Which member in each pair in [PROBLEM 9-68] is a better leaving group?
c. H₂O or H₂S
d. HO⁻ or HS⁻

Verified step by step guidance

1. Step 1: Understand the concept of a leaving group. A leaving group is an atom or group of atoms that can depart with a pair of electrons during a chemical reaction, such as a substitution or elimination reaction. A good leaving group is typically weakly basic and stable after leaving.
2. Step 2: Compare the acidity of the conjugate acids of the leaving groups. The strength of a leaving group is inversely related to the basicity of the group. The weaker the base, the better the leaving group. For example, the conjugate acids of H2O and H2S are H3O+ and H3S+, respectively. Similarly, the conjugate acids of HO− and HS− are H2O and H2S, respectively.
3. Step 3: Analyze the stability of the leaving group after departure. Stability is influenced by factors such as electronegativity, size, and polarizability. Larger and more polarizable atoms (like sulfur) tend to stabilize the negative charge better than smaller atoms (like oxygen).
4. Step 4: For part (c), compare H2O and H2S. H2S is a weaker base than H2O because sulfur is less electronegative and more polarizable than oxygen. Therefore, H2S is a better leaving group than H2O.
5. Step 5: For part (d), compare HO− and HS−. HS− is a weaker base than HO− for the same reasons as above (sulfur is less electronegative and more polarizable than oxygen). Thus, HS− is a better leaving group than HO−.

Key Concepts

Leaving Groups

Leaving groups are atoms or groups of atoms that can depart from a molecule during a chemical reaction, typically in nucleophilic substitution or elimination reactions. A good leaving group is stable after departure, often being a weak base. The ability of a leaving group to stabilize its negative charge or form a stable molecule is crucial in determining its effectiveness.

Acid-Base Strength

The strength of an acid or base is a key factor in determining the quality of a leaving group. Stronger acids have weaker conjugate bases, which tend to be better leaving groups. For example, H2O is the conjugate base of a strong acid (H3O+), making it a better leaving group than H2S, which is the conjugate base of a weaker acid (H2S).

Nucleophilicity vs. Leaving Group Ability

Nucleophilicity refers to the ability of a species to donate an electron pair to form a bond, while leaving group ability is about how well a species can depart from a molecule. Generally, a good nucleophile is a poor leaving group and vice versa. Understanding this relationship helps in predicting reaction pathways and the stability of intermediates.