For each pair, choose the nucleophile that would react most quickly in an S_N2 reaction (assume H₂O is the solvent).
(c)

Verified step by step guidance

1. Step 1: Recall the key factors that influence nucleophilicity in an SN2 reaction. Nucleophilicity depends on the ability of a nucleophile to donate electrons, which is influenced by charge, electronegativity, and polarizability.
2. Step 2: Compare the electronegativity of oxygen (O) and sulfur (S). Oxygen is more electronegative than sulfur, meaning it holds onto its electrons more tightly, making sulfur a better nucleophile in terms of electron donation.
3. Step 3: Consider polarizability. Sulfur is larger and more polarizable than oxygen, which allows it to form a stronger interaction with the electrophile during the SN2 reaction. This makes sulfur a faster nucleophile in SN2 reactions.
4. Step 4: Evaluate the solvent effect. In H2O (a polar protic solvent), nucleophiles with lower electronegativity and higher polarizability, like sulfur, are less hindered by solvation and react more quickly.
5. Step 5: Conclude that the nucleophile with the sulfur atom (S⁻) will react more quickly in an SN2 reaction compared to the nucleophile with the oxygen atom (O⁻).

Key Concepts

Nucleophilicity

Nucleophilicity refers to the ability of a species to donate an electron pair to an electrophile, forming a chemical bond. In Sₙ2 reactions, stronger nucleophiles react more quickly due to their higher electron density and better ability to stabilize the transition state. Factors influencing nucleophilicity include charge, electronegativity, and solvent effects.

Sₙ2 Mechanism

The Sₙ2 (substitution nucleophilic bimolecular) mechanism involves a single concerted step where the nucleophile attacks the electrophile while simultaneously displacing a leaving group. This reaction is characterized by a backside attack, leading to inversion of configuration at the chiral center. The rate of the reaction depends on both the nucleophile and the substrate.

Solvent Effects

The choice of solvent can significantly influence the rate of Sₙ2 reactions. Polar aprotic solvents, like acetone or DMSO, enhance nucleophilicity by stabilizing the nucleophile without solvation, allowing for faster reactions. In contrast, polar protic solvents, such as water, can hinder nucleophilicity by forming hydrogen bonds with the nucleophile, thus slowing the reaction.