Step 1: Understanding the Concept:
The Sandmeyer reaction is a prominent chemical reaction utilized to synthesize aryl halides or aryl nitriles from reactive aryl diazonium salts. The critical distinguishing feature of this specific reaction is the necessary use of cuprous ($\text{Cu}^{\text{I}}$) salts as reagents/catalysts.
Step 2: Key Formula or Approach:
Approach: Identify the characteristic copper-based salt reagents used specifically in the Sandmeyer reaction and distinguish them from variants like the Gattermann reaction.
Step 3: Detailed Explanation:
Let's analyze the given reagent options:
- (A) CuCN/KCN: This is a classic, defining reagent for the Sandmeyer reaction. When a diazonium salt is treated with cuprous cyanide ($\text{CuCN}$) dissolved in potassium cyanide ($\text{KCN}$), the diazonium group is nucleophilically replaced by a cyano ($-\text{CN}$) group, forming an aryl nitrile. Similarly, the use of $\text{CuCl/HCl}$ and $\text{CuBr/HBr}$ defines the Sandmeyer reaction for introducing $-\text{Cl}$ and $-\text{Br}$ respectively.
- (B) Cu (Powder) /HBr: When finely divided copper powder is used with a halogen acid ($\text{HBr}$ or $\text{HCl}$) instead of a cuprous salt to convert diazonium salts to aryl halides, the reaction is known as the Gattermann reaction, which is a modified variation of the Sandmeyer reaction.
- (C) $\text{H_3\text{PO}_3$:} Usually, hypophosphorous acid ($\text{H}_3\text{PO}_2$) is used as a mild reducing agent to replace the diazonium group with a hydrogen atom ($-\text{H}$), converting the diazonium salt back to a simple arene (deamination).
- (D) $\text{CH_3\text{CH}_2\text{OH}$:} Ethanol is also used as a mild reducing agent for deamination, replacing the diazonium group with hydrogen while being oxidized to acetaldehyde.
Step 4: Final Answer:
CuCN/KCN is the correct reagent for the Sandmeyer reaction.