The number of halobenzenes from the following that can be prepared by Sandmeyer's reaction is _________

Solution

Chemistry ma’am · 2 steps

Sandmeyer means a copper(I) halide, and only CuCl and CuBr are used, so count just Cl and Br.

1. What Sandmeyer is. Sandmeyer's reaction replaces the -N₂⁺ group of benzenediazonium salt with a halogen using a copper(I) halide in the matching acid. The Cu⁺ salt is what makes it Sandmeyer.

   C₆H₅N₂⁺Cl⁻ →(CuX/HX) C₆H₅X + N₂ (X = Cl, Br)

   Halobenzene	How it is made from the diazonium salt	Sandmeyer?
   I: C₆H₅F	HBF₄, then heat (Balz–Schiemann)	No
   II: C₆H₅Cl	CuCl/HCl	Yes
   III: C₆H₅Br	CuBr/HBr	Yes
   IV: C₆H₅I	KI alone, no copper needed	No
   V: C₆H₅At	Not a Sandmeyer product; astatine is radioactive and rare	No

2. So. Only chlorobenzene and bromobenzene come from Sandmeyer's reaction, so the answer is 2.

The common slip: 3. Counting iodobenzene as a Sandmeyer product gives 3. It is made with KI alone, with no copper(I) salt, so it is not Sandmeyer.

The idea to keep. From a diazonium salt: Cl and Br need CuX (Sandmeyer), I needs only KI, and F needs HBF₄ then heat (Balz–Schiemann). No copper, no Sandmeyer.

Answer from the NTA final answer key. Solution by PrepWiser.