Which is a better nucleophile?
e. HO⁻ or ^-NH₂ in NH₃
f. HO⁻ or ^-NH₂ in DMSO

Verified step by step guidance

1. Step 1: Understand the concept of nucleophilicity. Nucleophilicity refers to the ability of a species to donate a pair of electrons to an electrophile. It is influenced by factors such as charge, electronegativity, solvent effects, and steric hindrance.
2. Step 2: Analyze the nucleophiles in question. Both HO⁻ (hydroxide ion) and NH₂⁻ (amide ion) are negatively charged, which generally makes them strong nucleophiles. However, their relative nucleophilicity depends on the solvent and their intrinsic properties.
3. Step 3: Consider the solvent in part (e), which is NH₃ (ammonia). NH₃ is a polar protic solvent, meaning it can form hydrogen bonds with nucleophiles. Polar protic solvents stabilize smaller, more electronegative nucleophiles like HO⁻ through hydrogen bonding, reducing their nucleophilicity. NH₂⁻, being less electronegative and larger, is less stabilized by hydrogen bonding and thus remains a stronger nucleophile in NH₃.
4. Step 4: Consider the solvent in part (f), which is DMSO (dimethyl sulfoxide). DMSO is a polar aprotic solvent, meaning it does not form hydrogen bonds with nucleophiles. In polar aprotic solvents, nucleophilicity is less affected by solvent stabilization, and the intrinsic properties of the nucleophiles dominate. NH₂⁻ is still a better nucleophile than HO⁻ because nitrogen is less electronegative than oxygen, making NH₂⁻ more willing to donate its lone pair of electrons.
5. Step 5: Summarize the findings. In NH₃ (polar protic solvent), NH₂⁻ is a better nucleophile than HO⁻ because it is less stabilized by hydrogen bonding. In DMSO (polar aprotic solvent), NH₂⁻ is also a better nucleophile than HO⁻ due to its lower electronegativity and greater intrinsic nucleophilicity.

Key Concepts

Nucleophilicity

Nucleophilicity refers to the ability of a species to donate an electron pair to an electrophile, forming a chemical bond. It is influenced by factors such as charge, electronegativity, and solvent effects. Stronger nucleophiles are typically negatively charged or have lone pairs that can be readily donated. Understanding nucleophilicity is essential for predicting reaction mechanisms in organic chemistry.

Solvent Effects

The solvent can significantly influence nucleophilicity by stabilizing or destabilizing the nucleophile. Protic solvents, like ammonia (NH3), can solvate nucleophiles through hydrogen bonding, often reducing their reactivity. In contrast, aprotic solvents, such as dimethyl sulfoxide (DMSO), do not solvate anions as effectively, allowing nucleophiles to remain more reactive. Recognizing the role of solvents is crucial for evaluating nucleophilic strength in different environments.

Comparative Nucleophilicity

When comparing nucleophiles, factors such as charge and steric hindrance must be considered. For example, hydroxide (HO−) is generally a stronger nucleophile than amide (−NH2) in protic solvents due to its negative charge. However, in aprotic solvents, the relative nucleophilicity can change based on the solvent's ability to stabilize the nucleophiles. Analyzing these comparisons helps in predicting which nucleophile will be more effective in a given reaction.