Tiêu đề 32. SANDMEYER’S REACTION.pdf ✅

PHARMD GURU Page 1  Alkyl halides are more often prepared from the corresponding alcohols by reacting the alcohols with PCl3, SOCl2 or concentrated HCl / ZnCl2.  The aryl halides cannot be prepared from phenols by these methods.  Direct halogenation of aromatic compounds yields a mixture of isomers from which it is difficult to isolate the desired isomer.  Sandmeyer reaction affords a useful method for introducing a halogen substituent at the desired position of an aromatic ring.  The method involves the conversion of an aromatic primary amine into an aryl diazonium salt by treatment with nitrous acid in the presence of mineral acids (usually HCl or H2SO4) at low temperature (0–5 °C).  Subsequent decomposition of the diazonium salt by heating with cuprous chloride (or) bromide in the presence of an excess amount of the corresponding halogen acid gives aryl chloride or bromide.  The overall reaction is, thus, the replacement of the amino group of the aromatic amine by halogens. MECHANISM:  It seems that cuprous copper has the power to reduce diazonium ion to aryl radical by oxidation–reduction involving one-electron transfer. The copper first acts as reducing and then as oxidizing agent. SANDMEYER’S REACTION