Question

The correct order of following nucleophiles in acetone (Bu means butyl) is \(\mathrm{Bu}_{4} \stackrel{\oplus}{\mathrm{N}} \mathrm{Cl}^{\ominus}, \mathrm{Bu}_{4}^{\oplus}{\mathrm{N}} \mathrm{Br}^{\ominus}, \quad \mathrm{Bu}_{4} \stackrel{\oplus}{\mathrm{N}}^{\ominus}\) (I) \(\quad\) (II) \(\quad\) (III) (A) I \(>\) II \(>\) III (B) III > II > I (C) I > III > II (D) \(\mathrm{II}>\mathrm{I}>\mathrm{III}\)

Short answer

The correct order of nucleophiles in acetone is (B) III > II > I.

Step-by-step solution

Nucleophilicity in a polar aprotic solvent

Acetone is a polar aprotic solvent. Polar aprotic solvents do not have acidic protons, and their dipole moment is generally high, which allows them to solvate positive charges well. In this scenario, negatively charged nucleophiles are more preferentially solvated than positively charged ones. Solvation decreases the nucleophilicity of the nucleophile.

Determine the effect of anions on nucleophilicity

The nucleophiles are all quaternary ammonium salts with different anions. The more basic the anion, the more likely it is to form stronger interactions with the positive charges of the solvent, decreasing its nucleophilicity. Anions in decreasing order of basicity: 1. \(\mathrm{Cl}^{\ominus}\) 2. \(\mathrm{Br}^{\ominus}\) 3. \(\mathrm{N^{-}}\) This means that the nucleophile with \(\mathrm{Cl}^{\ominus}\) will experience the strongest solvation effect, followed by the one with \(\mathrm{Br}^{\ominus}\).

Ordering the nucleophiles

Based on the above analysis, we can now order the nucleophiles in terms of their nucleophilicity in acetone: 1. \(\mathrm{Bu}_{4} \stackrel{\oplus}{\mathrm{N}}^{\ominus}\) (III) (least solvated, most nucleophilic) 2. \(\mathrm{Bu}_{4}^{\oplus}{\mathrm{N}} \mathrm{Br}^{\ominus}\) (II) (intermediate solvation, intermediate nucleophilic) 3. \(\mathrm{Bu}_{4} \stackrel{\oplus}{\mathrm{N}} \mathrm{Cl}^{\ominus}\) (I) (most solvated, least nucleophilic)

Select the answer

Now, we can identify the correct answer among the given options: (A) I \(>\) II \(>\) III (B) III > II > I (C) I > III > II (D) \(\mathrm{II}>\mathrm{I}>\mathrm{III}\) The correct order of the given nucleophiles in acetone is (B) III > II > I.

Key concepts

Polar Aprotic Solvent

Acetone is an example of a polar aprotic solvent. These solvents cannot donate hydrogen bonds since they lack acidic hydrogen atoms. This absence of hydrogen bonding makes them very effective in dissolving ionic compounds.

Polar aprotic solvents possess a high dielectric constant and a notable dipole moment. This allows them to stabilize positive charges efficiently. However, they do not solvate negatively charged ions as effectively.

This property makes polar aprotic solvents optimal for reactions involving nucleophiles, particularly those that are negatively charged. These nucleophiles remain less solvated, which often increases their reactivity compared to what would be observed in polar protic solvents.

Solvation Effect

The solvation effect refers to the interaction between solvent molecules and solute ions or molecules. In polar aprotic solvents like acetone, the solvation of positive ions is strong due to the solvent's dipole interactions.

However, negatively charged nucleophiles are less solvated, making them more reactive in these environments. Solvation reduces a nucleophile's reactivity. Thus, the more solvated a nucleophile, the less effective it is in reactions.

This is crucial in determining nucleophilicity order, as seen with the quaternary ammonium salts. The extent of solvation directly affects their reactivity in nucleophilic substitution reactions.

Quaternary Ammonium Salts

Quaternary ammonium salts are molecules that contain a nitrogen atom bound to four alkyl groups. In our example, these salts are paired with different anions like \(\mathrm{Cl}^{\ominus}\), \(\mathrm{Br}^{\ominus}\), and \(\mathrm{N}^{\ominus}\).

The anions play a significant role in determining the overall nucleophilicity of the salt. Because these anions differ in basicity, they interact differently with the solvent, affecting their solvation and thus their reactivity.

Understanding the nature of these salts and their interactions with polar aprotic solvents allows us to predict and order their nucleophilicity as seen in various reaction contexts.

Anion Basicity

Anion basicity refers to the ability of an anion to accept protons. The more basic an anion, the stronger its attraction to positive charges. In polar aprotic solvents, this basicity impacts solvation.

Stronger bases like \(\mathrm{Cl}^{\ominus}\) tend to be more solvated, reducing their nucleophilicity. Conversely, weaker bases like \(\mathrm{N}^{\ominus}\) are less affected by solvation, making them more nucleophilic.

Understanding this concept is key to predicting reaction outcomes, as it helps in organizing anions based on their reactivity. This is crucial for students attempting to solve related textbook exercises, like determining nucleophile order.