Chemistry · Amines · NEET
Sandmeyer works for chlorobenzene and bromobenzene because CuCl and CuBr can deliver Cl and Br to the ring. But there is no useful cuprous fluoride reagent that does this cleanly, and simply heating a diazonium salt in HF gives poor yields and dangerous side reactions. So fluorine is handled differently: the -N2+ group is first locked up as a stable, insoluble tetrafluoroborate salt, which is then heated dry. This two-step trick is the Balz-Schiemann reaction.
HBF4 supplies the BF4- (tetrafluoroborate) ion. When you add HBF4 to benzene diazonium chloride, the BF4- pairs with the -N2+ cation to give benzene diazonium tetrafluoroborate, a solid that can be filtered, dried, and stored. The fluorine atom that ends up on the ring actually comes from this BF4- ion when the salt is heated. So HBF4 is both the fluorine source and the thing that makes the diazonium salt stable enough to isolate.
On gentle dry heating (thermal decomposition), the salt falls apart into three products: fluorobenzene (C6H5F), nitrogen gas (N2), and boron trifluoride (BF3). The N2 leaving is the same driving force seen in all diazonium replacement reactions. NEET wants you to remember all three products, not just fluorobenzene.
Yes. The first step, making the diazonium salt from aniline with NaNO2 and HCl, is still done cold at 273-278 K (0-5 C) because aromatic diazonium salts are unstable when warm. Only after you convert it to the fluoroborate salt and dry it do you apply heat. Students mix this up: cold to MAKE the diazonium, heat to DECOMPOSE the fluoroborate.
Chlorine, bromine, iodine, and the CN group are all put onto the ring by fairly simple diazonium replacements (Sandmeyer/Gattermann for Cl and Br, plain KI for I). Fluorine has no such direct route, so the Balz-Schiemann salt-then-heat method is its own named reaction. For NEET, link F to Balz-Schiemann, Cl/Br to Sandmeyer or Gattermann, and I to KI.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Step 1: benzene diazonium chloride + HBF4 gives benzene diazonium tetrafluoroborate (a solid salt). Step 2: heat the dry salt to give fluorobenzene + N2 + BF3.
Fluorobenzene (C6H5F), nitrogen gas (N2), and boron trifluoride (BF3). Remembering all three is a common NEET checkpoint.
Because there is no clean cuprous fluoride reagent to transfer F to the ring. Trapping the diazonium as a BF4- salt and heating it is the reliable route to aryl fluorides.
At 273-278 K (0-5 C, ice-cold) with NaNO2 and HCl, because aromatic diazonium salts decompose if warm. Heat is applied only later, to the dried fluoroborate salt.
Yes. Like other diazonium replacements, loss of very stable N2 drives the reaction. The N2 leaving is what makes the fluorine take its place on the ring.
