Chemistry · Amines · NEET
Direct halogenation works well only for Cl and Br (with a catalyst). Iodination is very slow and reversible, and direct fluorination is too violent and hard to control. So aryl iodides and aryl fluorides are made through the diazonium salt instead: C6H5N2+ + KI gives iodobenzene, and the fluoroborate salt on heating (Balz-Schiemann) gives the aryl fluoride. This is the single biggest reason diazonium salts are important.
You start from aniline (made from nitrobenzene), diazotise it at 273-278 K to get C6H5N2+, and then replace N2+ with whatever group you want. Because N2 leaves as a gas, the reaction is driven forward and gives a clean product at a fixed position on the ring. This gives access to F, Cl, Br, I, CN, OH, NO2 and even H (deamination) from one common intermediate.
The -NH2 (and later -N2+) group fixes the position on the ring in advance. You can use the amino group to direct incoming groups to specific positions, then remove or replace it through the diazonium salt at the end. This is how NCERT makes tricky targets like 1,3,5-tribromobenzene from aniline: brominate first, then remove the amino group by deamination.
Aromatic primary amines give stable diazonium salts at low temperature because the N2+ is stabilised by resonance with the ring. Aliphatic amines give diazonium salts that break down at once, so they are useless for synthesis. That stability is exactly why aromatic (aryl) diazonium salts became such a useful intermediate.
Yes: -Cl and -Br (Sandmeyer with CuCl/CuBr, or Gattermann with Cu/HX), -I (with KI), -F (Balz-Schiemann with HBF4 then heat), -CN (CuCN, Sandmeyer), -OH (warm water to 283 K, gives phenol), -NO2 (fluoroborate + NaNO2/Cu), and -H (deamination with H3PO2 or ethanol). Learning this list answers most NEET questions on this topic.
Statement I: Benzenediazonium salt is prepared by the reaction of aniline with acid at 273-278 K. It decomposes easily in the dry state. Statement II: Insertion of iodine into the benzene ring is difficult, and hence iodobenzene is prepared through the reaction of the benzenediazonium salt with KI. In the light of the above statements, choose the most appropriate answer.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
They are versatile intermediates that let the N2+ group be replaced by F, Cl, Br, I, CN, OH, NO2 or H, giving substituted aromatics that cannot be made by direct substitution of benzene.
Fluorine (via Balz-Schiemann) and iodine (via KI). Direct halogenation gives only chlorine and bromine easily.
273-278 K (0-5 C). Above this the salt breaks down, so diazotisation is always done cold in an ice bath.
No. Aliphatic diazonium salts are unstable and decompose immediately, so only aromatic amines like aniline give useful diazonium salts.
Diazotise aniline to the diazonium salt, then warm the solution to about 283 K so it hydrolyses and the N2+ is replaced by -OH, giving phenol.
