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Aryl Halide

June 5, 2013

The hydrocarbons containing any halogen atom is known as haloalkanes or alkyl halide. If any aromatic ring is directly bonded with halogen (fluorine, chlorine, iodine or bromine), then it is called as Aryl Halide. They can be classified on the basis of number of halogen atom in the organic compound (mono, di, poly) or on the basis of hybridization of carbon atom which is bonded with the halogen atom (alkyl, aryl, vinyl, allyl or alkynyl).

Mono-halo-alkanes can be further classified as primary halogeno-alkane, secondary and tertiary haloalkanes. In case of primary (1°); one halogen atom is bonded on the primary carbon atom while in secondary, the halo group is bonded in the secondary carbon atom and so on.

Let’s discuss some of the physical properties of these organic compounds. The boiling Points of these compounds depend upon the length of alkyl chain and the type of halogen. From chlorine to iodine; the boiling point increases. The trend can be observed in aryl halides also. This is due to intermolecular attractions; van-der Waals dispersion forces which increase with the length of carbon chain and decreases with the electro negativity of halogen. As the branching in the chain increases; the boiling point of compound decreases due to fall in the effectiveness of the dispersion forces.

The halogen compounds are less soluble in water due to strong van der Waals dispersion and dipole-dipole interactions between them with strong hydrogen bonds between water molecules. The cleavage of both combinations required a large amount of energy. But these organic compounds are quite soluble in organic solvent due to the formation of strong intermolecular attraction forces between molecules and solvent.

The strength of these attraction forces decreases from fluorine to iodine which further decreases the solubility of compound. The reactivity of halogen com[pound increases from fluorine to iodine; therefore aryl iodide is the most reactive and aryl fluoride is the least one. These compounds are generally involved in Nucleophilic substitution reactions as the cleavage of carbon-halogen bond forms positive and negative ions. The positive charged carbon atom reacts with nucleophile and pushing off the halogen atom.

There are many preparation methods for the synthesis of halogen compounds. For aromatic halogen compound; benzene diazonium halide is widely used and the reaction is known as Balz-Schiemann reaction or Schiemann Reaction which converts the aryl amines to diazonium salt by diazotisation. The diazonium salt converts in to aryl fluorides by thermal or photochemical decomposition. The aryl amine or aniline reacts with HBF4 and NaNO2 to form tetrafluoroborate diazonium salt which converts into fluorobenzene by thermal decomposition.

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