Some Ponts about Physical Properties of Benzene has given Below:-
- Aromatic hydrocarbons like benzene are colorless and have characteristic odor.
- Benzene is toxic and carcinogenic in nature.
- It is a non-polar molecule and exists in the form of colorless liquid and highly inflammable in nature.
- That is the reason, the bottle of benzene are marketed with the warning of toxic and flammable liquid.
- Because of the high percentage of carbon atom compare to alkanes, Benzene burns with sooty flame and less dense than water.
- The density of benzene is 0.8765 g/cm3 and melts at 278.7 K. The boiling point of benzene is 353.3 K temperature.
Chemical Properties of Benzene
Benzene undergoes substitution reactions in spite of the high degree of unsaturation. This behavior of benzene is called as aromaticity or aromatic character. Aromaticity of benzene can be easily explained on the basis of resonance structure of benzene. During additional reactions of benzene, it will lose its aromaticity, hence it’s preferred to undergo substitution instead of additional reaction.
In benzene there are three pi bonds located in hexagonal ring in alternate manner. These pi bonds get delocalized in ring and make molecule stable. The carbon atoms in benzene are sp2 hybridized and each carbon atom has one unhybridized p-orbital. These six unhybridized p-orbitals get delocalized above and below the plane of ring.
Since six pi electrons are delocalized over the whole ring, therefore the cyclically conjugated double bonds represent by a circle and the carbon-carbon bond length becomes equal. This structure of benzene is called a resonance hybrid of benzene and generally used to represents the benzene molecule.
Thus due to resonance and high electron density, benzene mainly undergoes electrophilic substitution reaction. It can show additional and oxidation reactions also in the presence of strong reagents. Some common chemical properties of benzene are as follows.
1. Electrophilic substitution reaction
The most common substitution with benzene is electrophilic substitution reaction which is a multi step reaction. The catalysts and co-reagents react to generate a strong electrophilic species in initial step of the substitution. Electrophile interacts with benzene with to form a cyclohexadienyl cation which is known as Wheland complex or the s complex or the arenium ion.
In second step, base involves in reaction and reacts with s complex to form substituted product through deprotonation.
Arenium ion is a stable intermediate due to delocalization of positive charge on ring.
Different electrophilic substitution reactions with electrophile, products and catalyst as follows.
Table: Various electrophilic substitution reactions of Benzene
Since the intermediate formed during the substitution reaction is not aromatic in nature, therefore reaction will continue until the aromaticity been regained.
Let’s discuss one of the examples of electrophilic substitution of benzene like halogenation. Chlorine or bromine reacts with benzene in the presence of Lewis acid like ferric or aluminium salts of corresponding halogen to form halobenzene.
Reaction takes place in two steps.
- Reaction of AlCl3 with chlorine molecule (Cl2) to form Chlorinium ion ( Cl+) which acts as electrophile and attack on benzene ring.
- The reaction of electrophile results in the formation of arenium ion as an intermediate which gets stabilized by resonance.
- In the last step base (AlCl4-) which generated in first step, reacts with intermediate to regain aromaticity of ring and form chlorobenzene.
2. Addition reaction of benzene
Benzene shows some of addition reactions like alkene and alkyne under more drastic condition to form additional products. These additional products are more stable and behave as saturated hydrocarbons. The most common addition reactions of benzene are hydrogenation and halogenation results the formation of Cyclohexane and benzene hexachloride respectively. Hydrogenation of benzene takes place in the presence of a catalyst like nickel or palladium at 475-500K temperature.
In the presence of sunlight, benzene shows additional reaction with halogens like chlorine or bromine to form additional products. For example, with chlorine, it forms benzene hexachloride which is also called as BHC or gamaxine and used as an insecticide. Since this halogenation takes place in the presence of light without any catalyst, therefore reaction follows free radical mechanism.
3. Oxidation of benzene
The combustion of benzene forms carbon dioxide and water like other hydrocarbons. It burns with sooty flame due to high carbon content compare to other hydrocarbons.
2C6H6 + 15O2 $\rightarrow$ 2CO2 + 6H2O + heat
While the controlled oxidation in the presence of catalyst like vanadium pentaoxide (V2O5) at 725 K temperature results maleic anhydride.The ozonolysis of benzene forms Glyoxal through the formation of benzene Triozonide as an intermediate.
- A nitro group can be introduced into benzene by using a nitrating mixture to form nitro benzene.
- The nitrating mixture is a mixture of concentrated nitric acid and concentrated sulfuric acid.
- Here sulfuric acid acts as catalyst and responsible for the formation of electrophile that is nitronium ion (NO2+).
- When benzene is treated with this nitrating mixture at a temperature below 50°C, it forms nitrobenzene. It’s an example of electrophilic substitution reaction of benzene and completed through the formation of arenium ion as an intermediate.
Since sulfuric acid is a strong acid than nitric acid, it gets protonated the nitric acid which causes the loss of a water molecule and form electrophile, nitronium ion. In the absence of sulfuric acid, it is not possible to protonate the nitric acid due to its acidic properties.
The reaction of benzene with concentrated nitric acid and sulfuric acid give nitro benzene. This reaction is known as nitration of benzene. It follows electrophilic substitution mechanism and completed in three steps.
- The presence of concentrated sulfuric acid activates the nitric acid to form a stronger electrophile; nitronium ion (NO2+). Since this is the reaction between two acids, therefore one acts as Bronsted acid and another as Bronsted base. Out of these two acids, sulfuric acid is a stronger one, hence acts as Bronsted acid and protonated nitric acid. The protonation of nitric acid results lose of water molecule and form nitronium ion.
- In the second step, electrophile attacks on benzene ring to form intermediate and lose the aromaticity.
- Further this intermediate reacts with base that is bisulphate ion (HSO4- ion) produce in first step. Base gets deprotonate the intermediate to form nitrobenzene and sulphuric acid which acts as a catalyst for reaction.
The replacement of hydrogen atom of benzene by a sulfonic acid group (-SO3H) is called as sulfonation of benzene. The reaction is carried out in the presence of concentrated sulfuric acid containing dissolved sulfur trioxide which is also known as fuming sulfuric acid. The sulfonation of benzene results the formation of benzene sulfonic acid.
In sulfonation of benzene, sulfur trioxide acts as electrophile produced from concentrated sulfuric acid. The reaction is reversible in nature; hence the rate of reaction can increase with increasing the electrophile content. The sulfonation of benzene can be carried out by two ways, either by refluxing the benzene with concentrated sulfuric acid for several hours or refluxing the warm benzene at 313 K temperature for 20-30 minutes.
The sulfonation of benzene is a multi step reaction completed in four steps through the formation of sigma complex as an intermediate. Sulfur trioxide acts as intermediate in reaction and produced by auto-protolysis of sulfuric acid. Reaction gets complete in following steps.
- Formation of electrophile: The auto-protolysis of sulfuric acid results in the formation of sulfur trioxide which acts as electrophile due to positively charged sulfur atom in polar sulfur trioxide molecule and reacts with benzene.
- Next step involve the attack of electrophile on benzene ring to form sigma complex which is a zwitterion in this reaction due to the presence of opposite charge on same molecule.
- The intermediate gets stabilized by the delocalization of charge on benzene ring.
- In last two steps, base that is dissolved SO3 remove proton from sigma complex to form an aromatic sulfonate which further protonated by HSO3+ to form benzene sulfonic acid and sulfur trioxide.
The hydrogenation of benzene is an example of addition reaction of benzene which occurs only under drastic condition like strong catalyst, high temperature or high pressure etc. This reaction is exothermic in nature and furnished in three steps through the formation of 1,3-cyclohexadiene, Cyclohexene and finally forms Cyclohexane.
Hydrogenation of benzene takes place in the presence of metal catalyst like nickel, palladium or platinum at high temperature around 475 - 500 K. The selective hydrogenation of benzene results the formation of Cyclohexene in the presence of Ru-based catalyst. Since Cyclohexene is more prone to hydrogenation to form Cyclohexane, the reaction carried out in the presence of water which slows down this conversion by dissolving benzene.
The heat released during hydrogenation is known as heat of hydrogenation which is used for calculation of resonance energy of benzene by comparing the heat of hydrogenation of cylohexene and 1,3-cyclohexadien. This comparison proves that benzene is more stable compare to other two Cyclohexene molecules by 36 kcal/mol of energy and this energy is called as resonance energy.
Just like the hydrogenation of alkenes and alkyne in the presence of metal catalyst, the hydrogenation of benzene also takes place on the surface of catalyst through the formation of weak Vander Waals interactions between metal surface and benzene which finally form benzene.
The hydrogenation of benzene occurs in stepwise manner and first forms 1, 3 - cyclohexadien which further hydrogenated to form Cyclohexene. Hence hydrogen added stepwise on the double bonds of benzene and finally converted into Cyclohexane.
The selective hydrogenation of benzene in the presence of Ru - Zn catalyst form Cyclohexene through mono layer dispersion on the catalyst surface.
Toxicokinetics involves the complete description of the dose of toxic substance uptake into the body that is absorption of toxic substance, movement of substance in body i.e. distribution, conversion of substance to toxic or non-toxic species by bio-transformation and finally the removal of substances from the body by excretion process.
- The total amount of toxic substance which is known as dose of substance can be explained by using the dose-response relationship.
- The high dose of substance can be toxic and poisonous for body. The higher dose of substance severs more response and exhibits the toxic effect. However, the effect of toxic substances depends upon the reactivity of substance as well as on individuals.
- Like other aromatic compounds, benzene is also toxic in nature and cause adverse effect. Its mainly present at automobile service stations, industrial emissions, in tobacco smoke and in exhaust from motor vehicles.
- Out of these sources, tobacco smoke contributes about 50% of the entire exposure. The workers employed in benzene production industries like storage or transport of benzene, coke and coal chemical manufacturing, rubber tire manufacturing and petroleum refining are mainly exposed by high level of benzene.
- Around 50% of the benzene in air is absorbed through lungs and absorbed through the gastrointestinal tract. This absorbed benzene is rapidly distributed in body and accumulates in fatty tissues.
- The metabolism of benzene produces many reactive metabolites in liver. The low level of these metabolites can rapidly excrete as conjugated urinary metabolites like muconic acid and S-phenyl mercapturic acid.
- Hence the urinary benzene level is a sensitive bio-marker of low level of exposure of benzene. The permissible level for benzene in blood is 0.06-0.05 μg/L.
- The acute exposure of benzene mainly affects the hematopoietic system, nervous system, and immune system. Since it is carcinogenic in nature, it can also responsible for acute myelogenic leukemia.
Benzene is mainly found in crude oil, gasoline and cigarette smoke. Because of various industrial processes like burning coal, tobacco smoke, gasoline leaks, it enters in air, water, and soil. Many natural sources like crude oil seeps, volcanoes and forest fires are also responsible for expose of benzene. Benzene is degradable substance up to a level by volatilization, bio-degradation under aerobic conditions or photo oxidation with hydroxyl radicals. In metropolitan areas, it is found around 0.58 ppb in air sample and less than 5 ppb in sediment sample. While the level of benzene found in surface water samples is around 100 µg/L.
The degradation of benzene in air, water and soil results the formation of other aromatic compounds like nitro benzene, nitro phenols, dihydroxybenzene etc.
Some Uses of Benzene has given Below:
- Benzene is the simplest aromatic compound and involves in preparation of many other aromatic compounds like cumene, aniline, ethylbenzene which are further involved in preparation of resins and polymers.
- Benzene is mainly used as industrial solvent like for decreasing metal and as gasoline additive which decreases the knocking and increases the octane rating.
- A small amount of benzene used for manufacturing of rubbers, dyes, insecticides, solvents, fumigants, gasoline, paint removers, drugs, lubricants, detergents, explosives and pesticides.
- Benzene is also used in tire and rubber manufacturing processes as solvent. It is also a component of adhesive used for the attachment of soles to shoes.
- Benzene used as solvent for auto repair to clean parts like hydraulic systems, fuel system components and brakes. As benzene can easily dissolve the greases and does not react with metal surface of engine.