Question

Consider the following carbocations:
1. \[{\text{C}}{{\text{H}}_{\text{3}}} - {}^ + {\text{C}}{{\text{H}}_{\text{2}}}\]
2. \[{\text{C}}{{\text{H}}_{\text{2}}} = {}^ + {\text{CH}}\]
3. \[{\text{C}}{{\text{H}}_2} = {\text{CH}} - {}^ + {\text{C}}{{\text{H}}_{\text{2}}}\]
4. \[{{\text{C}}_{\text{6}}}{{\text{H}}_{\text{5}}} - {}^ + {\text{C}}{{\text{H}}_{\text{2}}}\]
Stability of these carbocations in decreasing order is
A) (4)>(3)>(1)>(2)
B) (4)>(3)>(2)>(1)
C) (3)>(4)>(2)>(1)
D) (3)>(4)>(1)>(2)

Answer 1

Hint: We know that carbocation is a positively charged carbon atom. The carbocation which has resonating structures is more stable. Higher the resonance higher is the stability of the carbocation. As the number of alpha hydrogen atoms increases, the stability of carbocation increases.

Complete step-by-step answer:
We are given four carbocations as follows:
\[{\text{C}}{{\text{H}}_{\text{3}}} - {}^ + {\text{C}}{{\text{H}}_{\text{2}}}\]
\[{\text{C}}{{\text{H}}_{\text{2}}} = {}^ + {\text{CH}}\]
\[{\text{C}}{{\text{H}}_2} = {\text{CH}} - {}^ + {\text{C}}{{\text{H}}_{\text{2}}}\]
\[{{\text{C}}_{\text{6}}}{{\text{H}}_{\text{5}}} - {}^ + {\text{C}}{{\text{H}}_{\text{2}}}\]

The delocalization of electrons in the structure of the molecule is known as resonance. From the given carbocations, carbocation (3) and carbocation (4) have resonating structures.

As the number of covalent bonds increases, the resonance in a molecule increases. The number of covalent bonds in carbocation (4) is more than that in carbocation (3). Thus, carbocation (4) is more stable than carbocation (3).

The first carbon atom that attaches to a functional group is known as an alpha carbon. The hydrogen atoms attached to an alpha carbon are known as alpha hydrogens. As the number of alpha hydrogens increases the stability of carbocation increases. We can decide the stability of carbocation (1) and carbocation (2) based on the number of alpha hydrogens.

Carbocation (1) has 3 alpha hydrogen atoms while carbocation (2) has 2 alpha hydrogen atoms.
Thus, carbocation (1) is more stable than carbocation (2). Thus, the stability of these carbocations in decreasing order is: (4)>(3)>(1)>(2).

Thus, the correct answer is option (A) (4)>(3)>(1)>(2).

Note: When a carbon atom carrying a positive charge is attached to only one alkyl group it is known as a primary carbocation. When a carbon atom carrying a positive charge is attached to two other alkyl groups it is known as a secondary carbocation. When a carbon atom carrying a positive charge is attached to three other alkyl groups it is known as a tertiary carbocation. A tertiary carbocation is more stable than the secondary carbocation which is more stable than the primary carbocation.