Q42 / 180 · COMEDK 2021 · Chemistry
Which of the following is most stable?
- A$\mathrm{CH}_{2} \mathrm{~N}_{2}^{+} X^{-}$
- Correct answerB$\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{~N}_{2}^{+} X^{-}$
- C$\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{~N}_{2}^{+} X^{-}$
- D$\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{~N}_{2}^{+} X^{-}$
Solution
Stability of diazonium salts ($R-N_2^+X^-$) depends on the stability of the carbocation formed after loss of $N_2$.
$CH_2N_2^+X^-$ and $CH_3CH_2N_2^+X^-$: aliphatic $\Rightarrow$ resulting carbocations are highly unstable. These decompose rapidly.
$C_6H_5CH_2N_2^+X^-$: benzyl carbocation is resonance stabilized, but still relatively reactive.
$C_6H_5N_2^+X^-$: the diazonium group is directly attached to the aromatic ring. The lone pair on the nitrogen delocalizes into the ring $\Rightarrow$ the diazonium cation itself is resonance stabilized. This makes benzene diazonium salt the most stable, existing in solution at $0-5^\circ C$.
Correct option: (2)
$CH_2N_2^+X^-$ and $CH_3CH_2N_2^+X^-$: aliphatic $\Rightarrow$ resulting carbocations are highly unstable. These decompose rapidly.
$C_6H_5CH_2N_2^+X^-$: benzyl carbocation is resonance stabilized, but still relatively reactive.
$C_6H_5N_2^+X^-$: the diazonium group is directly attached to the aromatic ring. The lone pair on the nitrogen delocalizes into the ring $\Rightarrow$ the diazonium cation itself is resonance stabilized. This makes benzene diazonium salt the most stable, existing in solution at $0-5^\circ C$.
Correct option: (2)
Hence, the correct answer is option: 'B'.