Question

Most stable carbanion is (a) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}-\) (b) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2}\) (c) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CH}^{-}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2}-\)

Short answer

The most stable carbanion is (b) \(C_{6}H_{5}CH_{2}^{-}\).

Step-by-step solution

Understanding Carbanion Stability

Carbanions are carbon-based ions with a negative charge. Stability of a carbanion depends on several factors, with inductive effects, resonance, and hyperconjugation being the most significant. A carbanion is generally stabilized if its negative charge is distributed over a larger area or is shared by more than one atom through resonance.

Assessing Methyl Carbanion

The carbanion \((CH_{3})_{3}C^{-}\) has three methyl groups. This carbanion is not stabilized by resonance, and its electron density is concentrated on the carbon atom, making it less stable due to steric crowding from the three methyl groups.

Examining Benzyl Carbanion

The carbanion \(C_{6}H_{5}CH_{2}^{-}\) is a benzyl carbanion. It is highly stabilized due to resonance. The negative charge can resonate into the phenyl ring, spreading the charge across multiple atoms, increasing the stability significantly.

Evaluating Isopropyl Carbanion

The carbanion \((CH_{3})_{2}CH^{-}\) is isopropyl. This ion is slightly stabilized by hyperconjugation from adjacent methyl groups, but lacks resonance stabilization, making it less stable than benzyl but more stable than trimethyl carbanion.

Considering Ethyl Carbanion

The carbanion \(CH_{3}CH_{2}^{-}\) is an ethyl carbanion. Like the isopropyl carbanion, it relies on hyperconjugation for stability, with no resonance structures to distribute the negative charge, ranking its stability similarly to the isopropyl carbanion.

Conclusion on Stability

Considering the factors affecting carbanion stability, the benzyl carbanion (option b) has the most effective resonance stabilization, which offers the best distribution of the negative charge. This significantly enhances its stability compared to the other options.

Key concepts

Inductive Effects

The inductive effect is a fundamental principle in understanding carbanion stability. It refers to the shifting of electron density along a chain of atoms within a molecule, caused by differences in electronegativity. Carbanions, being negatively charged, are sensitively affected by these shifts.

In a molecule, when an electronegative atom is attached to the carbon, it tends to pull electron density towards itself through sigma bonds. This effect can stabilize or destabilize a carbanion depending on the context:

• In the case where an electronegative atom is close to the carbanion center, it might stabilize the negative charge by pulling it away.
• Conversely, if the carbanion is away from such atoms, any pushing of electron density towards it can destabilize the ion.

Therefore, understanding the surrounding atoms' electronegativity helps predict which structures might be more or less stable.
However, inductive effects alone are not enough to determine carbanion stability. They must be considered alongside other factors, such as resonance and hyperconjugation, to fully understand the stability landscape.

Resonance

Resonance plays a critical role in stabilizing carbanions. It involves the delocalization of electrons across a molecule, allowing a more even distribution of the negative charge. This can significantly enhance the stability of a carbanion.

For example, the benzyl carbanion ( C_{6}H_{5}CH_{2}^{-} ) benefits greatly from resonance. The negative charge on the carbanion can be shared across the phenyl ring, distributing the charge over several atoms. This delocalization reduces the potential energy associated with holding too much negative charge in one place, making the molecule more stable.
Key points about resonance:

• It allows for the sharing and spreading of electron density over multiple atoms.
• Resonance structures are often depicted using double-headed arrows to show the potential positions of electrons.
• The more resonance forms available, the more stable the carbanion generally is.

When analyzing carbanions like the benzyl carbanion, resonance provides a dominant form of stabilization, often outweighing other effects such as inductive and hyperconjugative effects.

Hyperconjugation

Hyperconjugation is markedly different from inductive effects and resonance. It involves the interaction of electrons in sigma bonds (typically C-H or C-C bonds) with an adjacent empty or partially filled p-orbital or a pi-orbital. This interaction helps to spread out electron density, albeit more subtly than resonance.

In carbanions such as (CH_{3})_{2}CH^{-} (isopropyl carbanion) and CH_{3}CH_{2}^{-} (ethyl carbanion), hyperconjugation occurs between the negative charge on the carbanion and neighboring C-H bonds.
Here’s how it stabilizes carbanions:

• The electron density from C-H bonds overlaps with the carbanion’s filled orbital.
• This overlap allows for a slight spreading of negative charge, adding stability.
• Although weaker than resonance, multiple hyperconjugative interactions can provide a meaningful stabilization effect.

Hyperconjugation is often cited as a reason why certain alkyl-substituted carbanions exhibit more stability compared to those without such interactions. Therefore, when assessing carbanion stability, hyperconjugation can provide fine adjustments to predictions and is vital for a comprehensive understanding.