Problem 59 - A Complete Resource Book in Chemistry for JEE Main

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

Step-by-Step Solution

Answer

Option (c) is correctly matched: 1EB_11 reaction follows Saytzeff’s rule, producing the more substituted alkene, but none of the given options matches precisely with Hoffmann's elimination....

Step 1: Understanding the Options

We need to identify which pair is correctly matched based on the names and rules provided. Each of these rules pertains to elimination reactions in organic chemistry.

Step 2: Reviewing Saytzeff's Rule

Saytzeff's rule states that during an elimination reaction, the more substituted alkene is usually favored as it is more stable. Therefore, option (a) is incorrect as it mentions the least substituted alkene.

Step 3: Reviewing Hoffmann's Rule

Hoffmann's rule predicts that during elimination, the less substituted, less stable alkene is often the product, especially in the presence of bulky bases. This means that option (b) is incorrect as it incorrectly associates Hoffmann with the most substituted alkene.

Step 4: Understanding the 1EB_{1}1 же критерии

In an 1EB1") has a different characteristic than Hoffmann elimination, as it typically follows Saytzeff’s rule, forming the more substituted alkene whenever possible. This makes option (d) incorrect.

Step 5: Understanding 1EB_{1}1eb...

E1cm reaction involves the formation of a positive charge intermediate, and the Hoflmarmnillesiaans! They do po3rm an alo view the eleirinotably with a bad neoldes aaron...

Key Concepts

Saytzeff Rule

In organic chemistry, the Saytzeff rule helps predict the outcome of elimination reactions. According to this rule, when a molecule undergoes an elimination reaction, the most stable alkene will form.

Stability in alkenes generally increases with the degree of substitution. This means alkenes with more substituents around the double bond are more stable. The stability is due to hyperconjugation and the electron-releasing effect of alkyl groups, which help distribute charge.

Thus, in the presence of favorable reaction conditions, such as smaller base size or higher temperature, the most substituted, or Saytzeff product, will predominate. Remember, stability is key when using Saytzeff's rule, and watching for reaction conditions can often lead to the correct prediction of product formation in elimination reactions.

Hoffmann Rule

The Hoffmann rule offers a different perspective when predicting products from an elimination reaction. It suggests that the less substituted alkene is the main product, especially when using bulky bases or in sterically hindered environments.

Reactants with larger, bulkier bases may not easily approach and extract protons from more substituted carbons due to steric hindrance. This limitation causes elimination to occur at less hindered, primary carbons, resulting in less substituted alkenes.

• Bulkier bases include tertiary amines and those with significant steric hindrance.
• The less substituted alkene, while less stable, is preferred due to easier formation.

The Hoffmann rule is particularly useful when designing reactions to yield less substituted alkenes intentionally or when specific reaction conditions naturally shift the pathway.

E1 Reaction

E1 reactions are a type of unimolecular elimination reaction commonly studied in organic chemistry. The term "E1" refers to two main steps in the reaction mechanism:

• Formation of a carbocation intermediate.
• Elimination of a proton to form the double bond.

Let's understand these steps:

**Carbocation Formation**: The first and rate-determining step involves the ionization of the substrate, usually by losing a good leaving group to form a positively charged carbocation. This intermediate is generally more stable in more substituted carbocations, which supports the Saytzeff rule in E1 contexts.

**Proton Elimination**: Once the carbocation is formed, a proton loss from an adjacent carbon results in the formation of a double bond, finalizing the alkene formation.

E1 reactions often happen in polar protic solvents increasing carbocation stabilization. An understanding of these steps is essential to predict reaction outcomes accurately, especially in showing why E1 reactions tend to favor Saytzeff products.