Problem 196 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\mathrm{CH}_{3} \mathrm{Br}+\mathrm{Nu} \longrightarrow \mathrm{CH}_{1}-\mathrm{Nu}+\mathrm{Br} \quad\) [2006] The decreasing order of the rate of the above reaction with nucleophiles \(\left(\mathrm{Nu}^{-}\right)\)a to \(\mathrm{d}\) is \(\left[\mathrm{Nu}=\begin{array}{lll}\text { a) } \mathrm{PhO} & \text { (b) } \mathrm{AcO}^{-} & \text {(c) } \mathrm{HO}^{-} & \left.\text {(d) } \mathrm{CH}_{3} \mathrm{O}^{-}\right]\end{array}\right.\) (a) \(\mathrm{d}>\mathrm{c}>\mathrm{a}>\mathrm{b}\) (b) \(\mathrm{d}>\mathrm{c}>\mathrm{b}>\mathrm{a}\) (c) \(a>b>c>d\) (d) \(b>d>c>a\)

Step-by-Step Solution

Answer

The correct order is (a) \( \mathrm{d} > \mathrm{c} > \mathrm{a} > \mathrm{b} \).

Step 1: Understand the Reaction Mechanism

This question involves a nucleophilic substitution reaction. The rate generally depends on the strength of the nucleophile. A stronger nucleophile will react faster.

Step 2: Identify the Nucleophiles

Analyze the given nucleophiles: (a) \( \mathrm{PhO}^{-} \), (b) \( \mathrm{AcO}^{-} \), (c) \( \mathrm{HO}^{-} \), and (d) \( \mathrm{CH}_{3} \mathrm{O}^{-} \). Each nucleophile has a different strength based on its structure.

Step 3: Compare Nucleophile Strength

\( \mathrm{CH}_{3} \mathrm{O}^{-} \) and \( \mathrm{HO}^{-} \) are strong nucleophiles due to being alkoxides and hydroxide ions. \( \mathrm{PhO}^{-} \) is stronger than \( \mathrm{AcO}^{-} \) due to resonance stabilization in \( \mathrm{AcO}^{-} \), making it less reactive.

Step 4: Determine the Order of Reactivity

The decreasing order of nucleophilic strength based on the structure is \( \mathrm{CH}_{3} \mathrm{O}^{-} > \mathrm{HO}^{-} > \mathrm{PhO}^{-} > \mathrm{AcO}^{-} \). Thus, option (a)\( \mathrm{d} > \mathrm{c} > \mathrm{a} > \mathrm{b} \) is correct.

Key Concepts

Nucleophile Strength

In nucleophilic substitution reactions, the strength of a nucleophile is crucial in determining the reaction rate. Generally, a stronger nucleophile will attack the electrophile more rapidly, leading to a faster reaction. Several factors influence nucleophile strength:

• **Charge:** Negatively charged nucleophiles tend to be stronger than their neutral counterparts because of their higher electron density.
• **Basicity:** Nucleophiles that are basic, such as alkoxide ions, typically show greater strength because they are more inclined to donate an electron pair.
• **Polarizability:** Larger atoms or ions are more polarizable, allowing them to donate their electron density more readily, which can enhance nucleophile strength.

In the provided exercise, the nucleophiles under comparison are different ions with varying nucleophile strength. Recognizing these differences helps predict reaction outcomes accurately.

Reaction Mechanism

Understanding the reaction mechanism is essential for grasping how nucleophilic substitution unfolds. In these reactions, a nucleophile substitutes a leaving group in an electrophilic molecule. For a typical SN2 mechanism:

• The nucleophile attacks the electrophilic carbon directly, while the leaving group departs simultaneously, forming a transition state.
• This mechanism results in one-step concerted reaction without any intermediates.
• Backside attack is characteristic of this reaction, leading to inversion of configuration at the carbon center undergoing substitution.

Such mechanisms are common in reactions involving strong nucleophiles, as highlighted in the problem focusing on different ions reacting with methyl bromide.

Alkoxide Ion

Alkoxide ions, such as \( \mathrm{CH}_{3}\mathrm{O}^{-} \) (methoxide), are potent nucleophiles due to their negative charge and high electron density.Characteristics of alkoxide ions that enhance their nucleophile strength include:

• **Stability:** Although charged, alkoxides are often stabilized by the surrounding solvent molecules which can enhance their reactivity.
• **Basicity:** Alkoxides are strong bases, which in turn makes them strong nucleophiles capable of efficiently attacking electrophilic centers.
• **Structure:** Small alkoxides like methoxide can easily access the electrophilic carbon due to reduced steric hindrance compared to bulkier groups.

In the given problem, the presence of methoxide is noted as having significant nucleophilic strength, comparing it favorably to other provided ions.

Resonance Stabilization

Resonance stabilization is a critical concept affecting nucleophile strength, as seen in nucleophiles like acetate \( \mathrm{AcO}^{-} \).Resonance can decrease nucleophile reactivity by delocalizing electrons:

• **Electron Delocalization:** In acetate, the negative charge is spread over multiple atoms, making it less concentrated and reducing nucleophilic strength.
• **Stabilization vs. Reactivity:** While resonance stabilizes a molecule, it can decrease the reactivity needed for an efficient nucleophilic attack.
• **Comparison with non-resonant nucleophiles:** Non-resonance stabilized ions like hydroxide \( \mathrm{HO}^{-} \) are stronger due to the concentration of negative charge on a single atomic center.

Acetate's resonance stabilizes it but also dampens its nucleophilic potency, making it less reactive than other nucleophiles without similar stabilization, such as methoxide and hydroxide ions, as highlighted in the exercise discussion.