Question

Question: Explain the following statement. Although $\mathbf{HC}\mathbf{\equiv }{\mathbf{C}}^{\mathbf{-}}$is more stable than${\mathbf{H}}_{\mathbf{2}}\mathbf{C}\mathbf{=}{\mathbf{C}}^{\mathbf{-}}$,$\mathbf{HC}\mathbf{\equiv }{\mathbf{C}}^{\mathbf{+}}$is less stable than ${\mathbf{H}}_{\mathbf{2}}\mathbf{C}\mathbf{=}{\mathbf{C}}^{\mathbf{+}}$.

Short answer

Answer

Among the given carbanions, the $\mathrm{HC}\equiv {\mathrm{C}}^{-}$is more stable than ${\mathrm{H}}_2\mathrm{C}={\mathrm{C}}^{-}$. Among the given carbocations, ${\mathrm{H}}_2\mathrm{C}={\mathrm{C}}^{+}$the ion is more stable than the$\mathrm{HC}\equiv {\mathrm{C}}^{+}$ion.

Step-by-step solution

Step-by-Step SolutionStep 1: Stability of the carbanions and carbocations

The carbanion in which the lone pair of electrons on a carbon atom is present in the orbital with a greater s character is most stable.

On the other hand, the carbocation in which the positive charge is due to the vacant orbital with less percentage of s character is most stable.

Formal charges

The negatively charged ion in which the carbon atom has a formal negative charge and is bonded to three other atoms or groups.

The positively charged ion in which the carbon atom has a formal positive charge and is bonded to three other atoms or groups.

Stability of the given carbanions and carbocations

Among the given carbanions, the $\mathrm{HC}\equiv {\mathrm{C}}^{-}$is more stable than .

The carbon atom with a negative charge in the $\mathrm{HC}\equiv {\mathrm{C}}^{-}$ion is sp hybridized and has the hybrid orbitals that have about 50% s character.

In the case of${\mathrm{H}}_2\mathrm{C}={\mathrm{C}}^{-}$ ion, the carbon atom having the negative charge is hybridized and has about 33.33% character. As a result, the$\mathrm{HC}\equiv {\mathrm{C}}^{-}$ ion is more stable.

Among the given carbocations, therole="math" localid="1648979664760" ${\mathrm{H}}_2\mathrm{C}\equiv {\mathrm{CH}}^{+}$ ion is more stable than$\mathrm{HC}\equiv {\mathrm{C}}^{+}$ the ion. This is because in , role="math" localid="1648979670575" ${\mathrm{H}}_2\mathrm{C}\equiv {\mathrm{CH}}^{+}$the carbon atom with a positive charge is sp hybridized, and the p orbital is vacant.

On the other hand, in$\mathrm{HC}\equiv {\mathrm{C}}^{+}$ ion, the carbon atom is sp hybridized, and the vacant orbital has a 50% s character, making it highly unstable.