Chemical and physical properties of arenes
There are molecules that contain a small number of hydrogen atoms, although their carbon skeleton is quite large. Such substances are considered highly unsaturated, however, they do not enter into reactions characteristic of alkenes or alkynes. For example, do not discolor solutions of potassium permanganate or bromine water. The first chemists from this group of compounds were substances with a pleasant odor, so they were called aromatic. There is another name - arenas. Nomenclature, isomerism, obtaining and their physical properties will be studied by us in this article.
What is benzene
The simplest representative of arenes is benzene. Molecular Formula - C6H6. Among other aromatic hydrocarbons, it has the most important practical importance. The compound was discovered in 1825 by the English physicist M. Faraday in the composition of the light gas. It is a colorless liquid, lighter than water (density 0.88 g / cm3).It has a characteristic smell, poisonous. Boiling point - 80, freezes at 5.5. The physical properties of arenes, as well as other organic substances, depend not only on the qualitative and quantitative composition, but also on the structure of their molecules.
Riddle of the aromatic core
The first structural formula of the substance was proposed by the German chemist Kekule. In it, the carbon atoms lie in the same plane and form between them a regular hexagon. The carbon particles in the molecule are connected by two types of covalent bonds: single and double. However, the experiments did not confirm the presence of pi bonds in benzene, since the substance was not oxidized by solutions of bromine water and potassium permanganate. The explanation of this contradiction is the electronic structure of matter. In its molecule there is a special type of chemical bond, called aromatic or one-and-a-half. It is represented by a common electron cloud consisting of six mutually overlapping p-electrons. Being located above and below the plane of the molecule, the bond has the form of a ring and is called the aromatic core. The one-and-a-half bond determines the physical properties of arenes, as well as their ability to substitution and addition reactions.
The mechanism of the replacement process
The aromatic nucleus system has an increased electron density and may be subject to attacks of electrophilic particles with a deficit of negative charges. The result of this effect will be the replacement of hydrogen atoms by other particles, for example, in the benzene molecule. The physical properties of arenes and the peculiarities of chemical processes are very different from those of alkanes or alkenes. Moreover, unlike other classes of organic compounds, arenas cannot be described by one general formula. So, for naphthalene, the formula will be as follows: CnH2n-12for benzene –CnH2n-6for diphenyl - CnH2n-14.
Chlorination, bromination and nitration of aromatic hydrocarbons occurs by a single mechanism of electrophilic substitution, in which chlorine, bromine or nitro groups play the role of an attacking electrophilic agent.
Why do addition reactions require harsh conditions?
The main feature of the structure of the molecules of aromatic compounds is the presence in their molecules of the benzene ring, consisting of six overlapping p-electrons. The main chemical characteristics and physical properties of arenes depend on it. To break a one-and-a-half bond, an additional significant portion of energy is needed.It is obtained by applying the following parameters: ultraviolet radiation, high temperature and pressure in the reaction mixture, as well as using catalysts. For example, chlorine attaches to benzene only when irradiated with ultraviolet light, forming hexachlorocyclohexane (hexachlorane). It is used for dressing wood during its long-term storage, as well as in agriculture as a means to combat insect pests.
C6H6 + 3Cl2 --- light ---> C6H6Cl6 - addition
If hydrogen atoms in the benzene molecule are replaced by hydrocarbon radicals, for example, methyl -CH3or ethyl -C2H5then the resulting aromatic hydrocarbons will not be a simple sum of the properties of alkanes and arenes. The physical properties and chemical reactions characteristic of these compounds will have a number of differences. So, toluene and ethylbenzene are colorless liquids with a peculiar odor, lighter than water and insoluble in it. But substances are well dissolved in organic solvents, for example, acetone or dichloromethane. They themselves can also be used as a solvent.Aromatic hydrocarbons also have specific characteristics. Surface tension, diffusion coefficient, viscosity are the physical properties of arenes, briefly describing their characteristics as organic solvents.
Under the action of nitric acid and in the presence of sulphate acid as a catalyst, arenas are nitrated. The reaction of benzene with a nitrous mixture will be as follows.
Nitrobenzene is a pale yellow oily liquid with a characteristic almond odor. It is used as a solvent and serves as a feedstock for the production of aniline.
In the reaction of nitric acid with toluene, an explosive is formed - trinitrotoluene or trotyl. Some other organic substances with benzene cores containing multiple bonds in the side chains are also used as pyrogenic compounds.
Getting and applying arenas
The physical properties of arenes: high heat capacity, flammability, ability to dissolve other substances, are widely used in various economic sectors.Benzene, toluene, naphthalene, anthracene and other aromatic hydrocarbons are obtained from petroleum or coal tar formed during the coking of coal. Since the industry’s demand for arenas is enormous, they are also mined by the method of catalytic aromatization of hydrocarbons that are part of petroleum and have an acyclic structure. In this case, a dehydrogenation reaction occurs - the elimination of hydrogen atoms, as a result of which, for example, benzene can be obtained from hexane. In the chemistry of organic synthesis continue to apply the method of producing benzene from acetylene. The reaction is carried out by heating ethine to 600 ° C and is called trimerization. The chemical and physical properties of arenes are used by chemistry in the production of plastics, varnishes, and dyes.
Benzene is used as an additive to the fuel. In agriculture, compounds - halogen derivatives of aromatic hydrocarbons, serve as insecticides - means to combat insect pests. Nitro compounds, for example, trinitrotoluene, are used in the military industry as explosives.Many arenas are good solvents.
In our article, we studied the chemical and physical properties of arenes, and identified the areas of application of these compounds in industry.