Hydrocarbons: Chemical Properties of Aromatic Hydrocarbons and Direct Influence of Functional Group (For CBSE, ICSE, IAS, NET, NRA 2022)

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Chemical Properties of Aromatic Hydrocarbons

Halogenation

  • The reaction in which a hydrogen atom of benzene is replaced by a halogen atom is called halogenation of benzene.
  • Halogenation takes place in the presence of iron, or ferric halides (, where X = Cl or Br) .
Chemical Properties: Halogenation
  • In case of iodination the HI formed can reduce iodobenzamide back to be benzene.
  • To prevent it is carrying out this reaction in the presence of or .
Chemical Properties: Halogenation

Nitration

  • The chemical reaction in which a hydrogen atom of benzene ring is replaced by group, is called nitration.
  • It is carried out in the presence of nitrating mixture, i.e.. a mixture of conc. and conc. .
Chemical Properties: Nitration

Sulphonation

The chemical reaction in which the hydrogen atom of benzene is replaced by – group when benzene is heated with fuming sulphuric acid (oleum) , is called Sulphonation.

Chemical Properties: Sulphonation

Friedel – Craft՚s Reactions

  • In Friedel-Craft՚s reaction, benzene is heated either with alkyl halide (alkylation) or acyl halide (acylation) in the presence of a catalyst (anhydrous ) .
  • The products formed are alkyl or acyl derivatives of benzene.

Alkylation

Friedel-Craft՚s Reactions: Alkylation

Acylation

Friedel-Craft՚s Reactions: Acylation

Direct Influence of Functional Group

  • In substituted aromatic compounds, the functional group (s) already present directs the next incoming group to a particular position in the aromatic ring. It is called direct influence of the group attached to the benzene.
  • For example, phenol on chlorination gives a mixture of ortho – chlorophenol and para – chlorophenol as groups is an ortho and para directing group.
  • In nitrobenzene, group is a meta directing group.

Carcinogenicity and Toxicity

There are several aromatic compound – many of them are very important for life while there are some others which are harmful.

Uses of Aromatic Compounds

  • Benzene is used as a solvent for several organic compounds and acts as a medium for carrying out synthetic reactions.
  • Toluene, a higher homologue of the benzene, finds its uses for dry-cleaning, as a solvent, and as a starting material for the manufacture of dyes, drugs, explosive (trinitrotoluene, T. N. T.) , benzaldehyde, benzoic acid etc.

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