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Question: Which of the following is most stable carbocation? (A) \(C{{H}_{3}}C{{H}_ {2}} ^ {+} \) (B) \(C...

Which of the following is most stable carbocation?
(A) CH3CH2+C{{H}_{3}}C{{H}_ {2}} ^ {+}
(B) CH3−C+=OC{{H}_{3}}-{{C}^ {+}} =O
(C) CH3−C+=NHC{{H}_{3}}-{{C}^ {+}} =NH
(D) CH3−C+=CH2C{{H}_{3}}-{{C}^ {+}} =C{{H}_ {2}}

Explanation

Solution

A carbocation is a molecule in which a carbon atom has a positive charge and three bonds. Carbocations Are Stabilized By Neighboring Carbon-Carbon Multiple Bonds. Carbocations adjacent to another carbon-carbon double or triple bond have special stability because of overlap between the empty p orbital of the carbocation with the p orbitals of the pi bond, allowing for charge to be shared between multiple atoms.

Complete step by step answer:
We have been provided with 4 carbocation: CH3CH2+C{{H}_{3}}C{{H}_{2}}^{+}, CH3−C+=OC{{H}_{3}}-{{C}^{+}}=O, CH3−C+=NHC{{H}_{3}}-{{C}^{+}}=NH and CH3−C+=CH2C{{H}_{3}}-{{C}^{+}}=C{{H}_{2}},
We need to tell which of them is the most stable carbocation;
So, for that:
A carbocation is an ion with a positively charged carbon atom.
The stability of carbocations increases as we go from primary to secondary to tertiary carbons. In other words, the neighbouring carbon pays the carbocation with electrons it steals from the hydrogens.
The stability of a carbocation depends on Resonance, Hyperconjugation, and Inductive effect.
The concept of resonance effect tells about the polarity induced in a molecule by the reaction between a lone pair of electrons and a pi bond. It also occurs by the interaction of 2 pi bonds in the adjacent atoms. Resonance in simple is the molecules with multiple Lewis structures.
Hyperconjugation refers to the delocalization of electrons with the participation of bonds of primarily σ\sigma -character.
The inductive effect is an effect regarding the transmission of unequal sharing of the bonding electron through a chain of atoms in a molecule, leading to a permanent dipole in a bond.
If the electron withdrawing groups like =NH,=O,=CH2=NH, =O, =C{{H}_ {2}} are directly bonded to the carbocation, they destabilize the carbocation. However, methyl groups increase the stability of carbonation due to its positive inductive effect. So, we can say that CH3CH2+C{{H}_{3}}C{{H}_ {2}} ^ {+} is the most stable carbocation.
Therefore, we can conclude that option (A) is correct.

Note: A tertiary carbocation forms the most quickly because it is the most stable. All carbocations are very reactive, so their relative reactivity doesn't matter much for the rate of a reaction.