Problem 72 - A Complete Resource Book in Chemistry for JEE Main

Question

The final product \(\mathrm{B}\) in the following reaction is \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}=\mathrm{CH}_{2} \underset{\left(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{CO}_{2}\right)_{2}}{\stackrel{\mathrm{NBS}, \mathrm{hv}}{\mathrm{CH}_{2} \mathrm{OH}}} \mathrm{A} \frac{\mathrm{CH}_{3} \mathrm{SNa}}{\longrightarrow} \mathrm{B}\) (a) methyl 1-methylallylsulphide (b) allyl methyl sulphide (c) n-butyl methyl sulphide (d) diallyl sulphide

Step-by-Step Solution

Answer

The correct answer is (b) allyl methyl sulphide.

Step 1: Identifying Reagents and Reaction Conditions

The initial part of the reaction involves 1-butene, \(\text{CH}_3\text{CH}_2\text{CH}=\text{CH}_2\), with the reagents N-Bromosuccinimide (NBS) and light (hv). This indicates a bromination reaction at the allylic position.

Step 2: Understanding Allylic Bromination

NBS in the presence of light typically introduces a bromine at the allylic position, which is the position adjacent to a carbon-carbon double bond. Thus, the product \(A\) would be \(\text{CH}_3\text{CH}_2\text{CHBrCH}_2\) after allylic bromination of 1-butene.

Step 3: Substitution Reaction with Sodium Methanethiolate

The next step involves a substitution reaction with \(\text{CH}_3\text{SNa}\), which is sodium methanethiolate. This reagent will substitute the bromine on the allylic carbon, replacing the bromine with a methylthio group \(\text{S}\text{CH}_3\), leading to product \(B\).

Step 4: Identify the Final Product

After substitution, the product \(B\) is \(\text{CH}_3\text{CH}_2\text{CH}(\text{S}\text{CH}_3)\text{CH}_2\), known as allyl methyl sulphide.

Key Concepts

Allylic Bromination

Allylic bromination is a special type of bromination that targets the allylic position in a molecule. This position is adjacent to a carbon-carbon double bond, often found in alkenes. What makes allylic bromination unique is its ability to predominantly and selectively introduce a bromine atom at this specific site.

• It requires a brominating agent, like N-Bromosuccinimide (NBS), and often a catalyst, usually in the form of light (hv).
• The presence of a double bond plays a crucial role in stabilizing the intermediate radicals formed during the reaction.

Understanding this reactivity helps chemists control the synthesis of complex molecules by modifying specific sites rather than affecting the whole structure.

Substitution Reaction

A substitution reaction involves replacing one functional group in a molecule with another. These reactions are fundamental in organic chemistry for transforming one compound into another. In the provided exercise, sodium methanethiolate acts as the nucleophile.
Its role is to attack and replace the bromine atom in the allylic bromide.

• Substitution reactions are categorized into either SN1 or SN2 reactions.
• In this case, the substitution with sodium methanethiolate follows an SN2 pathway where the nucleophile directly displaces the leaving group, which in this case is bromine.

This pathway is characterized by a one-step mechanism and is highly specific to the reaction conditions and types of reactants involved.

N-Bromosuccinimide (NBS)

N-Bromosuccinimide, commonly abbreviated as NBS, is a reagent used widely in organic chemistry for bromination reactions, especially allylic and benzylic brominations. NBS has several advantages that make it a preferred choice in such reactions.

• It provides a controlled release of bromine, which reduces the risk of over-bromination.
• It is more selective, allowing specific bromination at allylic positions, which is crucial for reactions like the one involving 1-butene.

Its practical application is seen in the exercise where NBS, in the presence of light, facilitates the efficient transformation of 1-butene to its allylic brominated counterpart.

Sodium Methanethiolate

Sodium methanethiolate (ext{CH}_3 ext{SNa}) is a strong nucleophile commonly utilized in substitution reactions.
As a source of methanethiolate ion, it plays a critical role in displacing halides like bromine in organic compounds.

• It introduces a methylthio group ( ext{S} ext{CH}_3) into the molecule upon reaction.
• Its effectiveness in substitution reactions like the one in this solution demonstrates its importance in the field of organic synthesis.

Here, it reacts with the allylic bromide to produce allyl methyl sulphide, showcasing its ability to effectively participate in SN2 reactions.

Alkene Chemistry

Alkenes are hydrocarbons characterized by the presence of at least one carbon-carbon double bond, providing them with distinct chemical properties. This double bond site serves as a hub for many types of reactions, including additions and substitutions.

• Alkenes can engage in reactions that either modify the double bond or target positions adjacent to it, such as in allylic bromination.
• This chemistry is the backbone of several organic reactions that require precise and controlled synthesis routes.

Understanding these pathways in alkenes allows chemists to construct more complex structures and achieve selective transformations, as evidenced in the process transforming 1-butene to allyl methyl sulphide.