Problem 129 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following statements are correct. (1) \(\overline{\mathrm{NH}}_{2}\) is better nucleophile than \(\mathrm{NH}_{3}\) but latter \(\left(\mathrm{NH}_{3}\right)\) is better nucleophile than \(\mathrm{NH}_{4}{\underline{\phantom{xx}}}^{+}\) (2) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{O}^{-}\)is better nucleophile than CC(=O)O (3) \(\mathrm{OH}\) - is better nucleophile than \(\mathrm{SH}\) and \(\mathrm{H}_{2} \mathrm{O}\), but \(\mathrm{H}_{2} \mathrm{O}\) is better nucleophile than \(\mathrm{H}_{3} \mathrm{O}^{+}\) (4) \(\mathrm{ClO}^{-}\)is weaker nucleophile than \(\mathrm{ClO}_{4}\) (a) 1,2 and 3 (b) 1,3 and 4 (c) 2, 3 and 4 (d) \(1,2,3\) and 4

Step-by-Step Solution

Answer

(a) 1, 2, and 3

Step 1: Evaluate Statement (1)

A nucleophile is determined by its ability to donate an electron pair. - \(\overline{\mathrm{NH}}_{2}\) is a better nucleophile than \(\mathrm{NH}_{3}\) because it has a negative charge, increasing its electron-donating ability. - \(\mathrm{NH}_{3}\) is a better nucleophile than \(\mathrm{NH}_{4}^{+}\) because \(\mathrm{NH}_{4}^{+}\) is positively charged and cannot donate electrons efficiently. Thus, this statement is correct.

Step 2: Evaluate Statement (2)

We need to compare the nucleophilicity of phenoxide ion \(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{O}^{-}\) and acetate ion \(\mathrm{CH}_{3}\mathrm{COO}^{-}\). - Phenoxide ion has a negative charge on an oxygen, which makes it a better nucleophile than the resonance-stabilized \(\mathrm{CH}_{3}\mathrm{COO}^{-}\), which can donate electron pairs less effectively as the electron density is delocalized. This statement is correct.

Step 3: Evaluate Statement (3)

Consider nucleophilicity of \(\mathrm{OH}^{-}\), \(\mathrm{SH}^{-}\), \(\mathrm{H}_{2}\mathrm{O}\), and \(\mathrm{H}_{3}\mathrm{O}^{+}\): - \(\mathrm{OH}^{-}\) is a better nucleophile than \(\mathrm{SH}^{-}\) due to higher charge density. - \(\mathrm{OH}^{-}\) is also better than water (\(\mathrm{H}_{2}\mathrm{O}\)). - \(\mathrm{H}_{2}\mathrm{O}\) is better than \(\mathrm{H}_{3}\mathrm{O}^{+}\) because \(\mathrm{H}_{3}\mathrm{O}^{+}\) is positively charged and thus a poor electron donor. Therefore, this statement is correct.

Step 4: Evaluate Statement (4)

For the nucleophilicity of \(\mathrm{ClO}^{-}\) versus \(\mathrm{ClO}_{4}^{-}\): - \(\mathrm{ClO}_{4}^{-}\) is a very weak nucleophile due to extensive resonance stabilization. - \(\mathrm{ClO}^{-}\) is less resonantly stabilized than \(\mathrm{ClO}_{4}^{-}\), and it should be a stronger nucleophile. Thus, this statement is incorrect.

Step 5: Conclusion: Determine Correct Statements

From the evaluations: - Statement (1) is true. - Statement (2) is true. - Statement (3) is true. - Statement (4) is false. Therefore, the correct statements are 1, 2, and 3.

Key Concepts

Electron Donors

An electron donor is a chemical entity that donates electrons to another entity, making it a key player in determining nucleophilicity. In chemistry, nucleophiles are often considered as electron donors because they tend to donate a pair of electrons to form new chemical bonds.
To understand nucleophilicity, one must recognize that the more available and high in energy an electron pair is, the greater the electron donating capacity of the entity:

• A negatively charged species, such as \( \overline{\mathrm{NH}}_{2} \), has more electrons to donate than neutral counterparts, like \( \mathrm{NH}_{3} \).
• Conversely, a positively charged species cannot supply electrons easily. \( \mathrm{NH}_{4}^{+} \) is an example, being less effective as an electron donor due to its positive charge.

Charge Density

Charge density refers to the distribution of electric charge over an atom or molecule. It is a significant factor affecting nucleophilicity. A higher charge density typically implies stronger electron-donating abilities.
Consider the examples:

• \( \mathrm{OH}^{-} \) possesses a high charge density due to its negative charge, and thus, it acts as an excellent nucleophile.
• Water, \( \mathrm{H}_{2}\mathrm{O} \), has a lower charge density because it carries no charge, making it less nucleophilic compared to \( \mathrm{OH}^{-} \).
• The effectiveness of \( \mathrm{OH}^{-} \) over \( \mathrm{SH}^{-} \) can also be explained because oxygen is more electronegative than sulfur, resulting in a more concentrated charge density even though sulfur can also donate electrons.

Resonance Stabilization

In the realm of chemistry, resonance stabilization involves the delocalization of electrons across a molecule, often weakening its nucleophilicity. When electrons are delocalized, they become "spread out," making them less available to participate in new bond formation.
Let's delve deeper:

• The acetate ion, \( \mathrm{CH}_{3}\mathrm{COO}^{-} \), exhibits resonance stabilization as its negative charge is delocalized over multiple atoms. This characteristic diminishes its electron-donating capability.
• In comparison, the phenoxide ion, \( \mathrm{C}_{6}\mathrm{H}_{5}\mathrm{O}^{-} \), while also stabilized by resonance, has its negative charge more localized, making it a stronger nucleophile.

Resonance can significantly affect the position of an atom within a molecule, influencing its interaction as an electron donor.

Chemical Reactivity

Chemical reactivity is the propensity of a substance to engage in chemical reactions. Nucleophilicity is a part of a molecule's overall chemical reactivity, largely influenced by its ability to donate electrons.
Key aspects include:

• A molecule with a higher nucleophilicity reacts more readily with electrophiles, making it highly chemically reactive. For example, \( \overline{\mathrm{NH}}_{2} \) reacts readily with various electrophiles due to its negative charge and ability to donate electrons.
• Whilst \( \mathrm{ClO}^{-} \) should logically be a stronger nucleophile than \( \mathrm{ClO}_{4}^{-} \) as the latter is very resonance-stabilized, false assumptions can lead to illogical conclusions, thus affecting chemical reactivity predictions.
• Understanding the intricate characteristics of a molecule, such as whether it is resonantly stabilized, is crucial for accurate predictions of its chemical reactivity and nucleophilicity.

The interplay of these factors highlights the importance of assessing chemical reactivity holistically, rather than in isolation.