Question

Consider the following carbanions: (1) \(\mathrm{CH}_{3}-\stackrel{\ominus}{\mathrm{C}} \mathrm{H}_{2}\) (2) \(\mathrm{CH}_{3}=\stackrel{\ominus}{\mathrm{C}} \mathrm{H}\) (3) \(\mathrm{CH} \equiv \overrightarrow{\mathrm{C}}\) Correct order of stability of these carbanions in decreasing order is (a) \(1>2>3\) (b) \(2>1>3\) (c) \(3>2>1\) (d) \(3>1>2\)

Short answer

The correct order of stability is (d) 3 > 1 > 2.

Step-by-step solution

Understand the Structure of Each Carbanion

Carbanions are carbon ions carrying a negative charge. In problem configuration:1. (1) \(\mathrm{CH}_{3}-\stackrel{\ominus}{\mathrm{C}} \mathrm{H}_{2}\) is a primary carbanion.2. (2) \(\mathrm{CH}_{3}=\stackrel{\ominus}{\mathrm{C}} \mathrm{H}\) forms a vinylic carbanion.3. (3) \(\mathrm{CH} \equiv \overrightarrow{\mathrm{C}}\) represents an acetylenic or alkynic carbanion.Understanding their structures helps in assessing their stability based on the hybridization and resonance effects.

Analyze Hybridization  Impact on Stability

Stability of carbanions is largely determined by the hybridization of the carbon bearing the negative charge. The more s-character in the orbital, the closer electrons are held to the nucleus, which increases stability. 1. (1) The carbanion has an sp3 hybridized carbon. 2. (2) The carbanion has sp2 hybridization. 3. (3) The sp hybridized carbon has 50% s character, making it potentially more stable than the others.

Consider Resonance and Inductive Effects

Examine if any resonance or inductive effects stabilize the carbanions. 1. No resonance is possible in either the primary carbanion or the acetylenic carbanion. 2. The vinylic carbanion (2) is less stable due to the lack of resonance and because sp2 hybridization is less stable than sp hybridization.

Rank the Stability of Carbanions

Based on the analysis of the hybridization and resonance, the order of stability in decreasing manner is determined. 1. (3) sp hybridized acetylenic carbanion has the greatest stability due to high s-character. 2. (1) sp3 hybridized primary carbanion is more stable than a vinylic carbanion due to less electron repulsion and localized charge. 3. (2) Vinylic carbanion is the least stable due to its sp2 hybridized form without resonance support. Correct order: (3) > (1) > (2)

Key concepts

Hybridization

Hybridization refers to the mixing of atomic orbitals in an atom to form new orbitals of equivalent energy. This concept is pivotal in understanding the structure of the carbanions in question because the hybridization affects the electron distribution around the carbon atom. For the carbanions given:

• Carbanion (1) is sp3 hybridized, meaning the carbon atom has 25% s-character and 75% p-character. This results in electrons being held further away from the nucleus, typically leading to lower stability.
• Carbanion (2) is sp2 hybridized, with 33% s-character and 67% p-character, offering slightly more s-character than sp3, which typically translates to better stability than sp3 hybridized carbanions.
• Carbanion (3) is sp hybridized, providing 50% s-character, offering the greatest electron density near the nucleus and typically the highest stability among these types.

The hybridization impacts how tightly or loosely the electrons are held, which significantly affects the carbanion's stability.

Resonance Effects

Resonance involves the delocalization of electrons across different atoms, contributing to increased stability in some cases. In resonance-stabilized structures, the negative charge can be spread across multiple atoms, reducing electron repulsion at a single site.

• Carbanion (1) does not have any resonance stabilization possibilities due to its structure.
• Carbanion (2) also lacks resonance stabilization. Despite its vinylic nature, there is no viable resonance pathway, impacting its overall stability.
• Carbanion (3) does not possess resonance effects as well, however, its superior stability stems from its hybridization rather than delocalized electrons.

Comparatively, the lack of resonance effects in these types means stability primarily depends on other factors like hybridization in this exercise.

S-character

The s-character refers to the proportion of the s orbital in the hybrid orbitals of a carbon atom. The higher the s-character, the more closely electrons are held to the nucleus. This higher attraction to the nucleus increases the stability of the carbanion.

• Carbanion (1), with sp3 hybridization, has 25% s-character, which is less effective in stabilizing negative charges.
• Carbanion (2) has 33% s-character because of its sp2 hybridization, offering moderate stabilization effects.
• Carbanion (3) displays the greatest s-character at 50% due to its sp hybridization, providing significant stability by holding electrons closely to the nucleus.

Thus, stability tends to increase with an increase in s-character, as more highly concentrated electron density near the nucleus mitigates charge instability.

Inductive Effects

Inductive effects involve the polarization of sigma bonds, usually due to electronegativity differences, which helps in the stabilization of charged species. The negative charge on a carbanion can be affected by the electron-donating or electron-withdrawing nature of adjacent groups.

• For carbanion (1), there aren't significant adjacent electronegative atoms to influence stability significantly through inductive effects.
• Carbanion (2) also lacks notable inductive stabilization factors, contributing, in part, to its lower stability.
• Finally, carbanion (3), although with minimal inductive influences, gains its main stability from high s-character rather than inductive effects.

When considering these effects, the inductive factor plays a lesser role in these carbanions, especially since their primary stabilizing force is based on hybridization.