Benzene is an aromatic organic compound which contains only carbon and hydrogen as elements. Due to specific electron arrangement in the benzene molecule (electron ring), Benzene shows following types of reactions and we will discuss them in detail.
The corresponding saturated alkane for number of carbon atoms (6) of benzene is hexane (C6H14). But Benzene has only six hydrogen atoms. Therefore benzene should be a very unsaturated compound. But benzene is subject to some addition reactions. As an example, by the catalytic hydrogenation, benzene gives cyclohexane. In this reaction six hydrogen atoms are added to the benzene molecule. Therefore we can assume there may three double bonds in the benzene molecule.
Alkenes discolourize the liquid bromine and cold alkali permanganate solution. But benzene does not response to these reagents. Therefore we see like benzene is not unsaturated. So, we can understand benzene is not unsaturated as much as alkenes.Thus, kekule structure cannot describe the chemical behaviour of benzene.
One or more hydrogen atoms in benzene ring get replaced by other atoms or groups. These substitution reactions are occurred through electrophilic substitution reaction mechanism. Under this reaction type, we will study following reactions.
A hydrogen atom in the benzene ring is replaced by a halogen atom (F, Cl, Br, I) is called halogenation. Lewis acids such as Ferric chloride (FeCl3) or Aluminium chloride (AlCl3) are used as a catalyst and they acts as the halogen carrier. This reaction occurs through a electrophilic substitution mechanism.
Alkyl halides react with benzene in the presence of anhydrous AlCl3, and gives alkyl benzene as a product. A hydrogen atom in the benzene ring is replaced by alkyl group.
benzene and chloromethane react in the presence of anhydrous AlCl3 to give methylbenzene / toluene.
Benzene and bromoethane react in the presence of anhydrous AlCl3 to give ethylbenzene.
Benzene reacts with acid chloride or acid anhydride and give acyl benzene in the presence of anhydrous AlCl3.
A hydrogen atom in the benzene ring is replaced by a nitro(-NO2) group is called the nitration. this reaction is carried out by heating benzene with the nitrating mixture of concentrated nitric ans sulfuric acids at 500C.
When hydrogen atom or atoms in the benzene ring are replaced by another atom(s) or group (X = Cl, Br, NO2, OH, NH2 etc), we name positions of carbon atoms of benzene ring as ortho, meta and para related to the position of X. Electrons density of those ortho para meta positions are different. Electrons density of some positions will increase and in some positions decrease. Therefore position of attaching another group to carbon ring(substituting instead of H) is changed according to the position of carbon atom. As a example, phenol, aniline increases electrons density in ortho, para locations. Electrons density of ortho para positions in nitrobenzene is less than meta position. This phenomenon can be explained by resonance effect.
This ortho, para, meta positions are important when we study about substituting reactions about benzene.
Phenol and aniline are ortho para directors. That means substitution mainly occurs in ortho para positions in the benzene ring.
Ortho para directors give major product as ortho-product and para product. As a minor product meta product is given. You will get a mixture of products.
Addition reactions occur under more drastic conditions(higher temperatures, higher pressures) because extra stability of the delocalized Π electrons.
Benzene reacts with hydrogen in the presence of nickel or platinum catalyst at 2000C, 3000C under pressure to form cyclohexane.
Benzene reacts with chlorine(Cl2) or bromine(Br2) in the presence of sunlight to form hexachloride (BHC).
Toluene (methyl benzene - C6H5CH3) is a good solvent and used instead of benzene due to toxicity of benzene). Toluene is a ortho para director and activate the benzene ring.
Toluene and bromine react in the presence of anhydrous FeBr3 and give ortho and para products mixture. These two products can be separated by fractional distillation.
The alkyl side chain in benzene can be oxidized by strong oxidizing agents such as hot H+/KMnO4 or hot H+/Na2Cr2O7 or concentrated HNO3. With strong oxidizing agent, the alkyl chain is oxidized into carboxylic acid group. But with mild oxidizing agents such as acidic manganese dioxide(MnO2) or chromyl chromide(CrO2Cl2), alkyl group is oxidized into aldehyde group.
Methylbenzene reacts with hot Potassium permanagante in acidic medium and gives benzoic acid as the product.
Ethylbenzene reacts with hotPotassium permanagante in acidic medium and gives benzoic acid as the product.
Xylenes reacts with hot H+/KMnO4 and gives phthalic acids as the products.
Toluene reacts with MnO2 or CrO2Cl2 and gives benzaldehyde.