JEE Main & Advanced Chemistry Hydrocarbons / हाइड्रोकार्बन Benzene \[\left( {{\mathbf{C}}_{\mathbf{6}}}{{\mathbf{H}}_{\mathbf{6}}} \right)\]

Benzene is the first member of arenes. It was first discovered by Faraday (1825) from whale oil. Mitscherllich (1833) obtained it by distillating benzoic acid with lime.  Hofmann (1845) obtained it from coal tar, which is still a commercial source of benzene.

 

(1) Structure of benzene : Benzene has a special structure, which is although unsaturated even then it generally behave as a saturated compound.

 

(i) Kekule's structure : According to Kekule, in benzene 6-carbon atoms placed at corner of hexagon and bonded with hydrogen and double bond present at alternate position.

 

(a) Evidence in favour of Kekule's structure

 

• Benzene combines with 3 molecules of hydrogen or three molecules of chlorine. It also combines with 3 molecules of ozone to form triozonide. These reactions confirm the presence of three double bonds.

 

• Studies on magnetic rotation and molecular refraction show the presence of three double bonds and a conjugated system.

 

• The synthesis of benzene from three molecule of acetylene also favour's Kekule's structure.

 

 

• Benzene gives cyclohexane by reduction with hydrogen.

 

 

(b) Objections against Kekule's formula

 

• Unusual stability of benzene.

 

• According to Kekule, two ortho disubstituted products are possible. But in practice only one ortho disubstituted product is known.

 

• Heat of hydrogenation of benzene is 49.8 kcal/mole, whereas theoretical value of heat of hydrogenation of benzene is 85.8 kcal/mole. It means resonance energy is 36 kcal/mole.

 

• \[C-C\] bond length in benzene are equal, (although it contains 3 double bonds and 3 single bonds) and are \[1.39\,\,\overset{o}{\mathop{A}}\,\].

Kekule explained this objection by proposing that double bonds in benzene ring were continuously oscillating between two adjacent positions. 

 

(2) Methods of preparation of benzene            

(i) Laboratory method :  

 

                              

 

(ii) From benzene derivatives

(a) From phenol :

 

 

(b) From chlorobenzene :

 

 

 

(c) By first preparing grignard  reagent of chlorobenzene and then hydrolysed  

 

\[\underset{\text{Chlorobenzene}}{\mathop{{{C}_{6}}{{H}_{5}}Cl}}\,\underset{\text{dry}\,\text{ether}}{\mathop{\xrightarrow{Mg}}}\,\underset{\text{chloride}}{\mathop{\underset{\text{Phenylmagnesium}}{\mathop{{{C}_{6}}{{H}_{5}}MgCl}}\,}}\,\xrightarrow{{{H}_{2}}O}\underset{\text{Benzene}}{\mathop{{{C}_{6}}{{H}_{6}}}}\,+Mg\ \ \ \ \begin{matrix}OH  \\Cl  \\\end{matrix}\]

 

(d) From benzene sulphonic acid :

 

 

(e) From benzene diazonium chloride :

 

              

 

(f) From acetylene :

 

• Cyclic polymerisation takes place in this reaction.

 

(g) Aromatisation : \[\underset{n-Hexane}{\mathop{{{C}_{6}}{{H}_{14}}}}\,\underset{\text{at high pressure}}{\mathop{\underset{500{}^\circ C}{\mathop{\xrightarrow{C{{r}_{2}}{{O}_{3}}/A{{l}_{2}}{{O}_{3}}}}}\,}}\,\underset{\text{Benzene}}{\mathop{{{C}_{6}}{{H}_{6}}}}\,+4{{H}_{2}}\]

 

(3) Properties of benzene

(i) Physical properties

 

(a) Benzene is a colourless, mobile and volatile liquid. It's boiling point is \[{{80}^{o}}C\] and freezing point is \[{{5.5}^{o}}C\]. It has characteristic odour.

 

(b) It is highly inflammable and burns with sooty flame.

 

(c) It is lighter than water. It's specific gravity at 20°C is 0.8788.

 

(d) It is immiscible with water but miscible with organic solvents such as alcohol and ether.

 

(e) Benzene itself is a good solvent. Fats, resins, rubber, etc. dissolve in it.

 

(f) It is a non-polar compound and its dipole moment is zero

 

(g) It is an extremely poisonous substance. Inhalation of vapours or absorption through skin has a toxic effect.

 

(ii) Chemical properties : Due to the presence of p electron clouds above and below the plane benzene ring, the ring serves as a source of electrons and is easily attacked by electrophiles (Electron loving reagents). Hence electrophilic substitution reaction are the characteristic reactions of aromatic compounds.

 

Substitution reactions in benzene are prefered rather than addition are due to the fact that in the former reactions resonance stabilised benzene ring system is retained while the addition reactions lead to the destruction of benzene ring. Principal reactions of benzene can be studied under three heads,

 

(a) Addition reactions

 

(b) Substitution reactions

 

(c) Oxidation reactions 

 

(a) Addition reactions : In which benzene behaves like unsaturated hydrocarbon.

 

Addition of hydrogen : Benzene reacts with hydrogen in the presence of nickel (or platinum) as catalyst at \[150{}^\circ C\] under pressure to form cyclohexane.

 

 

Addition of halogen :

 

 

Addition of ozone :

 

 

(b) Substitution reactions :           

Nucleophilic substitution :            

Unimolecular : Mostly uncommon in  aromatic substitution, there is only one example which obtain in benzene diazonium dichloride.

 

      

 

• Bimolecular  :    

        

 

        

    Example :

           

• Elimination-addition mechanism (Benzyne mechanism)                  

 

                           

 

• Electrophilic substitution reaction : Benzene undergoes this reaction because it is an electron rich system due to delocalized p- electrons.            

                       

     

 

Electrophile \[({{E}^{\oplus }})\]	Name	Source	Name of substitution reaction
\[C{{l}^{+}}\]	Chloronium	\[C{{l}_{2}}+AlC{{l}_{3}}\] or  \[FeC{{l}_{3}}\]	Chlorination
\[B{{r}^{+}}\]	Bromonium	\[B{{r}_{2}}+AlB{{r}_{3}}\] or \[FeB{{r}_{3}}\]	Bromination
\[NO_{2}^{+}\]	Nitronium	\[HN{{O}_{3}}+{{H}_{2}}S{{O}_{4}}\]	Nitration
\[S{{O}_{3}}\]	Sulphur trioxide	Conc. \[{{H}_{2}}S{{O}_{4}}\], Fuming sulphuric acid	Sulphonation
\[{{R}^{+}}\]	Alkyl carbonium	\[RX+Al{{X}_{3}}\,(X=Cl\]or \[Br),\,ROH+{{H}^{+}}\]	Friedel-Craft's (Alkylation)
\[R-\overset{+}{\mathop{C}}\,=O\]	Acyl carbonium	\[RCOCl+AlC{{l}_{3}}\]	Friedel-Craft's (Acylation)

 

• Free radical aromatic substitution : The aromatic substitution reactions which follow free radical mechanisms are very few and have limited synthetic value. But some typical example of these reactions are:            

 

\[{{(C{{H}_{3}})}_{3}}COOC-{{(C{{H}_{3}})}_{3}}\xrightarrow{\text{heat}}2{{(C{{H}_{3}})}_{3}}C\overset{\cdot }{\mathop{O}}\,\xrightarrow{{}}\overset{\cdot \,\,\,}{\mathop{2C{{H}_{3}}}}\,+2C{{H}_{3}}COC{{H}_{3}}\]        

 

                                         

 

The mechanism of chlorination of benzene at high temperature is similar to that of the free radical aliphatic substitution            

 

\[C{{l}_{2}}\xrightarrow{{}}\overset{.}{\mathop{Cl}}\,\,+\overset{.}{\mathop{Cl}}\,\] (Chain initiation)        

 

\[{{C}_{6}}{{H}_{6}}+\overset{.}{\mathop{Cl}}\,\xrightarrow{{}}\overset{.\,\,\,\,\,\,\,}{\mathop{{{C}_{6}}{{H}_{5}}}}\,+HCl\] (H- abstraction)

 

\[\overset{.\,\,\,\,\,\,\,\,}{\mathop{{{C}_{6}}{{H}_{5}}}}\,+C{{l}_{2}}\xrightarrow{{}}{{C}_{6}}{{H}_{5}}Cl+\overset{.}{\mathop{Cl}}\,\] (Chain propagation)            

 

(c) Oxidation : \[2{{C}_{6}}{{H}_{6}}+15{{O}_{2}}\xrightarrow{{}}12C{{O}_{2}}+6{{H}_{2}}O\] \[\Delta H=\]6530 kJ/mole            

 

When vapours of benzene and air are passed over vanadium pentoxide at \[450-{{500}^{o}}C\], maleic anhydride is obtained.

 

                               

 

• Strong oxidising agents converts benzene slowly into \[C{{O}_{2}}\] and water on heating.            

(d) Reduction :

 

 

(iii) Uses : (a) In dry cleaning (b) As a motor fuel when mixed with petrol. (c) As a solvent. (d) In the manufacture of gammexane (As insecticide). (e) In the preparation of nitrobenzene, chlorobenzene, benzene sulphonic acid, aniline, styrene, etc. Many of these are employed  for making dyes, drugs, plastics, insecticides, etc.