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

(c) E1 cb reaction, Hoffmann elimination is correct.

Step-by-step solution

Understanding Saytzeff Rule

The Saytzeff rule, also known as Zaitsev's rule, states that in an elimination reaction, the more substituted alkene (with more alkyl groups attached to the double-bonded carbons) is generally formed as the major product. This rule predicts the formation of the most stable alkene.

Understanding Hoffmann Rule

The Hoffmann rule is often used to describe situations where the least substituted alkene is the major product of an elimination reaction. This typically happens in situations involving bulky bases or when steric hindrance prevents the formation of more substituted alkenes.

Exploring E1 cb Reaction

An E1 cb (elimination unimolecular conjugate base) reaction is a method of producing alkenes where the leaving group departs and a base abstracts a proton from an adjacent carbon. It usually leads to a less substituted alkene, following Hoffmann-style elimination especially in cases of conjugated systems.

Exploring E1 Reaction

In an E1 (elimination unimolecular) reaction, the substrate first loses a leaving group, forming a carbocation intermediate. The next step is the departure of a hydrogen atom from the beta position to form an alkene. This typically results in the more substituted alkaline, following Saytzeff's rule.

Analyzing the Options

- Option (a) Saytzeff rule, least substituted alkene: Incorrect. Saytzeff rule results in the more substituted alkene. - Option (b) Hoffmann rule, most substituted alkene: Incorrect. Hoffmann rule results in the least substituted alkene. - Option (c) E1 cb reaction, Hoffmann elimination: Correct. E1 cb reactions often produce the less substituted alkene. - Option (d) E1 reaction, Hoffmann elimination: Incorrect. E1 reactions typically follow Saytzeff, leading to more substituted alkenes.

Key concepts

Saytzeff Rule

The Saytzeff rule, also known as Zaitsev's rule, is a guiding principle in organic chemistry that predicts the outcome of elimination reactions. When a molecule undergoes dehydrohalogenation, the elimination process can lead to the formation of different alkenes. According to Saytzeff's rule, the alkene that forms with the greatest number of substituents on the double-bonded carbons will predominate. This is because more substituted alkenes are generally more stable due to hyperconjugation and the electron-releasing effect of alkyl groups. These effects increase the stability of the olefin. In summary, Saytzeff’s rule is central in predicting that the major product of an elimination reaction will be the more substituted alkene.

Hoffmann Rule

In contrast to Saytzeff's rule, the Hoffmann rule predicts that elimination reactions can result in the formation of the least substituted alkene. This scenario often occurs when the reaction involves a bulky base, which hinders access to the more substituted beta-hydrogen atoms. As a result, the less substituted beta-hydrogen is removed, leading to the formation of the less substituted alkene. Steric hindrance plays a crucial role here by setting up a situation where the major product defies the usual stability principle used in Saytzeff's rule. In essence, the Hoffmann rule serves to predict outcomes in reactions where steric factors dominate over electronic effects.

Elimination Reactions

Elimination reactions are a type of organic reaction where a molecule loses atoms or groups, typically forming a double bond. These reactions are essential in organic synthesis for converting haloalkanes into alkenes. There are two main types of elimination reactions: E1 and E2. Both involve the removal of a hydrogen halide from an alkyl halide but follow different pathways and mechanisms based on reaction conditions and the substrate. Elimination reactions enable chemists to predictably create complex molecular structures by managing different variables such as the nature of the base, solvent, and temperature. Their versatility makes them a cornerstone of synthetic organic chemistry.

E1 Reaction

The E1 reaction, or unimolecular elimination reaction, is characterized by a two-step process. First, the substrate loses a leaving group, resulting in the formation of a carbocation intermediate. This step is the rate-determining step of the reaction. In the second step, a base abstracts a proton from a beta-carbon, leading to the formation of an alkene. E1 reactions typically follow Saytzeff's rule, favoring the formation of the more substituted and stable alkene. This type of reaction is common in tertiary substrates where the formation of stable carbocations is more feasible.

E1cb Reaction

The E1cb reaction, or Elimination Unimolecular Conjugate Base reaction, is unique in its mechanism. Unlike the E1 reaction, the first step involves the base abstracting a proton, forming a carbanion intermediate. Then, the leaving group departs in the second step. This sequence commonly arises in reactions where carbocations are unstable or not feasible, and typically leads to the formation of the less substituted alkene, in line with the Hoffmann rule. The E1cb mechanism is prominent in reactions involving conjugated systems or when the substrate has electron-withdrawing groups that stabilize the carbanion intermediate. Understanding this reaction type is essential for predicting reaction pathways in complex synthetic strategies.