Question

Which of the following is the most stable carbocation intermediate?
A.$\mathop {\text{C}}\limits^ \oplus {{\text{H}}_2} - {\text{N}}{{\text{H}}_2}$
B.$\mathop {\text{C}}\limits^ \oplus {{\text{H}}_2} - {\text{OH}}$
C.$\mathop {\text{C}}\limits^ \oplus {{\text{H}}_2} - {\text{Cl}}$
D.$\mathop {\text{C}}\limits^ \oplus {{\text{H}}_2} - {\text{C}}{{\text{H}}_3}$

Answer 1

Carbocation is an electron deficient species, if any electron donating group is present near the carbon then it will increase the electron density on the carbon and hence will increase the stability of the carbocation. Carbocation is most stabilized by mesomeric effect.

Complete step by step solution:
Carbocation which is stabilized by resonance or mesomeric effect will be more stable than the carbocation stabilized by hyperconjugation or inductive effect. Inductive effect contributes least towards stability of carbocation where mesomeric effect contributes most towards the stability of carbocation.
Methyl is an electron donating group through inductive effect whereas chloride, hydroxide and amine all show positive mesomeric effect. Option D stands eliminated here because the methyl group will stabilise the least.
Mesomeric effect arises due to the presence of functional groups with a lone pair of electrons. Now since all three are mesomeric groups and have lone pairs of electrons so we will look at the tendency of their electron donation. The more electronegativity of the donating atom lesser will be its tendency to donate electrons. Among the nitrogen, oxygen and chlorine, nitrogen is least electronegative and oxygen is most electronegative. Hence nitrogen will easily donate its lone pair of electrons compared to the others because the electron holding capacity of nitrogen will be less. The maximum mesomeric effect will be shown by ${\text{N}}{{\text{H}}_2}$ group. So the most stable carbocation will be: $\mathop {\text{C}}\limits^ \oplus {{\text{H}}_2} - {\text{N}}{{\text{H}}_2}$

Hence, the correct option is A.

Note:
A carbocation has a 6 electron in its valence shell. It acts as an electrophile because of its electron definition nature. The hybridization of a carbon is ${\text{s}}{{\text{p}}^2}$ and the shape is trigonal planar.