Textbook Question
For each pair, predict the stronger nucleophile in the SN2 reaction (using an alcohol as the solvent). Explain your prediction.
c. NH3 or PH3
d. CH3S– or H2S
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8. Elimination Reactions
Nucleophiles and Basicity
2:26 minutesProblem 68e,f
Textbook Question
Which member of each pair is a better nucleophile in methanol?
e. I− or Br−
f. Cl− or Br−
Verified step by step guidance1
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 depends on factors such as charge, electronegativity, solvent effects, and the size of the nucleophile.
Step 2: Consider the solvent. Methanol is a polar protic solvent, meaning it can form hydrogen bonds with nucleophiles. In polar protic solvents, larger, less electronegative ions are better nucleophiles because they are less tightly solvated and can more easily donate their electrons.
Step 3: Compare I⁻ and Br⁻. Iodide (I⁻) is larger and less electronegative than bromide (Br⁻). In a polar protic solvent like methanol, I⁻ is less solvated than Br⁻, making it a better nucleophile.
Step 4: Compare Cl⁻ and Br⁻. Chloride (Cl⁻) is smaller and more electronegative than bromide (Br⁻). In a polar protic solvent like methanol, Br⁻ is less solvated than Cl⁻, making Br⁻ a better nucleophile.
Step 5: Summarize the findings. In methanol, I⁻ is a better nucleophile than Br⁻, and Br⁻ is a better nucleophile than Cl⁻. This trend is due to the effects of solvation in a polar protic solvent.
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Video duration:
2mKey Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
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. In polar protic solvents like methanol, nucleophiles are often stabilized by solvation, which can affect their reactivity.
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Solvent Effects
The solvent can significantly impact nucleophilicity, especially in polar protic solvents like methanol. In such solvents, nucleophiles with larger, more polarizable anions are often better nucleophiles because they are less solvated compared to smaller, more electronegative anions. This solvation effect can hinder the reactivity of nucleophiles.
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Comparative Nucleophilicity
Comparative nucleophilicity involves evaluating the reactivity of different nucleophiles in a given solvent. For example, in the pairs I− vs. Br− and Cl− vs. Br−, the larger size and lower electronegativity of I− make it a better nucleophile than Br− in methanol, while Br− is generally a better nucleophile than Cl− due to similar considerations.
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