Benzene Preparing, reactions, physical characteristics

Physical properties of benzene

• colourless
• liquid compound in room temperature
• Melting point : 5.5⁰C
• Boiling point : 80⁰C
• There is a harsh smell
• Burns with high smoke
• A good solution for non-polar substances such as I₂, S....

Benzene's unsaturated behaviour

The corresponding saturated alkane for number of carbon atoms (6) of benzene is C₆H₁₄. But benzene has only six carbon atoms. Therefore benzene should be very unsaturated compounds. 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.

Kekule structure of benzene

Compare unsaturated of benzene and alkenes

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.

Preparation of Benzene

Prepare benzene from acetylene - actylene condensation

Send benzene through hot iron pipe. Here benzene is prepared by non aromatic compound. Three acetyle moles are send through hot Cu tube with catalyst condensation. Cu acts as the catalyst and benzene forms as the product.

Prepare benzene from benzoic acid

Benzoic acid is mixed with soda lime or solid Sodium hydroxide and heated. Benzene is given as carboxylic reduction. CaO and NaOH are used as sodalime mixture.

Prepare benzene from Phenol

Phenol and zinc dust is heated. -OH group is eliminated and benzene gives as the product.

Prepare benzene from diazonium salts

Benzene diazonium salts reacts with H₃PO₂ Or CH₃CH₂OH. Benzene is given as the product.

Prepare benzene from grignard reagent hydrolysis

Hydrolysis of any phenyl magnesium halide in acidic medium will give benzene as the product. If dilute sulfuric acid and phenyl magnesium chloride, benzene forms.

Substitution reactions 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.

• Halogenation
• Nitration
• Alkylation or friedel craft alkylation
• Acetylene or friedel craft acylation

Halogenation of benzene

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 catalyst and they acts as the halogen carrier. This reaction occurs through a electrophilic substitution mechanism.

benzene and chlorine with AlCl₃

benzene and bromine with FeBr₃

Alkylation or friedel crafts alkylation - Alkyl halides and benzene reaction

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 with anhydrous aluminium chloride

benzene and chloromethane react in the presence of anhydrous AlCl₃ to give methylbenzene / toluene.

benzene and bromoethane with anhydrous aluminium chloride

benzene and bromoethane react in the presence of anhydrous AlCl₃ to give ethylbenzene.

Acylation or friedel craft acylation - benzene and acid chloride reaction

Benzene reacts with acid chloride or acid anhydride and give acyl benzene in the presence of anhydrous AlCl₃.

Benzene and ethanoyl chloride with anhydride aluminium chloride

Benzene and with Acetic anhydride with aluminium chloride

Nitration - concentrated nitric and sulfuric acids with benzene reaction

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.

Ortho, para, meta positions of benzene

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 - ortho para directors

Phenol and aniline are ortho para directors. That means substitution mainly occurs in ortho para positions in the benzene ring.

Products of ortho para directors

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.

Ortho para directive compounds

• Phenol
• Aniline
• Toluene
• Chlorobenzene

Meta directive compounds

• Benzoic acid
• Nitrobenzene
• Benzaldehyde

Addition reactions of benzene

Addition reactions occur under more drastic conditions(higher temperatures, higher pressures) because extra stability of the delocalised Π electrons.

Addition of hydrogen(catalytic hydrogenation)

Benzene reacts with hydrogen in the presence of nickel or platinum catalyst at 200⁰C, 300⁰C under pressure to form cyclohexane.

Addition of halogens

Benzene reacts with chlorine(Cl₂) or bromine(Br₂) in the presence of sunlight to form hexachloride (BHC).

Methyl benzene or Toluene reactions

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 reaction

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.

Oxidation of alkyl side chain

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 and hot H⁺/KMnO₄

Methylbenzene reacts with hot H⁺/KMnO₄ and gives benzoic acid as the product.

Ethylbenzene and hot H⁺/KMnO₄

Ethylbenzene reacts with hot H⁺/KMnO₄ and gives benzoic acid as the product.

Xylenes and hot H⁺/KMnO₄

Xylenes reacts with hot H⁺/KMnO₄ and gives phthalic acids as the products.

Toluene and MnO₂ or CrO₂Cl₂

Toluene reacts with MnO₂ or CrO₂Cl₂ and gives benzaldehyde.

Questions of Benzene