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How to determine the molar mass of benzene : theoretical recommendations

The molar mass of matter is the ratio of the mass of substance to the number of moles of the substance. The molar mass of each chemical compound can be determined by specific experiments that are aimed at the determination of the density of its vapors and/or other specific experimental techniques. However, the molar mass of each existing or even hypothetical chemical compound can be determined by examining its molecular structure. Therefore, the molar mass of complex molecules can be determined by summing the molar masses of their constituent elements. In our case, this process is quite easy. According to the molecular structure of benzene, it contains six carbon atoms and six hydrogen atoms. The molar mass of carbon is 12 g/mol and the molar mass of hydrogen is 1 g/mol. Thus, we have to multiply the number of atoms of each element by its mass and sum the resulting numbers. According to our calculations, the molar mass of benzene is 78 g/mol. Naturally, diverse highly specialized chemical reference supply us with a more accurate value of the molar mass of benzene - 78.11 g/mol. However, in ordinary computations that do not require extremely high precision in results, we can use an approximate value with an eye to simplifying our calculations.

Benzene: the main chemical and physical properties

Benzene (C6H6, PhH) is an organic chemical compound. Under standard conditions (T=25 0C (298 K) and P=101.3 kPa) it is a colorless liquid with a specific sweet smell. In fact, benzene appertains to the series of organic compounds that are widely known outside the narrow circle of specialists because its structure is one of the best models for those who begin to study organic chemistry. Benzene is the simplest aromatic hydrocarbon. Therefore, it is of little wonder that all students who are learning organic chemistry and stereochemistry start their studies with the study of properties and structure of this chemical compound. In addition, one of the most popular and effective methods of study among a lion’s share of pedagogues is to give undergraduates an objective to write a 500 word essay about specific chemical and physical properties of benzene, methods of its industrial production and/or different aspects of its use in the industry. Nowadays, benzene is widely used as a part of gasoline. It is also the feedstock for the production of various drugs, a great variety of modern plastics, synthetic rubber and dyes. Although benzene is part of crude oil. However, for industrial use, it is synthesized from other components. Benzene also appertains to the series of chemical compounds that are toxic and carcinogenic. Students must comply with the high level of personal and public safety when working with this substance. Thereby, it is not used for laboratory work in schools and colleges. In order to show all safety precautions of work with benzene, teachers often give their students some particular assignments, such as composing narrative essays about this problem.

Benzene is a colorless liquid with a pungent odor. Its melting point is 5.5 ° C (278.8 K) and its boiling point is 80.1 ° C (353.25 K). The density of benzene is 0.879 g/cm³ (under the standard conditions). Just like other different unsaturated hydrocarbons, benzene burns strongly smoky flame. In fact, benzene is not only toxic but also explosive. It can form explosive mixtures with air. Benzene also mixes well with ether, gasoline and other organic solvents. Its solubility in water is 1.79 g/l (at 25 ° C). In addition, it should be noted that it forms with water an azeotrope with a boiling point of 69.25 ° C (91% benzene). The molar mass of benzene is 78.1 g/mol.

This chemical compound is not highly reactive. Nevertheless, benzene demonstrates a specific range of chemical reactions due to its chemical structure. In fact, the most typical class of reactions of benzene is a substitution reaction. Benzene reacts with alkenes, chloroalkanes, halogens, nitric and sulfuric acids. This fact can be easily explained by the study of its specific spatial structure. Six atoms of carbon form a ring, which can be considered as a highly stable structure. Obviously, it is a much harder objective to break chemical bonds between carbon atoms than to substitute hydrogen atoms that are located outside the carbon ring. Of course, under some specific conditions we can easily destroy this structure, but usually various types of reactions with benzene that lead to disruption of the benzene ring are held in harsh conditions (high temperature and/or enormously high pressure).

The industrial production and use of benzene: the main spheres of use

Nowadays, a significant part of the produced benzene is widely used for the synthesis of other organic compounds. Here is a concise register of these products together with approximate percent of benzene that is spent in each individual case:

  • About 50% of benzene is converted to ethylbenzene (benzene alkylation with ethylene).
  • About 25% of benzene is converted into cumene (alkylation of benzene with propylene).
  • About 10-15% of benzene is hydrogenated in order to produce cyclohexane.
  • About 10% benzene is used during the production of nitrobenzene.
  • About 2-3% of benzene is converted to different linear alkylbenzenes.
  • About 1% of benzene is used in the synthesis of chlorobenzene.

In addition, the significantly lower amounts of benzene are used in the synthesis of different organic compounds that are not so widely used in various spheres of the contemporary chemical industry. Benzene also can be used as a solvent (occasionally or in specific extreme cases, due to the high toxicity of this compound). Moreover, benzene is a part of gasoline. In 1920 - 1930, benzene were added in naphtha in order to increase its octane number, but up to the 1940s, such mixtures could not compete with the high-octane gasoline. Due to the high toxicity of benzene, its content in the fuel is limited to 1% in accordance with modern standards of safety and different modern nature conservation standards. Of course, the above statistics is insufficient for a serious and scrupulous academic research, such as writing a compare and contrast essay about all modern methods of use of such interesting and industrially significant compound as benzene in the contemporary chemical industry, but it is quite enough to demonstrate the main spheres of its use.

Biological effects of benzene: precautions and possible physiological effects

 

In long-term inhalation of benzene vapors poisoning does not occur immediately. Hereby, until recently, the instructions of work with benzene were not particularly regulated. Of course, this situation has changed and nowadays all workers and scientists who are closely engaged in work with benzene are aware of all the possible effects. High doses of benzene can cause nausea and dizziness, and in some severe cases, the poisoning can be fatal. The first sign of benzene poisoning is often euphoria. It also should be noted that due to the low molar mass of benzene, benzene vapors can be absorbed through intact skin and that benzene in its liquid form significantly irritates the skin. If the human body is subjected to prolonged exposure of benzene in small quantities, the consequences can be very serious, prolonged and, unfortunately, permanent. Benzene is a potent carcinogen. Different scientific researches demonstrate the strict and obvious connection between the poisoning of benzene and the emergence of various cancer diseases, such as bone marrow disease, acute leukemia (myelogenous, lymphoblastic), chronic myeloid leukemia, myelodysplastic syndrome, etc. Additionally, benzene can cause various intoxicating effects on human and even lead to serious addiction. Chronic poisoning of benzene also leads to serious consequences. In severe cases, there are usually observed headaches, extreme fatigue, shortness of breath, dizziness, weakness, nervousness, drowsiness or insomnia, indigestion, nausea, sometimes vomiting, lack of appetite, increased urination, menstruation; often there can be observed persistent bleeding from the mucous membranes of the mouth and a nose that lasts for hours and even days. In addition, there can be observed blood in the feces and/or uterine bleeding. In such cases, the prognosis is always serious. Currently, there exists a great number of different case study examples that are dedicated to this topic of modern toxicology in which one can find complete information about severe cases of benzene poisoning. For example, in rare cases, death can be caused by various secondary infections: gangrenous inflammation of the periosteum and necrosis of the jaw, severe ulcerative inflammation of the gums, general sepsis with septic endometritis.

History of the study of benzene: a brief historical reference

 

For the first time the benzene mixture, which is resulting from the distillation of coal tar, was described by the German chemist Johann Glauber in his book Furni novi philosophici that was first published in 1651. Obviously, that was just a simple description of the specific unknown substance. In those times, chemists had no idea about the unique chemical structure of the organic compounds, their molar and molecular masses. Thereby, Johann Glauber did not determine the molar mass of benzene, its boiling point or density. However, he was the first scientists who precisely described the main physical properties of benzene. Benzene as an individual substance was described by Michael Faraday, who isolated this substance in 1825 from a coal gas condensate, which is produced by coking coal. Soon, in 1833, Eilhard Mitscherlich (a famous German physicist and chemist) received a benzene during the dry distillation of the calcium salt of benzoic acid. In truth, this is the reason why the resulting substance was called a benzene. However, the main interest to chemists is not the chemical properties of benzene, such as the melting point of the structure, the molar mass of benzene or its index of refraction, which is, in principle, quite commonplace, but its unique chemical structure. By the 1860s, it was widely known by the lion’s share of scientists who were engaged in the evolution and development of organic chemistry, that the ratio of carbon atoms and hydrogen atoms in the molecule is similar to that of benzene acetylene. Withal, it was determined that its empirical formula is CnHn. A great number of famous and talented chemists, such as James Dewar, studied this problem. One can easily find a great number of different book reviews in the scientific literature of that time, which were dedicated to the mystery of benzene’s structure. In fact, this chemical enigma was unraveled by August Kekulé, who in 1865 managed to offer the right cyclic formula of this compound. The industrial preparation of benzene from acetylene is usually associated with Marceline Berthelot, whose works were started in 1851. However, the Berthelot method does not allow obtaining pure benzene. The result of the reaction is always a complex mixture of different organic compounds. In fact, only in 1948, was discovered a suitable catalyst – nickel, which is used in order to reduce the reaction temperature.

Nowadays, there are exist several fundamentally different ways of production of benzene. The first method is cooking coal. This process has been the first historically and served as a major source of benzene until the Second World War. Currently, the benzene fraction that is obtained by this method is less than 10%. Nowadays, the method of catalytic reforming gasoline fractions of petroleum is the primary source of benzene in the USA. However, the most popular method of production of benzene is the pyrolysis of gasoline and petroleum fractions. Nowadays, up to 50% benzene is produced by this technique. In addition, this method allows us to receive toluene and xylenes along with benzene.

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