Question

Which one of the following pairs is correctly matched? (a) Saytzeff rule, least substituted alkene (b) Hoffmann rule, most substituted alkene (c) \(\mathrm{E}_{1}\) cb reaction, Hoffmann elimination (d) \(\mathrm{E}_{1}\) reaction, Hoffmann elimination

Short answer

None of the given pairs are correctly matched.

Step-by-step solution

Understand the Terms

Before matching the terms, we need to understand what each one means. Saytzeff's rule states that in an elimination reaction, the more substituted alkene is generally the major product. The Hoffman rule is the opposite, where the less substituted alkene is the major product. An E1 reaction typically involves the formation of a carbocation intermediate, leading to more substituted products. Meanwhile, an \(\mathrm{E}_{1}\)cb reaction (Elimination, unimolecular, conjugate base) involves a carbanion intermediate and is less common.

Evaluate Option (a)

Option (a) states Saytzeff rule, least substituted alkene. According to Saytzeff's rule, the major product of an elimination reaction is the more substituted alkene. Therefore, option (a) is incorrectly matched.

Evaluate Option (b)

Option (b) states Hoffmann rule, most substituted alkene. The Hoffmann rule actually predicts the formation of the least substituted alkene; hence option (b) is incorrectly matched.

Evaluate Option (c)

Option (c) states \(\mathrm{E}_{1}\) cb reaction, Hoffmann elimination. The \(\mathrm{E}_{1}\)cb reaction involves a carbanion and does not follow Hoffmann elimination, which yields the least substituted alkene through a different mechanism.

Evaluate Option (d)

Option (d) states \(\mathrm{E}_{1}\) reaction, Hoffmann elimination. An \(\mathrm{E}_{1}\) reaction proceeds through a carbocation intermediate and normally obeys the Saytzeff rule, not Hoffmann, which incorrectly matches option (d).

Conclusion

Since none of the options seem to be correctly matching with the rules or types of eliminations mentioned, revise or verify if there's an error in the matching list.

Key concepts

Elimination Reactions

Elimination reactions are a fundamental type of reaction in organic chemistry where elements of a molecule are removed or "eliminated," typically resulting in the formation of a double bond. These reactions play a crucial role in synthesizing alkenes.

There are various types of elimination reactions, each with its unique mechanism. The type of mechanism employed can greatly affect the outcome of the reaction. This is particularly important when determining the structure of the resulting product (often an alkene). Common elimination mechanisms include E1, E2, and even less common ones like E1cb.

Generally, elimination reactions involve the removal of atoms or groups from carbon atoms, enhancing the reactivity and complexity of organic molecules. Understanding the underlying mechanism is critical as it allows chemists to predict the final product's stability and structure.

Saytzeff Rule

The Saytzeff rule, sometimes spelled as Zaitsev, is a principle used to predict the major product in elimination reactions. According to this rule, the product that forms the most substituted alkene, meaning the alkene with more alkyl groups attached to the double-bonded carbon atoms, is usually favored.

This is because more substituted alkenes tend to be more stable due to hyperconjugation and the inductive effect, which provide stabilization through electron donation.

• The Saytzeff rule is generally applicable in thermal or acidic conditions.
• It is especially relevant for E1 reactions, where a carbocation intermediate allows for rearrangements leading to more substituted alkenes.

An understanding of this rule is paramount for predicting the outcome of alkene-forming elimination reactions in organic synthesis.

Hoffmann Rule

In contrast to the Saytzeff rule, the Hoffmann rule governs the formation of the least substituted alkene during an elimination reaction. This scenario typically arises when steric hindrance or specific reaction conditions favor the formation of the less stabilized but more accessible alkene.

The Hoffmann rule typically applies when bulky bases are used in the reaction, as these bases preferentially remove the least hindered hydrogen.

• It often leads to the kinetic product rather than the thermodynamically most stable product.
• This rule is applied more commonly in E2 reactions where a strong, bulky base is present.

Understanding the Hoffmann rule is essential for designing reactions where specific alkene products are desired.

E1 Reaction

An E1 reaction, or unimolecular elimination, is characterized by a two-step mechanism. It first involves the formation of a carbocation intermediate, followed by the elimination step where an alkene is formed.

This type of reaction is typical for substrates that can easily form stable carbocations, such as tertiary alkyl halides or alcohols. Due to the carbocation's presence, there is potential for rearrangement, resulting in a Saytzeff product.

• E1 reactions occur under acidic conditions and often coincide with substitution reactions (SN1).
• The slow rate-determining step is the formation of the carbocation.

Despite its slower rate compared to bimolecular eliminations like E2, understanding E1 is crucial for synthesizing various organic compounds.

E1cb Reaction

The E1cb reaction, standing for Elimination, unimolecular, conjugate base, distinguishes itself by involving a carbanion intermediate rather than a carbocation, as seen in E1 reactions.

This mechanism is more likely to occur in molecules where the hydrogen atom to be abstracted is adjacent to a strong electron-withdrawing group, which stabilizes the carbanion.

• E1cb involves three primary steps: base-induced deprotonation, formation of the carbanion, and elimination of the leaving group to form the alkene.
• The E1cb mechanism ensures the formation of alkenes generally in cases where the substrate provides suitable stabilization for the creation of a conjugate base.

The E1cb mechanism does not typically follow the Hoffmann elimination path, making it unique among elimination reactions. Understanding this reaction type is key in specific synthetic applications where carbanion formation is viable.