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Preparation of Benzene and its Homologues

  1. From alkynes
    Alkynes polymerize at high temperatures to yield arenas e.g. benzene is obtained from ethyne.

  1. Decarboxylation of aromatic acids
    In laboratory benzene is prepared by heating sodium benzoate with sodalime.

  1. Reduction of diazonium salts
    In presence of hypophosphorus acid, benzene diazonium chloride is converted into benzene (diazo group is replaced by H)


  1. Friedel - Crafts reaction
    Benzene can yield arenas on treating with alkyl halide in presence of anhydrous aluminium chloride.



  1. Wurtz-Fittig reaction
    Arenes can be obtained by action of sodium metal on a mixture of aryl halide and alkyl halide in either.


  1. From Grignard reagents
    Arenes can be prepared by reacting Grignard reagent with alkyl halide. e.g.



Physical and Chemical properties of Arenes

Aromatic hydrocarbons are unsaturated cyclic compounds. They are also known as arenes. They are highly unreactive towards chemical reagents.

Benzene and its homologues are better solvents as they dissolve a larger number of compounds. This is because the electron cloud makes it polar to some extent and therefore even polar molecules are attracted towards it. This helps in dissolving in them a large number of compounds.

The reasons for the inertness of benzene and its homologues is due to the presence of π -electron clouds above and below the plane of the ring of carbon atoms. Therefore, nucleophilic species (electron-rich species, such as Cl-, OH-, CN-, etc) cannot attack the benzene ring due to the repulsion between the negative charge on the nucleophile and the delocalized π -electron cloud. However, eletrophiles (such as H+, Cl+, NO2+, etc) can attack the benzene ring and for this reason benzene and its homologues undergo electrophilic reactions.

Reactions of benzene will be considered as a typical example of the reactions of aromatic compounds. Benzene undergoes substitution, addition and oxidation reactions.

Aromatic hydrocarbons do not easily undergo addition reactions. Under drastic conditions, such as high temperature, the presence of a catalyst, etc. hydrocarbons undergo substitution reactions in preference to addition reactions. The hydrogen atoms of benzene and its homologues can be replaced by nitro (-NO2), halogen (-X), sulphonic acid group
(-SO3H), etc.

However, arenes also undergo a few addition reactions under more drastic conditions, such as increased concentration of the reagent, high pressure, high temperature, the presence of a catalyst, etc.

Substitution Reactions

  1. Halogenation
    Benzene (and its homologues) reacts with chorine or bromine in the presence of a catalyst, such as anhydrous ferric chloride or anhydrous aluminium chloride to give halogen-substituted benzene.

Fluorine is very reactive and, hence, this method is not used for the preparation of fluorobenzene.
Iodobenzene is prepared by carrying out of reaction in the presence of oxidizing agent such as concentrated nitric acid, mercuric oxide, etc.

  1. Nitration
    Benzene forms nitrobenzene when treated with a mixture of concentrated nitric acid and concentrated sulphuric acid (nitration mixture) at 330 K.

  1. Sulphonation
    The introduction of a sulphuric acid group (-SO2.OH) is called sulphonation. Benzene sulphonic acid is formed when benzene is heated with fuming sulphuric acid (concentrated sulphuric acid containing some sulphur trioxide). Thus, the hydrogen atom in benzene is replaced by a sulphonic acid group.

Chlorosulphonic acid can also be used for sulphonation of benzene.

  1. Alkylation
    The alkyl group (-R) can be substituted in aromatic hydrocarbons. Alkylation of aromatic hydrocarbons is carried out by Friedal-crafts reaction. In this reaction benzene is heated with an alkyl halide in the presence of anhydrous aluminium chloride (or other Lewis acids, such as anhydrous ferric chloride, boron trifluoride, etc.).

  1. Acylation
    The introduction of RCO-group is acylation. Aromatic hydrocarbons undergo acylation by Friedel-Crafts reaction. The product found is an aromatic ketone.

Addition Reaction of Arenes

Benzene and its homologues undergo some addition reactions similar to alkenes and alkynes. However, extremely drastic conditions are required for carrying out addition reactions in arenes. For example, benzene can be chlorinated and hydrogenated.
  1. Addition of chlorine
    Benzene, when treated with excess of chlorine in the presence of sunlight or ultraviolet light, gives an addition product, benzene hexachloride (BHC). It is used as an insecticide.

  1. Addition of hydrogen
    Hydrogen adds on to benzene when heated (475K) under pressure in the presence of a nickel catalyst to cyclohexane (hexahydrobenzene). Similarly, toluene gives methylcyclohexane (hexahydrotoluene).


Arenes can be oxidised to different products depending upon their structure and conditions of the reaction.
  1. Combustion
Aromatic hydrocarbons burn with a luminous and sooty flame.
2C6H6 +  15O2 12CO2   +   6H20
C6H5.CH3 +   9O2 7CO2 + 4H2O
  1. Side-chain oxidation
Arenes with a side chain, on oxidation with strong oxidizing agents, such as alkaline potassium permanganate, give carboxylic acid. However, a hydrocarbon without a side chain remains unaffected.

In case there are more carbon atoms in the side chain, on oxidation all the carbon atoms get oxidized to carbon dioxide except the one that is directly attached to the aromatic ring.

  1. Catalytic oxidation
    Benzene can be catalytically oxidized to an aliphatic compound maleic anhydride. Benzene, when heated with excess of air at 800 k in the presence of vanadium pentoxide (V2O5) gives maleic anhydride.

Orientation in Benzene ring

The substituent group in Mono substituted Benzene directs the incoming group either to O and p- positions or m - position.

Ortho and para directing groups increase the electron density on the ring while m-directing groups decrease the electron density on the benzene ring.

Ortho and para directing groups: NH2 - NHR; - NR2; - OCH3, CH3; C2H5, OH, halogens etc.
m - directing groups: Ex: -NO2; -COOH; -SO3H; -CN;
- CHO; - COR etc.

The orientation and reactivity of the substituents can be explained by (i) Resonance and (ii) inductive effect.


(i) Ortho and p-directing

Resonance in Nitrobenzene

ii) m - directing

In general the o,p directing groups increase the electron density on the benzene ring (activating) and the m-directing groups decrease the electron density on the benzene ring (deactivating).

Carcinogenicity and Toxicity

Under prolonged exposure of some organic compounds, derived form coal tare cause toxicity and produce cancer. Through it is difficult to predict carcinogenic activity of hydrocarbons and their derivatives, the presence of some groups like -CH3; -OH; -CN etc have been found to influence carcinogenic activity.

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