Question

The most stable carbonium ion among the following is:
(A)- ${{\text{C}}_{\text{6}}}{{\text{H}}_{\text{5}}}{\text{ - C}}{{\text{H}}_{\text{2}}}{\text{ - }}\mathop {\text{C}}\limits^{\text{ + }} {{\text{H}}_{\text{2}}}$
(B)- ${\text{C}}{{\text{H}}_3}{\text{ - }}\mathop {\text{C}}\limits^{\text{ + }} {{\text{H}}_{\text{2}}}$
(C)- ${\left( {{{\text{C}}_{\text{6}}}{{\text{H}}_{\text{5}}}} \right)_3}{\text{ - }}\mathop {\text{C}}\limits^{\text{ + }} $
(D)- ${{\text{C}}_{\text{6}}}{{\text{H}}_{\text{5}}}{\text{ - }}\mathop {\text{C}}\limits^{\text{ + }} {{\text{H}}_{\text{2}}}$

Answer 1

Hint: Stability of carbonium ion increases as the no. of attach alkyl group on the carbonium carbon atom increases. Or in other words we can also say that on the addition of a group on the carbonium ion which increases positive charge on the carbon atom stability of that carbonium ion also increases.

Complete step by step answer:
The order of stability of carbonium ion is as follow:
\[{1^0} < {2^0} < {3^0}\]
In the above question following molecules are given and their stability is as follow:
-In option (A) ${{\text{C}}_{\text{6}}}{{\text{H}}_{\text{5}}}{\text{ - C}}{{\text{H}}_{\text{2}}}{\text{ - }}\mathop {\text{C}}\limits^{\text{ + }} {{\text{H}}_{\text{2}}}$ molecule is given, and in this molecule carbonium ion carbon is present in primary state because only one alkyl group is attach to the carbonium carbon. So, it has least stability.
-In option (B) ${\text{C}}{{\text{H}}_3}{\text{ - }}\mathop {\text{C}}\limits^{\text{ + }} {{\text{H}}_{\text{2}}}$ molecule is given, and in this molecule carbonium ion carbon is present in primary state because only one alkyl group is attach to the carbonium carbon. So, it has least stability.
-In option (C) ${\left( {{{\text{C}}_{\text{6}}}{{\text{H}}_{\text{5}}}} \right)_3}{\text{ - }}\mathop {\text{C}}\limits^{\text{ + }} $ molecule is given, and in this molecule carbonium ion carbon is present in tertiary state because three alkyl group is attach to the carbonium carbon. So, it shows the highest stability.
-In option (D) ${{\text{C}}_{\text{6}}}{{\text{H}}_{\text{5}}}{\text{ - }}\mathop {\text{C}}\limits^{\text{ + }} {{\text{H}}_{\text{2}}}$ molecule is given, and in this molecule carbonium ion carbon is present in primary state because three alkyl group is attached to the carbonium carbon. So, it shows the highest stability.
So, the most stable carbonium ion among the above given molecules is ${\left( {{{\text{C}}_{\text{6}}}{{\text{H}}_{\text{5}}}} \right)_3}{\text{ - }}\mathop {\text{C}}\limits^{\text{ + }} $.

Hence, option (C) is correct.

Note: Here some of you may count hydrogen atoms as the attached alkyl group, that will lead to give the wrong answer to the question. So, always make an expanded structure with showing bonds of given molecules.