Problem 90 - A Complete Resource Book in Chemistry for JEE Main

Question

Consider the following nucleophiles: \(\mathrm{H}_{2} \mathrm{O}, \mathrm{CH}_{3} \mathrm{COO}, \quad \overline{\mathrm{OH}}, \mathrm{CH}_{3} \mathrm{O}\) \(\begin{array}{llll}\text { I } & \text { II } & \text { III } & \text { IV }\end{array}\) The correct order of decreasing nucleophilicity is, (a) \(\mathrm{I}>\mathrm{II}>\mathrm{III}>\mathrm{IV}\) (b) \(\mathrm{IV}>\mathrm{III}>\mathrm{II}>\mathrm{I}\) (c) \(\mathbb{I V}>\mathrm{I}>\mathrm{II}>\mathrm{III}\) (d) \(\mathrm{IV}>\mathrm{II} \geq \mathrm{III}>\mathrm{I}\)

Step-by-Step Solution

Answer

The correct order of decreasing nucleophilicity is (b) IV > III > II > I.

Step 1: Identify Nucleophilicity Factors

Nucleophilicity is influenced by charge, electronegativity, solvent, and steric hindrance. Generally, a negative charge increases nucleophilicity, while high electronegativity decreases it. Larger atoms or ions are typically better nucleophiles in polar protic solvents due to solvation effects.

Step 2: Analyze Each Nucleophile

- \(\overline{\mathrm{OH}}\) is negatively charged, making it a strong nucleophile.- \(\mathrm{CH}_{3}\mathrm{O}\) is also negatively charged and generally more nucleophilic than \(\overline{\mathrm{OH}}\) due to methyl group donation.- \(\mathrm{CH}_{3}\mathrm{COO}\) has a resonance-stabilized carboxylate, reducing its nucleophilicity.- \(\mathrm{H}_{2}\mathrm{O}\) is neutral and the least nucleophilic because it can donate protons.

Step 3: Arrange in Decreasing Order of Nucleophilicity

Based on the analysis:1. \(\mathrm{CH}_{3}\mathrm{O}\) is the most nucleophilic because of its strong electron donation without resonance.2. \(\overline{\mathrm{OH}}\) follows, as it is negatively charged, but less so than methoxide due to the absence of an additional activating alkyl group.3. \(\mathrm{CH}_{3}\mathrm{COO}\), although anion, is stabilized by resonance which decreases its reactivity.4. \(\mathrm{H}_{2}\mathrm{O}\) is the least nucleophilic due to its neutral charge. Thus, the order is IV > III > II > I.

Step 4: Match with Given Options

Compare the determined order (IV > III > II > I) with the provided choices:- Option (b) matches this order (\(IV > III > II > I\)). Therefore, option (b) is the correct answer.

Key Concepts

Factors affecting nucleophilicity

Nucleophilicity refers to the ability of an atom or molecule to donate a pair of electrons and form a new chemical bond. Several factors can influence how effective a nucleophile is.

1. **Charge:** Negatively charged ions typically make better nucleophiles compared to their neutral counterparts. This is because ions with a negative charge have an excess of electrons that they can offer readily.

2. **Electronegativity:** Atoms with lower electronegativity commonly possess higher nucleophilic ability. This is due to their tendency to release electrons more easily than highly electronegative atoms, which prefer to hold onto their electron pairs.

3. **Solvent:** The nature of the solvent can significantly alter nucleophilicity. Polar protic solvents, which have hydrogen bonding capabilities (like water or alcohol), can impede nucleophiles by solvating them, thereby reducing their reactivity. Conversely, in polar aprotic solvents (like acetone or dimethyl sulfoxide), nucleophiles are less hindered by solvation and often appear stronger.

4. **Steric hindrance:** Bulky nucleophiles face difficulty in approaching an electrophilic center, thus reducing their effectiveness. Therefore, the less crowded a nucleophile is, the more nucleophilic it tends to be.

Order of nucleophilic strength

Determining the order of nucleophilic strength can help understand chemical reactivity in various environments. Not every nucleophile is the same; their ability to donate electrons can vary widely.

In our example:

• **Methoxide ( \( \mathrm{CH}_{3}\mathrm{O} \))** - This is a very strong nucleophile since it carries a negative charge and lacks complicating factors like resonance, making electron donation straightforward.
• **Hydroxide ( \( \overline{\mathrm{OH}} \))** - While also negatively charged, it shows slightly less nucleophilicity compared to methoxide due to the absence of an additional alkyl group enhancing electron donation.
• **Acetate ( \( \mathrm{CH}_{3}\mathrm{COO} \))** - Although initially it might appear stronger due to its charge, the resonance stabilization of acetate lessens its reactivity.
• **Water ( \( \mathrm{H}_{2}\mathrm{O} \))** - Neutral in charge, water is the least effective in acting as a nucleophile.

Given these considerations, in decreasing order of nucleophilic strength, we have \( \mathrm{CH}_{3}\mathrm{O} > \overline{\mathrm{OH}} > \mathrm{CH}_{3}\mathrm{COO} > \mathrm{H}_{2}\mathrm{O} \).

Nucleophile examples

Nucleophiles are a diverse group, with many examples spanning organic and inorganic chemistry. Understanding common examples helps one predict chemical reactions.

**Methoxide ( \( \mathrm{CH}_{3}\mathrm{O} \))**

Originates from methanol and is an example of a negatively charged, highly reactive nucleophile. Its simple structure allows it easy access to electrophilic centers.

**Hydroxide ( \( \overline{\mathrm{OH}} \))**

This ion is a well-known nucleophile found in basic solutions. Its small size and charge make it highly reactive, suitable for a variety of reaction mechanisms including saponification and nucleophilic substitution.

**Acetate ( \( \mathrm{CH}_{3}\mathrm{COO} \))**

A resonance-stabilized carboxylate ion, acetate acts as a reasonable nucleophile, often participating in reactions like esterification.

**Water ( \( \mathrm{H}_{2}\mathrm{O} \))**

Serving more as a weak nucleophile due to its neutrality, water can partake in reactions through more gradual mechanisms such as hydrolysis.

Charged vs neutral nucleophiles

Understanding the difference between charged and neutral nucleophiles is crucial for predicting their performance in reactions. Charged nucleophiles usually exhibit higher reactivity due to their electron-rich nature.

**Charged Nucleophiles:**

• Carry a full or partial negative charge, such as hydroxide ( \( \overline{\mathrm{OH}} \)) and methoxide ( \( \mathrm{CH}_{3}\mathrm{O} \)).
• Tend to be more aggressive in seeking out electrophilic centers to donate electrons.
• The negative charge enhances their nucleophilic capacity, allowing for more efficient bond formation.

Conversely, **Neutral Nucleophiles:**

• Examples include molecules like water ( \( \mathrm{H}_{2}\mathrm{O} \)), which lack a charge and thus display lower nucleophilicity.
• They can offer electron pairs but lack the charge-associated push that charged nucleophiles have.
• Often require stronger activation to partake in chemical reactions.

Considering these differences helps to predict reaction outcomes in organic synthesis and other chemical processes.