Problem 150 - A Complete Resource Book in Chemistry for JEE Main

Question

Match the following: List I (Reactants) 1\. \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right), \mathrm{OH} \stackrel{\text { Nalfr. } \mathrm{H}_{1} \mathrm{SO}_{4} \mathrm{~A}}{\longrightarrow}\) 2\. \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{COH} \frac{\mathrm{Com} \cdot \mathrm{HCl}}{\text { Roan } \mathrm{km}}\) 3\. \(\mathrm{CH}_{3} \mathrm{CH}(\mathrm{OH})\left(\mathrm{CH}_{2}\right)_{2} \mathrm{CH}_{3} \stackrel{\mathrm{Hat}_{2}}{\longrightarrow}\) 4\. \(\mathrm{Me}_{2} \mathrm{CHCH}_{2} \mathrm{OH} \stackrel{\mathrm{soc}_{2}}{\longrightarrow}\) List II (Alkyl halides) A. \(\mathrm{CH}_{3} \mathrm{CHBr}\left(\mathrm{CH}_{2}\right)_{2} \mathrm{CH}_{3}\) B. \(\mathrm{Me}_{2} \mathrm{CHCH}_{2} \mathrm{Cl}\) C. \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCl}\) D. \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{3} \mathrm{Br}\) The correct matching is \(\begin{array}{llll}1 & 2 & 3 & 4 \\ \text { (a) } \mathrm{C} & \mathrm{D} & \mathrm{B} & \mathrm{A}\end{array}\) (b) \(\mathrm{C} \quad \mathrm{D} \quad \mathrm{A} \quad \mathrm{B}\) (c) \(\mathrm{D} \quad \mathrm{C}\) A \(\quad\) B (d) \(\mathrm{D} \quad \mathrm{C}\) B \(A\)

Step-by-Step Solution

Answer

The reactions match up as (d): \( D, C, A, B \).

Step 1: Analyze Reaction 1

Examine the reactant \( \mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)\mathrm{OH} \). This is a primary alcohol that reacts with \( \mathrm{H}_{2}\mathrm{SO}_{4} \), typically giving rise to dehydration and forming an alkene, but here it's converting to an alkyl halide using \( \mathrm{NaBr} \). Based on typical reactions, it is converted into \( \mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{3}\mathrm{Br} \). Thus, Reaction 1 matches with \( D \).

Step 2: Analyze Reaction 2

The reactant \( \left(\mathrm{CH}_{3}\right)_{3}\mathrm{COH} \) is a tertiary alcohol. When treated with \( \mathrm{HCl} \), it undergoes substitution to form a tertiary alkyl halide \( \left(\mathrm{CH}_{3}\right)_{3}\mathrm{CCl} \). Therefore, Reaction 2 matches with \( C \).

Step 3: Analyze Reaction 3

The reactant is \( \mathrm{CH}_{3}\mathrm{CH}(\mathrm{OH})\left(\mathrm{CH}_{2}\right)_{2}\mathrm{CH}_{3} \), a secondary alcohol. It's treated with \( \mathrm{HBr} \), resulting in a secondary bromide \( \mathrm{CH}_{3}\mathrm{CHBr}\left(\mathrm{CH}_{2}\right)_{2}\mathrm{CH}_{3} \). Therefore, Reaction 3 matches with \( A \).

Step 4: Analyze Reaction 4

Examine the reactant \( \mathrm{Me}_{2}\mathrm{CHCH}_{2}\mathrm{OH} \). This is a primary alcohol. With the reagent \( \mathrm{SOCl}_{2} \), it forms \( \mathrm{Me}_{2}\mathrm{CHCH}_{2}\mathrm{Cl} \). Therefore, Reaction 4 matches with \( B \).

Step 5: Compile the Matches

Based on the analysis, the correct matches are: Reaction 1 with \( D \), Reaction 2 with \( C \), Reaction 3 with \( A \), and Reaction 4 with \( B \). Compare this to the given options.

Key Concepts

Alcohol Reactions

Alcohols are versatile molecules in organic chemistry and can undergo various chemical reactions. One of the commonly observed reactions of alcohols is their conversion into alkyl halides. This happens through a substitution reaction where the hydroxyl group (-OH) is replaced by a halide ion (such as chloride, bromide, or iodide). The nature of the alcohol — whether primary, secondary, or tertiary — significantly affects how the reaction proceeds.
Primary alcohols can be transformed into alkyl halides through a process where the reaction typically involves acid catalysis, such as the use of sulfuric acid, combined with a halide source like sodium bromide. This is because primary alcohols are less hindered and therefore more reactive towards nucleophilic substitution. In contrast, tertiary alcohols convert into alkyl halides more readily using hydrogen halides like HCl due to the stability of the resulting carbocation intermediate formed during the reaction.
It's essential to understand that alcohol reactions depend on both the mechanism of the reaction and the structure of the alcohol involved. Reaction conditions, such as temperature and solvent, can also influence the reaction's outcome. Nonetheless, the transformation of alcohols to alkyl halides is a key reaction in organic synthesis.

Nucleophilic Substitution

Nucleophilic substitution is a fundamental type of reaction in organic chemistry where a nucleophile, which is an electron-rich species, replaces a leaving group attached to a carbon atom. In the context of alcohol reactions forming alkyl halides, the hydroxyl group in alcohols serves as the leaving group which gets substituted by a halide ion.
There are two main types of nucleophilic substitution mechanisms:

• **S_N1 Reactions**: These are unimolecular reactions characterized by two steps where the leaving group departs forming a carbocation, which is then attacked by the nucleophile. This mechanism is common for tertiary alcohols due to their ability to stabilize the carbocation intermediate.
• **S_N2 Reactions**: These are bimolecular reactions occurring in a single step where the nucleophile attacks the substrate at the same time as the leaving group leaves. S_N2 reactions are common for primary alcohols since they are less sterically hindered.

Understanding the mechanism of nucleophilic substitutions is crucial in predicting and controlling the outcome of these reactions. Reaction conditions, such as the strength of the nucleophile and the leaving group, also play a significant role.

Organic Chemistry Reactions

Organic chemistry is the study of the structure, properties, composition, reactions, and preparation of carbon-containing compounds. The reactions in organic chemistry are numerous and varied, with different types enabling the transformation of one organic compound into another.
One of the overarching themes in organic chemistry reactions is the understanding of functional group transformations as they define the reactivity and chemical behavior of molecules. Reactions like oxidation, reduction, addition, elimination, and substitution are foundational.
In the context of converting alcohols to alkyl halides via nucleophilic substitution, it’s critical to grasp how different conditions can shift the reaction pathway. The reagents used (like HCl for tertiary alcohols or SOCl₂ for primary alcohols) and the type of alcohol involved (primary, secondary, or tertiary) guide which substitution mechanism predominates.
Organic chemistry is not just about individual reactions but about how these reactions operate within the context of methodical synthesis. Understanding these concepts facilitates the design of synthetic pathways to yield desired compounds effectively.