Problem 6 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The increasing order of nucleophilicity of the following nucleophiles is: (i) \(\mathrm{CH}_{3} \mathrm{CO}_{2}^{-}\) (ii) \(\mathrm{H}_{2} \mathrm{O}\) (iii) \(\mathrm{CH}_{3} \mathrm{SO}_{3}^{-}\) (iv) \(\mathrm{OH}\) (a) \((\mathrm{i})<(\mathrm{iv})<(\mathrm{iii})<(\mathrm{ii})\) (b) \((\) ii \()<(\) iii \()<(\) iv \()<(\) i) (c) \((\mathrm{iv})<(\mathrm{i})<(\mathrm{iii})<(\mathrm{ii})\) (d) \((\) ii \()<(\) iii \()<(\) i \()<\) (iv)

Step-by-Step Solution

Answer

The increasing order of nucleophilicity is: (ii) < (iii) < (i) < (iv), so option (d) is correct.

Step 1: Understand nucleophilicity

Nucleophilicity refers to the ability of a species to donate an electron pair to an electrophile. It generally increases with increasing negative charge and less steric hindrance. General trends show that a stronger base and less stable anion usually act as a stronger nucleophile.

Step 2: Assess the nucleophilicity of each nucleophile

Let's evaluate each nucleophile:1. \(\mathrm{CH}_{3} \mathrm{CO}_{2}^{-}\) is an acetate ion, a weak base and a stable anion due to resonance.2. \(\mathrm{H}_{2} \mathrm{O}\) is a neutral molecule and is not very nucleophilic.3. \(\mathrm{CH}_{3} \mathrm{SO}_{3}^{-}\) is a mesylate ion, also a stable anion due to resonance, typically weaker than acetate.4. \(\mathrm{OH}^{-}\) is a hydroxide ion, a strong base and unstable due to lack of resonance, making it a strong nucleophile.

Step 3: Order the nucleophiles by nucleophilicity

Based on the assessment:1. \(\mathrm{H}_{2} \mathrm{O}\) is the weakest as it is neutral.2. \(\mathrm{CH}_{3} \mathrm{SO}_{3}^{-}\) is weaker than acetate because of mesylate's resonance stability.3. \(\mathrm{CH}_{3} \mathrm{CO}_{2}^{-}\) comes next due to resonance stabilization compared to hydroxide.4. \(\mathrm{OH}^{-}\) is the strongest nucleophile as a strong base without resonance stabilization.

Step 4: Match your order to the given options

Our increasing order is:\(\mathrm{H}_{2} \mathrm{O} < \mathrm{CH}_{3} \mathrm{SO}_{3}^{-} < \mathrm{CH}_{3} \mathrm{CO}_{2}^{-} < \mathrm{OH}^{-}\)This matches option (d): \((\text{ii})< (\text{iii})< (\text{i})< (\text{iv})\)So, the correct answer is option (d).

Key Concepts

Nucleophile

A nucleophile is a species that readily donates an electron pair to form a chemical bond with an electrophile. This process is fundamental in many chemical reactions, such as nucleophilic substitution and addition. Key characteristics that define a good nucleophile include:

• Having a negative charge, which often enhances nucleophilicity.
• Being less sterically hindered, allowing easier approach to the electrophile.
• Possessing a strong electron density that can be shared.

In the context of the exercise, we looked at substances like hydroxide and acetate ions to determine their nucleophilic potential. It's essential to know that while all bases can be nucleophiles, not all nucleophiles are strong bases. This distinction helps us understand reactions better.

Electron Pair Donation

The concept of electron pair donation is at the heart of nucleophilicity. When a nucleophile attacks an electrophile in a chemical reaction, it is actually donating a pair of electrons to form a new chemical bond. This is why nucleophiles are often referred to as 'electron-rich' species.

• A negatively charged ion, like \[ \mathrm{OH}^{-} \], often has a lone pair of electrons that it can donate.
• Neutral molecules can also act as nucleophiles if they have lone pairs, but they are generally weaker compared to charged ions.

Understanding how electron pair donation works helps in predicting the course of many chemical reactions. It's crucial to remember that the stronger the nucleophile, the more readily it can donate its electron pair, driving the reaction forward.

Base Strength

Base strength is a related concept that often intersects with nucleophilicity. A base is defined by its ability to accept protons (\[ \mathrm{H}^{+} \]), but many strong bases also make good nucleophiles. The stronger the base, the more likely it is to donate its electron pair, contributing to its nucleophilicity.

• For instance, \[ \mathrm{OH}^{-} \]is a very strong base because it effectively accepts protons, and this strength also makes it a potent nucleophile.
• Conversely, \[ \mathrm{CH}_{3} \mathrm{CO}_{2}^{-} \]and \[ \mathrm{CH}_{3} \mathrm{SO}_{3}^{-} \]are weaker bases because they are stabilized by resonance, which makes them less eager to donate an electron pair compared to \[ \mathrm{OH}^{-} \].

By considering base strength, we can better predict which species will act as more effective nucleophiles in a given reaction. It’s worth noting that while stronger bases are often stronger nucleophiles, this isn't a strict rule and can vary depending on the reaction and conditions.

Resonance Stability

Resonance stability is a critical factor affecting nucleophilicity. It refers to the delocalization of electrons across multiple atoms, stabilizing the molecule or ion. When resonance stability is high, the species is generally less reactive as a nucleophile.

• For example, acetate ion \[ \mathrm{CH}_{3} \mathrm{CO}_{2}^{-} \] gains stability from resonance, which lowers its nucleophilicity since the electron density is spread out.
• Similarly, the mesylate ion \[ \mathrm{CH}_{3} \mathrm{SO}_{3}^{-} \]is also stabilized by resonance, making it a weaker nucleophile compared to species without such stabilization.

Resonance can be beneficial for the stability of molecules, but it typically reduces their nucleophilic ability since the electrons are less freely available to form new bonds. Understanding the role of resonance in stability helps in anticipating how these species will behave in chemical reactions.