BENZENE: HISTORY, STRUCTURE, PROPERTIES, DERIVATIVES, USES - CHEMISTRY - 2024

The benzene is an organic compound consisting of one of the simplest of all aromatic hydrocarbons. Its chemical formula is C ₆ H ₆, of which it is known that the ratio between carbon and hydrogen is equal to 1; that is, for each carbon there is a hydrogen linked to it.

Although its physical appearance is that of a colorless liquid, it is naturally found in petroleum and petroleum products. Its smell is very characteristic, as it resembles a mixture of glue, bitumen and gasoline; on the other hand, it is a volatile and flammable liquid.

Bottle with benzene. Source: Air1404

The image above shows a container or bottle with benzene, presumably of non-analytical purity. If uncovered, the benzene vapors would immediately spread throughout the laboratory. For this reason this liquid, commonly used as a simple solvent, is handled inside a fume hood.

Being its formula C ₆ H ₆, the chemists of the XIX century raised numerous possible structures that would correspond to said C / H ratio equal to 1. Not only this, but the benzene molecule had to have special bonds in such a way that it could be explained its unusual stability against addition reactions; typical for alkenes and polyenes.

It was thus that their links represented an enigma for chemists of that time; until the property called aromaticity was introduced. Before considered a hexacyclotriene (with three C = C bonds), benzene is much more than that, and is another of many examples of synergy in chemistry.

In organic chemistry, benzene is a classic symbol, the structural basis for multiple polyaromatic compounds. From its hexagon, endless derivatives are obtained via aromatic electrophilic substitution; a ring from whose edges the structure that defines new compounds is woven.

In fact, its derivatives are due to the vast industrial uses for which they need benzene as their raw material. From the preparation of glues and textile fibers, to plastics, rubbers, pigments, drugs and explosives. On the other hand, benzene is naturally found in volcanoes, wildfires, gasoline, and in cigarette smoke.

History

Discovery and names

Its discovery dates back to 1825, usually attributed to Michael Faraday, when he collected and did experiments with a remaining oil product of the gas used for lighting. This liquid contained a C / H ratio close to 1, which is why he called it 'carbureted hydrogen'.

Chemist Auguste Laurent named the strange hydrocarbon 'pheno', derived from the Greek word 'phaínein' which means bright (because it was obtained after the gas burned). However, this name was not accepted by the scientific community and prevailed only as 'phenyl', to refer to the radical derived from benzene.

From benzoin gum the chemist Eilhard Mitscherlich, nine years later, managed to produce the same compound; Therefore, there was another source for the same hydrocarbon, which he baptized as 'benzine'. However, they also did not consider the appropriate name to assume that it was an alkaloid, such as quinine.

Thus, they replaced the name 'benzine' with 'benzol'. However, again there were contradictions and discrepancies due to the fact that the term 'benzol' confused the hydrocarbon for an alcohol. It was then that the name 'benzene' was born, used first in France and England.

Industrial production

Neither lighting gas nor benzoin gum were appropriate sources for generating benzene on a large scale. Charles Mansfield, working with August Wilhelm von Hofmann, succeeded in 1845 to isolate benzene (twenty years after its discovery) from coal tar, a side product of coke production.

This is how the industrial production of benzene from coal tar began. The availability of benzene in massive quantities facilitated the study of its chemical properties and allowed it to be related to other compounds with similar reactivities. August Wilhelm von Hofmann himself coined the word 'aromatic' for benzene and its related compounds.

Historical structures

August Kekulé's dream

Friedrich August Kekulé is attributed the hexagonal and cyclical structure of benzene around the year 1865, arising from a strange dream with Uroboros, the snake that bites its own tail by drawing a circle. Thus, he believed that benzene could be considered as a hexagonal ring, and other chemists raised possible structures, which are shown below:

Structures for the benzene ring proposed throughout history. Source: Jü

Some of the higher structures could account for the stability of benzene.

Book, rings and prism

Note that the third structure is not even a ring but a triangular prism, proposed by Albert Ladenburg in 1869; to his left, one in the shape of an open book, proposed by Sir James Dewar in 1867; and to its right, one with all the hydrogens directed towards the center of the ring, proposed by Henry Edward Armstrong in 1887.

The first structure, proposed by Adolf Karl Ludwig Claus in 1867, is also quite peculiar, since the CC links are crossed. And the last one was Kekulé's “serpentine” ring, dreamed of in 1865.

What was the "winner"? The fifth structure (from left to right), proposed in 1899 by Johannes Thiele.

In this, the resonance hybrid was considered for the first time, which combined the two Kekulé structures (turn the first ring on the right to observe it) and extraordinarily explained the delocalization of electrons and, with it, the until then unusual stability of the benzene.

Structure of benzene

Aromatic benzene ring. Source: Benjah-bmm27

Above is the structure proposed by Thiele using a model of spheres and bars.

The benzene molecule is flat, with the hydrogen atoms pointing outward from the sides of the ring. All carbon atoms have sp ² hybridization, with a p orbital available to establish the aromatic system in which six electrons delocalize.

These sp ² carbons are more electronegative than hydrogens, and therefore, the former withdraw electron density to the latter (C _sp2 ^δ- -H ^{δ +}). Consequently, the center of the ring has a higher concentration of electrons than its sides.

More precisely, the aromatic system can be represented as a cloud or electronic pad expanded on both sides of the hexagonal ring; and in the middle, on the sides or edges, an electronic deficiency made up of hydrogens with a positive partial charge.

Thanks to this distribution of electric charges, benzene molecules can interact with each other through dipole-dipole forces; H ^{δ +} atoms are attracted to the aromatic center of a neighboring ring (this will be represented below).

Also, the aromatic centers can be stacked one on top of the other to favor the induction of the instantaneous dipoles.

Resonance

Structures and resonance hybrid of benzene. Source: Edgar181 from Wikipedia.

The two Kekulé structures are shown at the top of the image, and below them, the resonance hybrid. Since the two structures occur over and over again at the same time, the hybrid is represented by a circle drawn in the middle (similar to that of a "hexagonal donut").

The hybrid circle is important because it indicates the aromatic character of benzene (and that of many other compounds). Furthermore, he points out that the links are not as long as CC, nor are they as short as C = C; rather, their length is between both extremes. Thus, benzene is not considered a polyene.

The foregoing has been demonstrated by measuring the lengths of the CC (139 pm) bonds of benzene, which are slightly more elongated than the CH (109 pm) bonds.

Crystals

Orthorhombic crystal structure of benzene. Source: Ben Mills

Benzene is a liquid at room temperature. Its intermolecular forces mean that, despite not having such a pronounced dipole moment, it can hold its molecules together in a liquid that boils at 80ºC. When the temperature drops below 5ºC, the benzene begins to freeze: and thus its corresponding crystals are obtained.

Benzene rings can adopt defined structural patterns in their solid. Their dipoles cause them to “tilt” to the left or right, forming rows capable of being reproduced by an orthorhombic unit cell. Thus, benzene crystals are orthorhombic.

Note in the upper image that the tilting of the rings favors the interactions between the H ^{δ +} and the aromatic centers, mentioned in previous subsections.

Properties

Molecular mass

78.114 g / mol.

Physical appearance

Colorless liquid with a gasoline-like odor.

Boiling point

80 ° C.

Melting point

5.5 ° C.

ignition point

-11ºC (closed cup).

Auto ignition temperature

497.78 ° C.

Density

0.8765 g / mL at 20 ° C.

Solubility

One liter of boiling water can barely dissolve 3.94 g of benzene. Its apolar character makes it practically immiscible with water. However, it is miscible with other solvents, such as ethanol, ethers, acetone, oils, chloroform, carbon tetrachloride, etc.

Vapor density

2.8 relative to air (that is, almost three times as dense).

Vapor pressure

94.8 mm Hg at 25 ° C.

Heat of combustion

-3267.6 kJ / mol (for liquid benzene).

Heat of vaporization

33.83 kJ / mol.

Surface tension

28.22 mN / m at 25 ° C.

Refractive index

1.5011 at 20 ° C.

Derivatives

The hydrogens of benzene can be replaced by other groups or atoms. There may be one or more substitutions, increasing the degree of substitution until none of the original six hydrogens remain.

For example, assume benzene as Ph-H, where H is any of its six hydrogens. Remembering that the center of the ring has a higher electron density, it attracts electrophiles, which attack the ring to replace H in a reaction called electrophilic aromatic substitution (SEAr).

If this H is replaced by an OH, we will have the Ph-OH, phenol; being replaced by a CH ₃, Ph-CH ₃, toluene; if it is NH ₂, Ph-NH ₂, aniline; or if it is CH ₂ CH ₃, Ph-CH ₂ CH ₃, ethylbenzene.

The derivatives can be the same or more toxic than benzene, or on the contrary they can become so complex that they have a desirable pharmacological effect.

Applications

It is a good solvent for a wide variety of compounds, present for example in paints, varnishes, adhesives and coatings.

Likewise, it can dissolve oils, fats or waxes, which is why it has been used as an extracting solvent for essences. This property came to be used by Ludwig Roselius in 1903 to decaffeinate coffee, an operation already in disuse due to the toxicity of benzene. Likewise, it was used in the past to degrease metals.

In one of its classic uses, it does not act as a solvent but as an additive: increasing the octane number of gasoline, substituting lead for this purpose.

Derivatives of benzene can have different uses; some serve as pesticides, lubricants, detergents, plastics, explosives, perfumes, dyes, glues, drugs, etc. If a benzene ring is observed in its structure, it is quite probable that its synthesis has started from benzene.

Among its most important derivatives are: cumene, xylene, aniline, phenol (for the synthesis of phenolic resins), benzoic acid (preservative), cyclohexane (for the synthesis of nylon), nitrobenzene, resorcinol and ethylbenzene.

Nomenclature

The nomenclature of benzene derivatives varies depending on the degree of substitution, what the substituent groups are, and their relative positions. Thus, benzene can undergo mono, di, tri, tetra, etc., substitutions

When the two groups are attached to adjacent carbons, the designation 'ortho' is used; if there is a carbon in between separating them, 'meta'; and if the carbons are in opposite positions, 'para'.

The images below show examples of benzene derivatives with their respective names governed by the IUPAC. They are also accompanied by common or traditional names.

Monoderivatives of benzene. Source: Gabriel Bolívar.

Other derivatives of benzene. Source: Gabriel Bolívar.

Note that in trisubstituted benzene the ortho, para, and meta indicators are no longer useful.

Toxicity

Benzene is a compound that must be handled with care. Given its particular smell, the immediate negative effects can be suffocation, dizziness, headaches, tremors, drowsiness, nausea, and even death (with high exposure). If ingested, in addition to the aforementioned, it can cause severe stomach pain and seizures.

In addition, the long-term effects under constant exposure to this liquid are carcinogenic; increases the chances that the individual will suffer from some type of cancer, especially blood cancer: leukemia.

In the blood, it can decrease the concentration of red blood cells, causing anemia, and also affect the bone marrow and liver, where it is assimilated by the body to generate even more toxic benzene derivatives; for example, hydroxyquinone. It also accumulates in the kidneys, heart, lungs, and brain.

References

1. Morrison, RT and Boyd, RN (1987). Organic Chemistry. (5th Edition). Addison-Wesley Iberoamericana.
2. Carey, FA (2008). Organic Chemistry. (6th Edition). McGraw-Hill, Interamerica, Editores SA
3. Graham Solomons TW, Craig B. Fryhle. (2011). Organic Chemistry. Amines. (10th edition.). Wiley Plus.
4. National Center for Biotechnology Information. (2019). Benzene. PubChem Database. CID = 241, Recovered from: pubchem.ncbi.nlm.nih.gov
5. Wikipedia. (2019). Benzene. Recovered from: en.wikipedia.org
6. Garcia Nissa. (2019). What is Benzene? - Uses, Structure & Formula. Study. Recovered from: study.com
7. Centers for Disease Control and Prevention. (April 4, 2018). Facts about benzene. Recovered from: emergency.cdc.gov
8. World Health Organization. (2010). Exposure to benzene: a major public health concern.. Recovered from: who.int
9. Fernández Germán. (sf). Benzene nomenclature problems. Organic Chemistry. Recovered from: quimicaorganica.org