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Preparation of Haloarenes

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Chloroarenes and bromoarenes are prepared by reaction of halogens (Chlorine and bromine) with benzene derivatives in the presence of lewis acids e.g., Fe, FeCl3, AlCl3 etc.

Preparation of haloarenes in the presence lewis acids

If halogen is used in large quantity, ortho and para isomers are formed because halogen is an o,p-directing group.

Preparation of haloarenes in the presence lewis acids : excess halogen leads to the formation of o,p isomers

Note : Ortho and para isomers can be easily separated due to large difference in their melting points.

If we use I2 instead of Cl2 or Br2, the reaction becomes reversible because HI produced during the reaction is a good reducing agent and converts aryl iodide back to the aromatic hydrocarbon.

Reaction of iodine with benzene is reversible

To shift the equilibrium in forward, some oxidizing agents like HNO3, HIO3, HIO4 or HgO are used. The oxidising agents either oxidise HI to I2 or convert it into HgI2.

2HI + 2HNO3 → 2NO2 + 2H2O + I2

5HI + HIO3 → 3I2 + H2O

2HI + HgO → HgI2 + H2O

Note : Fluoro compounds are not prepared by this method because the reaction is highly exothermic and causes explosion.

Preparation of haloarenes from diazonium salt

Diazonium salts (R-N2X) can alternatively be used to prepare haloarenes.

Benzenediazonium chloride

benzene diazonium salt

Diazotization reaction

Diazotization reaction is used to prepare diazonium salt. In diazotization reaction, aniline is dissolved in dilute HX with an aqueous solution of sodium nitrite at 0 to 5°C.

Preparation of diazonium salt by diazotisation reaction

Sandmeyer's reaction

Aryl chlorides and aryl bromides are prepared by Sandmeyer's reaction. In Sandmeyer's reaction, diazonium salt reacts with copper halides dissolved in HX to form haloarenes.

Sandmeyer's reaction

Preparation of iodoarenes

Iodoarenes are prepared by warming diazonium salt in aquous KI. Copper halides are not needed in this reaction.

Preparation of iodoarenes from diazonium salt using aquous potassium iodide

Gattermann's reaction

Haloarenes (Chloro and bromoarenes) can alternatively be prepared by gattermann's reaction. Gattermann's reaction is a modification of Sandmeyer's reaction.

In Gattermann's reaction, instead of cuprous halide (CuCl or CuBr), copper powder is used.

Gattermann's reaction
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