Which is a better leaving group, HO⁻ or H2O? Explain your answer.

Ch. 12 - Substitution and Elimination: Reactions of Haloalkanes

Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition

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Mullins 1st Edition

Ch. 12 - Substitution and Elimination: Reactions of Haloalkanes

문제 71

11장, 문제 71

Which is a better leaving group, HO⁻ or H₂O? Explain your answer.

검증된 단계별 안내

Step 1: Understand the concept of a leaving group. A leaving group is an atom or group of atoms that can detach from the parent molecule during a chemical reaction, typically in substitution or elimination reactions. A good leaving group is stable after it leaves, often as a neutral molecule or a weak base.

Step 2: Compare the stability of HO⁻ (hydroxide ion) and H₂O (water) after they leave. HO⁻ is a negatively charged ion, while H₂O is a neutral molecule. Neutral molecules are generally more stable than charged species, making H₂O more stable than HO⁻.

Step 3: Consider the basicity of the leaving groups. HO⁻ is a strong base, meaning it is less stable and more reactive. H₂O, on the other hand, is a weak base and more stable. Weak bases tend to be better leaving groups because they are less likely to react after leaving.

Step 4: Evaluate the role of solvation and stabilization. In a polar solvent, H₂O can be further stabilized due to hydrogen bonding and dipole interactions, whereas HO⁻, being charged, may experience less stabilization compared to the neutral H₂O.

Step 5: Conclude that H₂O is a better leaving group than HO⁻ because it is neutral, more stable, and less basic, making it more favorable for detachment in a chemical reaction.

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주요 개념

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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 one that can stabilize the negative charge or is able to exist stably in solution after leaving. The ability of a leaving group to depart is influenced by its bond strength and the stability of the resulting species.

Basicity and Leaving Group Ability

The basicity of a leaving group is inversely related to its ability to leave. Strong bases, like HO⁻ (hydroxide ion), are poor leaving groups because they are unstable and highly reactive in solution. In contrast, weaker bases, such as H₂O (water), are better leaving groups because they can stabilize the negative charge more effectively after departure, making them more favorable in substitution reactions.

Stability of Leaving Groups

The stability of a leaving group after it departs is crucial in determining its effectiveness. Water (H₂O) is a stable molecule that can exist freely in solution, while hydroxide (HO⁻) is a highly reactive ion. The stability of the leaving group influences the reaction's kinetics and thermodynamics, with more stable leaving groups facilitating faster and more favorable reactions.