Which of the following is the better leaving group in a polar aprotic solvent?
(c) Br ⁻ vs. I ⁻

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Identify the concept: The problem involves comparing the leaving group ability of Br⁻ and I⁻ in a polar aprotic solvent. Leaving group ability is influenced by the stability of the leaving group after it departs and the solvent environment.

Recall the general trend: In polar aprotic solvents, the leaving group ability is primarily determined by the size and polarizability of the ion. Larger, more polarizable ions tend to be better leaving groups because they can stabilize the negative charge more effectively.

Compare the sizes of Br⁻ and I⁻: Iodide (I⁻) is larger and more polarizable than bromide (Br⁻). This means I⁻ can better stabilize the negative charge after leaving, making it a better leaving group.

Consider the solvent effect: Polar aprotic solvents do not strongly solvate anions, so the intrinsic properties of the leaving group (size and polarizability) dominate. This reinforces the idea that I⁻ is the better leaving group in this environment.

Conclude based on the analysis: Based on size, polarizability, and the nature of polar aprotic solvents, I⁻ is the better leaving group compared to Br⁻.

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Key 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 a pair of electrons. The ability of a leaving group to stabilize the negative charge after departure is crucial; better leaving groups are typically weaker bases. In nucleophilic substitution reactions, the quality of the leaving group significantly influences the reaction rate and mechanism.

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 but not anions, making them favorable for reactions involving nucleophiles. In polar aprotic environments, the nucleophilicity of anions is enhanced, affecting the reactivity of leaving groups.

Nucleophilicity

Nucleophilicity refers to the ability of a species to donate an electron pair to an electrophile, forming a chemical bond. In polar aprotic solvents, nucleophilicity is influenced by the size and charge of the nucleophile. Larger anions, such as iodide (I⁻), are generally more nucleophilic than smaller ones, like bromide (Br⁻), due to their ability to better stabilize the transition state during a reaction.

Which of the following is the better leaving group in a polar aprotic solvent?(c) Br ⁻ vs. I ⁻