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write about gatterman-coach reaction.also define etard's reaction.
Gattermann-Koch reaction is used for direct introduction of the -CHO group in benzene ring .It is a modification of the Friedal-Craft's reaction .In this reaction ,a mixture of COand HCl gases is passed through benzene containing anhydrous AlCl3 and CuCl .

                             CO    +HCl __________>     HCOCl
                                                                        formyl chloride

The oxidation of toluene with chromyl chloride (CrO2Cl2 ) in CCl4 or CS2 ,to form benzaldehyde is called Etard's reaction .The chromyl chloride first forms a brown complex  with toluene ,which when decomposed with water gives benzaldehyde .


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