Problem 202 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is the correct order of decreasing \(\mathrm{SN}^{2}\) reactivity? \(\quad\) [2007] (a) \(\mathrm{RCH}_{2} \mathrm{X}>\mathrm{R}_{2} \mathrm{CHX}>\mathrm{R}_{3} \mathrm{CX}\) (b) \(\mathrm{R}_{3} \mathrm{CX}>\mathrm{R}_{2} \mathrm{CHX}>\mathrm{RCH}_{2} \mathrm{X}\) (c) \(\mathrm{R}_{2} \mathrm{CHX}>\mathrm{R}_{3} \mathrm{CX}>\mathrm{RCH}_{2} \mathrm{X}\) (d) \(\mathrm{RCH}_{2} \mathrm{X}>\mathrm{R}_{3} \mathrm{CX}>\mathrm{R}_{2} \mathrm{CHX}\)

Step-by-Step Solution

Answer

Option (a) \( \mathrm{RCH}_{2} \mathrm{X} > \mathrm{R}_{2} \mathrm{CHX} > \mathrm{R}_{3} \mathrm{CX} \) is the correct order.

Step 1: Understanding SN2 Reactivity

In SN2 reactions, the reactivity is influenced by steric hindrance. Less hindered (less crowded) reactants tend to be more reactive in SN2 reactions because the transition state can be more easily accessed by the nucleophile.

Step 2: Identify the Steric Effects

Let's analyze steric effects for each category: \( \mathrm{RCH}_{2}\mathrm{X} \) is the least sterically hindered, \( \mathrm{R}_{2} \mathrm{CHX} \) is more hindered, and \( \mathrm{R}_{3} \mathrm{CX} \) is the most hindered due to the number of substituents around the central carbon atom.

Step 3: Predict SN2 Reactivity Order

Following the steric hindrance analysis, the SN2 reactivity decreases as steric hindrance increases. Thus, the order from highest to lowest reactivity should be \( \mathrm{RCH}_{2}\mathrm{X} > \mathrm{R}_{2} \mathrm{CHX} > \mathrm{R}_{3} \mathrm{CX} \).

Step 4: Match with Options

Among the given options, option (a) \( \mathrm{RCH}_{2} \mathrm{X} > \mathrm{R}_{2} \mathrm{CHX} > \mathrm{R}_{3} \mathrm{CX} \) matches the predicted order based on steric hindrance and decreasing SN2 reactivity.

Key Concepts

Steric Hindrance

Steric hindrance is a crucial factor in determining the reactivity of molecules in organic chemistry, especially in nucleophilic substitution reactions. Imagine trying to squeeze through a crowded room; the more space there is, the easier it becomes. Similarly, in SN2 reactions, the nucleophile is like a guest trying to access the core of the molecule. The more 'crowded' or hindered the molecule is, the harder it is for the nucleophile to approach.

In the context of SN2 reactions, the "crowding" comes from bulky substituents around the central atom, usually carbon. Here's how steric hindrance plays out in SN2 reactions:

• Primary alkyl halides like \( \mathrm{RCH}_2\mathrm{X} \) have the least steric hindrance. They are wide open for nucleophiles to attack.
• Secondary alkyl halides such as \( \mathrm{R}_2\mathrm{CHX} \) present moderate crowding, making nucleophilic attack a bit challenging.
• Tertiary alkyl halides like \( \mathrm{R}_3\mathrm{CX} \) have the most steric hindrance, acting as a roadblock for any nucleophile trying to attack.

Understanding steric hindrance can explain why certain reactions proceed smoothly while others do not, highlighting its significance in predicting reactivities.

Organic Chemistry

Organic chemistry is the study of carbon-containing compounds. It's a broad field with applications ranging from developing everyday materials like plastics to synthesizing life-saving drugs. At its core, it involves understanding how these compounds react and transform.

Reactions in organic chemistry can be classified into different types such as addition, elimination, and substitution. One of the substitution reactions is the SN2 reaction, standing for bimolecular nucleophilic substitution. This type of reaction involves the simultaneous attack of the nucleophile and departure of the leaving group.

• Universal principles such as electron flow, nucleophilicity, and leaving group ability play major roles in these reactions.
• Organic chemists also have to consider structural aspects of molecules, like steric hindrance, which can greatly influence reaction pathways.

This dynamic field hinges on mastering both the theoretical and practical aspects of organic molecules and their interactions, aiming to create innovations and solve problems in various industries.

Nucleophilic Substitution Reactions

Nucleophilic substitution reactions are pivotal in organic chemistry. They involve a `nucleophile', which is an electron-rich chemical species that seeks out `positive' or electron-poor areas in a compound to form a new bond. These reactions can occur through different mechanisms, mainly SN1 and SN2.

The SN2 mechanism (bimolecular nucleophilic substitution) is characterized by its one-step process where a nucleophile attacks the substrate, leading to the simultaneous displacement of the leaving group:

• It requires a strong nucleophile and a substrate that is not too hindered. This makes primary substrates like \( \mathrm{RCH}_2\mathrm{X} \) very favorable for such reactions.
• The rate of an SN2 reaction depends on both the concentration of the nucleophile and the electrophile; hence, it is termed bimolecular.
• These reactions are often conducted in polar aprotic solvents, which increase the nucleophilicity by not stabilizing the nucleophile excessively.

A deeper understanding of SN2 reactions helps predict how molecules will react and guides the synthesis of complex organic compounds.