Chapter 9 / 9.9 Chemical Reactions

🎯 Quick Overview

Benzenediazonium chloride shows up as a colourless crystal that dissolves in water. Keep it cold to stay safe—warm water quickly breaks it down, and dry crystals fall apart even faster. Benzenediazonium fluoroborate, on the other hand, won’t dissolve in water and stays steady at room temperature. 🌡️:contentReference[oaicite:0]{index=0}

⚡ Two Big Reaction Paths

A. Reactions that kick out nitrogen (N₂)

1. Swap for Cl^–, Br^– or CN^– (Sandmeyer)
   \( \text{ArN}_2^+\text{Cl}^- + \text{CuCl} \rightarrow \text{ArCl} + \text{N}_2 \uparrow \)
   The copper(I) ion helps these swaps happen fast and clean. Sandmeyer wins on yield! 🔄:contentReference[oaicite:1]{index=1}
2. Add Cl or Br with an acid blast (Gattermann)
   Same idea as Sandmeyer, but you mix a halogen acid (HCl or HBr) with copper powder. Yields dip a bit versus Sandmeyer. 😅:contentReference[oaicite:2]{index=2}
3. Drop in I^–
   Potassium iodide slots iodine right into the ring:
   \( \text{ArN}_2^+\text{Cl}^- + \text{KI} \rightarrow \text{ArI} + \text{KCl} + \text{N}_2 \uparrow \) 🧂:contentReference[oaicite:3]{index=3}
4. Slide in F^–
   First make \(\text{ArN}_2^+\text{BF}_4^-\). Heat it and boom—aryl fluoride forms, nitrogen pops out, and BF₃ escapes. 🔥:contentReference[oaicite:4]{index=4}
5. Reduce to plain arene (H)
   Hypophosphorous acid (H₃PO₂) or ethanol strips away the diazo group, leaving just the arene plus harmless by-products:
   \( \text{ArN}_2^+\text{Cl}^- + \text{H}_3\text{PO}_2 \rightarrow \text{ArH} + \text{H}_3\text{PO}_3 + \text{N}_2 \uparrow \) ✂️:contentReference[oaicite:5]{index=5}
6. Hydrolyse to phenol (OH)
   Warm the salt to about 283 K and water jumps in to give phenol plus bubbling N₂. 💧:contentReference[oaicite:6]{index=6}
7. Switch to –NO₂
   Heat the fluoroborate with aqueous NaNO₂ and copper to plug a nitro group into the ring. 🚀:contentReference[oaicite:7]{index=7}

B. Reactions that keep the diazo group

Coupling reactions → azo dyes 🎨
The diazo partner (\(\text{ArN}_2^+\text{Cl}^-\)) teams with an activated ring (like phenol or aniline) at its para spot. You get bright azo compounds such as p-hydroxyazobenzene or p-aminoazobenzene:
\( \text{ArN}_2^+\text{Cl}^- + \text{PhOH} \rightarrow \text{p-HO–Ph–N=N–Ar} + \text{HCl} \)
They show lovely colors thanks to an extended conjugation line across \(-\mathrm{N = N}-\). 🎉:contentReference[oaicite:8]{index=8}

🌟 Why These Reactions Matter

• They let you pop in –F, –Cl, –Br, –I, –CN, –OH, or –NO₂ even when direct methods fail.
• Aryl fluorides and iodides are tough to make directly–diazonium tricks make them easy.
• Cyanobenzene? Skip messy nucleophilic swaps—go through diazonium!
• Azo dyes splash color into textiles, indicators, and biological stains.

:contentReference[oaicite:9]{index=9}

🔑 Important Concepts for NEET

1. Sandmeyer reaction: swift insertion of Cl, Br, or CN into aromatic rings.
2. Gattermann reaction: alternative halogen insertion route (Cl or Br).
3. Coupling reaction: crafting colorful azo dyes—watch for para-directed attack.
4. Hydrolysis of diazonium salts to phenols—an easy phenol prep.
5. Using diazonium salts to access hard-to-reach aryl fluorides and iodides.