Question

Most stable carbonium ion is

• A. ${{\overset{+}{\mathop{\text{C}}}\,}_{2}}{{H}_{5}}$
• B. ${{(C{{H}_{3}})}_{3}}\overset{+}{\mathop{C}}\,$
• C. $({{C}_{6}}{{H}_{5}})\overset{+}{\mathop{C}}\,$
• D. ${{C}_{6}}{{H}_{5}}\overset{+}{\mathop{C}}\,{{H}_{2}}$

Answer 1

Answer: (C)

$({{C}_{6}}{{H}_{5}})\overset{+}{\mathop{C}}\,$

Answer 2

${{\overset{+}{\mathop{C}}\,}_{2}}{{H}_{5}}$ and $C{{H}_{3}}-\underset{C{{H}_{3}}}{\overset{C{{H}_{3}}}{\mathop{{{\underset{|}{\overset{|}{\mathop{C}}}\,}^{\oplus }}}}}\,$ are stabilised by Hyperconjugation In ${{\text{C}}_{\text{6}}}{{\text{H}}_{\text{5}}}\overset{\oplus }{\mathop{\text{C}}}\,{{\text{H}}_{\,\text{2}}}$ benzyl group is resonance stabilised. In the triphenyl methyl carbonium ion, the n electrons of all three benzene rings are delocalized with the vacant p-orbital of the central carbon atom. So, it is resonance stabilized. Therefore, it is the most stable of the given carbonium ions. The more the number of resonatic structures more will be stability.

Concept: Stability of Alkyl Cations

The stability can be explained on the basis of the inductive effect.

• The alkyl groups show an electron-releasing inductive effect (+I effect) in the direction of the +ve charge in carbonation.
• Through the +I effect, an alkyl group thus disperses the +ve charge, thereby stabilizing the carbonation.
• The greater the number of alkyl groups attached to the +ve carbon, the greater would be the dispersal of charge.

Thus, greater would be the stability of the carbocation.