Question

The reaction intermediate in \(\mathrm{E}_{1 \mathrm{Cb}}\) is (a) Carbonium ion (b) Carbon free radical (c) Carbanion (d) Carbene

Short answer

(c) Carbanion

Step-by-step solution

Understanding E1CB Reaction

The E1cB elimination reaction is a specific type of elimination reaction where a carbanion intermediate is formed. E1cB stands for Elimination Unimolecular conjugate Base. Understanding the mechanism is crucial to identifying the intermediate.

Identifying Reaction Intermediate

In the E1cB mechanism, a base abstracts a proton and a carbanion intermediate is formed, which then eliminates a leaving group. This indicates that the reaction intermediate for E1cB reactions is a carbanion.

Eliminating Incorrect Options

Options (a), (b), and (d) can be eliminated as incorrect since (a) refers to a positively charged carbon ion (carbonium ion), (b) is a neutral radical (carbon free radical), and (d) is a neutral species with a divalent carbon (carbene), none of which form in E1cB reactions.

Key concepts

Carbanion: The Key Intermediate

In the \( \text{E}_{1 \text{cB}} \) elimination reaction, the star player is the carbanion. A carbanion is a negatively charged carbon atom, denoted by the presence of an extra pair of electrons. This intermediate is pivotal because it sets the stage for the rest of the reaction to proceed effectively. How is it formed? Well, it all starts when a base comes into the scene and snatches a proton from a carbon atom neighboring the leaving group.
This action leaves behind this negatively charged carbanion. It's worth noting because the stability of a carbanion can significantly influence how fast and efficiently the reaction can progress.

• The carbanion has a lone pair of electrons.
• It acts as a reactive intermediate, not a stable compound on its own.
• This aspect is crucial for the mechanism to occur in the specific \( \text{E}_{1 \text{cB}} \) pathway.

Understanding the Reaction Mechanism

Exploring the \( \text{E}_{1 \text{cB}} \) reaction mechanism involves diving into the sequence of steps that guide the transformation from reactants to products. This elimination reaction type is unique and distinct from other common elimination mechanisms like \( \text{E}_{1} \) and \( \text{E}_{2} \). The mechanism kicks off when a base abstracts a hydrogen, typically from a carbon adjacent to a leaving group.
The creation of a carbanion is the hallmark of this mechanism, serving as a temporary stepping stone before the final elimination step occurs. Once the carbanion is formed, it paves the way for the second crucial step: the departure of the leaving group, which results in the formation of a double bond in the final product.

• The mechanism unfolds in two main stages: proton removal and leaving group elimination.
• This reaction is usually facilitated by a strong base.
• The formation of a negatively charged intermediate (the carbanion) offers a window for the reaction's progression.

Exploring the Elimination Reaction

Elimination reactions are an integral category in organic chemistry where elements of the starting material are "eliminated" to form a double bond, producing unsaturated compounds. Within this group, the \( \text{E}_{1 \text{cB}} \) stands out, not as common as some of its peers but certainly crucial in specific contexts. The presence of a strong base and poor leaving groups typically characterizes these reactions.
Unlike more classic elimination types, \( \text{E}_{1 \text{cB}} \) depends heavily not on the leaving group's propensity to exit immediately, but on the stability of the carbanion intermediate. This allows certain difficult eliminations to proceed where other mechanisms might stall.

• In an elimination reaction, the removal of atoms or groups results in the formation of a new double bond.
• In \( \text{E}_{1 \text{cB}} \), the formation of the intermediate is a crucial step.
• This mechanism can also be useful in understanding difficult elimination scenarios.