CBSE Class 12 Chemistry 2023: question 28

Hints

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Step-by-step solution

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1. (a) Sandmeyer reaction: the diazonium group is replaced by Cl, Br or CN using the corresponding cuprous salt:
   ${\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{N}}_2^{+}\mathrm{C}{\mathrm{l}}^{-}\stackrel{\mathrm{C}{\mathrm{u}}_2\mathrm{C}{\mathrm{l}}_2/HCl}{\to }{\mathrm{C}}_6{\mathrm{H}}_5\mathrm{Cl}+{\mathrm{N}}_2$
   (similarly $\mathrm{C}{\mathrm{u}}_2\mathrm{B}{\mathrm{r}}_2/HBr$ gives ${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{Br}$; $CuCN/KCN$ gives ${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{CN}$).

   1 mark

2. (b) In aqueous solution basicity depends on the +I effect of alkyl groups and on solvation (H-bonding) of the protonated amine with water. +I effect alone would make the tertiary amine strongest.

   1 mark

3. $(\mathrm{C}{\mathrm{H}}_3{)}_2\mathrm{N}{\mathrm{H}}_2^{+}$ has two N–H hydrogens to form H-bonds with water and is better hydrated (stabilised) than $(\mathrm{C}{\mathrm{H}}_3{)}_3\mathrm{N}{\mathrm{H}}^{+}$, which has only one N–H and is crowded by three methyl groups (steric hindrance to solvation). The combined effect makes $(\mathrm{C}{\mathrm{H}}_3{)}_2\mathrm{NH}$ more basic.

   1 mark

Answer: (a) C₆H₅N₂⁺Cl⁻ + Cu₂Cl₂/HCl → C₆H₅Cl + N₂; (b) greater solvation (H-bonding) of (CH₃)₂NH₂⁺ with +I effect makes (CH₃)₂NH more basic

Revise the chapter

• Chapter: Amines (notes, formulas, where marks are lost and every past question)
• Topic: Basicity of Amines