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

Question

Which of the following statements are correct. (1) \(\overline{\mathrm{N}} \mathrm{H}_{2}\) is better nucleophile than \(\mathrm{NH}_{3}\) but latter \(\left(\mathrm{NH}_{3}\right)\) is better nucleophile than \(\mathrm{NH}_{4}^{+}\) (2) \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{O}^{-}\)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) \(\mathrm{ClO}^{-1}\) 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

Correct statements are 1, 2, and 3, so the answer is (a).

Step 1: Evaluate Statement 1

Examine the nucleophilicity of the given species: \(\overline{\mathrm{N}} \mathrm{H}_{2}\), \(\mathrm{NH}_{3}\), and \(\mathrm{NH}_{4}^{+}\). Nucleophilicity generally increases with negative charge and decreases with positive charge. Therefore, \(\overline{\mathrm{N}} \mathrm{H}_{2}\) (negatively charged) is a better nucleophile than \(\mathrm{NH}_{3}\) (neutral), and \(\mathrm{NH}_{3}\) is a better nucleophile than \(\mathrm{NH}_{4}^{+}\) (positively charged). Statement 1 is correct.

Step 2: Evaluate Statement 2

Examine \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{O}^{-}\) and compare it with the ester structure represented by \(CC(=O)O\). Phenoxide ions (like \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{O}^{-}\)) are generally more nucleophilic than esters because the negative charge on phenoxide can be delocalized over a benzene ring, enhancing nucleophilicity, compared to the resonance-stabilized neutral ester. Statement 2 is correct.

Step 3: Evaluate Statement 3

Compare the nucleophilicity of \(\mathrm{OH}^{-}\), \(\mathrm{SH}^{-}\), and \(\mathrm{H}_{2} \mathrm{O}\). Hydroxide ion \(\mathrm{OH}^{-}\) is a better nucleophile than \(\mathrm{SH}^{-}\) due to being smaller and having a concentrated charge, and is inherently more nucleophilic than neutral \(\mathrm{H}_{2} \mathrm{O}\). Also, \(\mathrm{H}_{2} \mathrm{O}\) is a better nucleophile than the protonated form \(\mathrm{H}_{3} \mathrm{O}^{+}\) due to the lack of positive charge. Statement 3 is correct.

Step 4: Evaluate Statement 4

Compare \(\mathrm{ClO}^{-1}\) and \(\mathrm{ClO}_{4}^{-}\). Perchlorate \(\mathrm{ClO}_{4}^{-}\) is more stable and less nucleophilic due to extensive resonance stabilization and charge dispersion compared to \(\mathrm{ClO}^{-1}\), which is a stronger nucleophile due to having more localized negative charge. Statement 4 is incorrect as \(\mathrm{ClO}^{-1}\) should be the better nucleophile.

Step 5: Determine Correct Combination of Statements

From the analysis: statements 1, 2, and 3 are correct. Statement 4 is incorrect. Therefore, the correct choice is (a) 1, 2, and 3.

Key Concepts

Nucleophilicity

Nucleophilicity is a fundamental concept in chemistry that describes the ability of a chemical species to donate a pair of electrons to an electrophile during a reaction. It is a property that is closely related to, but distinct from basicity. Nucleophiles are electron-rich species, which means they have electrons ready to form new bonds with electron-poor species.
There are several factors that affect nucleophilicity:

• Charge: Negatively charged species make better nucleophiles as they have an electron pair readily available for donation.
• Electronegativity: Less electronegative atoms are better nucleophiles because they hold onto their electrons less tightly, making them more ready to donate those electrons.
• Solvent effects: The solvent can either stabilize or destabilize the nucleophile, affecting its reactivity. Polar protic solvents tend to hinder nucleophilicity because they can stabilize the nucleophile through hydrogen bonding.

Negative Charge

Negative charge plays a significant role in determining the strength of a nucleophile. Generally, more negative charge leads to higher nucleophilicity because it indicates an excess of electrons, which are necessary for making new chemical bonds.
In the context of the exercise,

• The species \(\overline{\mathrm{N}} \mathrm{H}_{2}\)\ is more nucleophilic than neutral \(\mathrm{NH}_{3}\) because the extra negative charge provides a greater drive to share electrons.
• Contrastingly, positively charged species like \(\mathrm{NH}_{4}^{+}\) are poor nucleophiles since they lack excess electrons.

As you can see, negative charge is a very helpful factor to enhance nucleophilicity.

Resonance Stabilization

Resonance stabilization significantly affects the nucleophilicity of a species by distributing the electron density over multiple atoms or groups. When a negative charge is delocalized due to resonance, it reduces the nucleophile's reactivity since the electron density isn't concentrated in one area.
Here's how it works:

• In the case of the phenoxide ion \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{O}^{-}\), the negative charge can be spread over the benzene ring, making it a beneficial nucleophile. The resonance offers stability which supports nucleophilic reactions against electrophiles.
• On the contrary, the presence of resonance in \(\mathrm{ClO}_{4}^{-}\) as seen in the exercise reduces nucleophilicity because the electron density is dispersed across the entire molecule, weakening the ability to donate electrons for bond formation.

Charge Dispersion

Charge dispersion is a concept closely related to resonance stabilization. It involves the spreading out of charge over a larger volume or over several atoms, which can lower the energy of a molecule and affect its reactivity.
Understanding charge dispersion can help explain why some species behave as stronger nucleophiles than others.

• An example from the exercise highlights this: \(\mathrm{ClO}_{4}^{-}\) shows extensive charge dispersion due to resonance, which spreads the negative charge and lessens its reactivity, making it a weaker nucleophile. In contrast, \(\mathrm{ClO}^{-1}\) has less charge dispersion and therefore, more concentrated electron density, making it a stronger nucleophile.
• Thus, less dispersed negative charges often increase a species' ability to act as a nucleophile since the electron density is more concentrated and thus more available for reactions.