiate them, since they are all called dimethylbenzene, the prefixes ortho (o-), meta (m-) and para (p-) are used.
Therefore, and again, from left to right we have: ortho-dimethylbenzene, meta-dimethylbenzene, and para-dimethylbenzene. However, the traditional name for this compound is xylene, so the names come to be: ortho-xylene, meta-xylene, and para-xylene..
If there are more than two different substituents, the carbons are enumerated following the same nomenclature rules as for all hydrocarbons.
For aromatic hydrocarbons with more than one benzene ring, the traditional names become even more important. This is because the systematic names that describe their structures are cumbersome and difficult to memorize. Consider the example of naphthalene:
Naphthalene is also known commercially as naphthalene or white camphor. However, its systematic name is: Bicyclo [4,4,0] deca-1,3,5,7,9 pentaene. And this only for a compound with two fused benzene rings; the nomenclature becomes much more complicated for compounds with three or more rings.
Aromatic hydrocarbons contain rings that obey Huckel's rules. That is, their rings must have atoms with sp hybridizationstwo, be as flat as possible, and have a number of delocalized π electrons equal to 4n + 2. For example, benzene is aromatic because it has 6 delocalized electrons to n= 1 (4 · 1 + 2 = 6).
C / H ratios for aromatic hydrocarbons are high or greater than 1. For example, for benzene, C6H6, its C / H is equal to 6/6 or 1. While, for naphthalene, C10H8, its C / H is equal to 10/8 or 1.25. What does this mean? That these hydrocarbons are very “carbonated” compared to other compounds.
Precisely because of their high C / H ratios, when aromatic hydrocarbons are burned they give off yellowish flames, a product of the carbon particles formed (soot).
Aromatic hydrocarbons can replace their hydrogen atoms with any other substituent. This is achieved by two types of organic reactions: electrophilic aromatic substitution (SEAr), or nucleophilic aromatic substitution (SNAr)..
Aromatic hydrocarbons, as their name suggests, are characterized by having sweet or strong odors. This property allowed them to be differentiated, in principle, from gaseous hydrocarbons and some waxy paraffins..
Aromatic hydrocarbons, strictly speaking, concern only those that are composed of carbon and hydrogen. There should be no heteroatoms (O, P, N, S, etc.). Therefore, the following examples to be mentioned leave out compounds such as pyridine, furan or phenol.
Above we have ten examples of aromatic hydrocarbons. Note that they all have at least one benzene ring. Their names are:
A: Toluene
B: Styrene
C: Mesitylene
D: Dureno
E: Biphenyl
F: 2-Phenylhexane
G: 2-Methylnaphthalene
H: Anthracene
I: Phenanthrene
J: Pyrene
Of all of them, toluene is the one with the highest industrial value.
Most of the applications of aromatic hydrocarbons consist in serving as raw material for the synthesis, or production, of products with high commercial value. Almost all of them start from benzene, which is subjected to various organic reactions to obtain functional derivatives.
Cyclohexane is produced from benzene, which is later transformed into other compounds to synthesize nylon 6 or nylon 66. On the other hand, styrene, which consists of the monomer of polystyrene, can also be obtained from benzene. Thus, we have polymers necessary for the production of fibers and plastics.
From benzene, likewise, bisphenol A can be synthesized, with which, following various synthetic routes, epoxy resins, glues, adhesives, and even paints are produced..
Alkylbenzenes with long side chains are used for the production of detergents; including alkylbenzenesulfonates.
TNT can be produced from toluene: trinitrotoluene, one of the best known explosives.
From toluene, benzoic acid is obtained as a derivative, an essential compound for the formulations of many perfumes, medicines and foods..
Benzene, toluene and xylene (BTX) are the most used organic solvents in petroleum research.
Aromatic hydrocarbons are also part of the composition of gasoline, having the main role to raise its octane number or octane number..
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