Textbook Question
Which member in each pair in [PROBLEM 9-68] is a better leaving group?
e. I− or Br−
f. Cl− or Br−
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7. Substitution Reactions
Good Leaving Groups
1:59 minutesProblem 25a
Textbook Question
Which of the following is the better leaving group in a polar aprotic solvent?
(a) HO⁻ vs. F⁻
Verified step by step guidance1
Understand the concept of leaving groups: A leaving group is an atom or group of atoms that can depart with a pair of electrons during a chemical reaction. A better leaving group is typically more stable after leaving, often due to its ability to stabilize the negative charge.
Recall the effect of the solvent: In a polar aprotic solvent, the solvent does not donate hydrogen bonds to stabilize ions. This means that the leaving group's stability depends more on its intrinsic properties rather than solvent interactions.
Compare the basicity of the leaving groups: Basicity is inversely related to leaving group ability. A weaker base is a better leaving group because it is more stable after leaving. Hydroxide ion (HO⁻) is a stronger base than fluoride ion (F⁻), as oxygen is less electronegative than fluorine.
Consider the electronegativity of the atoms: Fluorine is more electronegative than oxygen, meaning it can better stabilize the negative charge when it becomes F⁻. This makes F⁻ a better leaving group than HO⁻ in general.
Conclude based on the solvent and properties: In a polar aprotic solvent, F⁻ is the better leaving group compared to HO⁻ because it is less basic and more stable as an ion due to its high electronegativity.
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1mKey Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Leaving Groups
Leaving groups are atoms or groups of atoms that can depart from a molecule during a chemical reaction, taking with them the electrons that were shared with the molecule. A good leaving group is typically stable after departure, which allows for smoother reaction pathways. Common examples include halides and water, while poor leaving groups include hydroxide (OH⁻) and fluoride (F⁻).
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The 3 important leaving groups to know.
Polar Aprotic Solvents
Polar aprotic solvents are solvents that have a significant dipole moment but do not have hydrogen atoms bonded to electronegative atoms, which means they cannot form hydrogen bonds. These solvents can stabilize cations and polarize anions, enhancing the reactivity of nucleophiles. Examples include acetone and dimethyl sulfoxide (DMSO), which are often used in reactions involving strong nucleophiles.
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Nucleophilicity and Basicity
Nucleophilicity refers to the ability of a species to donate an electron pair to an electrophile, while basicity measures the tendency of a species to accept protons. In polar aprotic solvents, nucleophilicity is often enhanced for anions because these solvents do not solvate anions as effectively as protic solvents. This distinction is crucial when comparing the leaving groups HO⁻ and F⁻, as their stability and reactivity can vary significantly in different solvent environments.
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06:21Understanding the difference between basicity and nucleophilicity.
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