Textbook Question
What is the major product of each of the following reactions?
c.
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7. Substitution Reactions
Substitution Comparison
3:09 minutesProblem 20d
Textbook Question
Would the following nucleophiles be more likely to participate in an SN1 or SN2 reaction?
(d) NH3
Verified step by step guidance1
Step 1: Understand the difference between Sₙ1 and Sₙ2 mechanisms. Sₙ1 (unimolecular nucleophilic substitution) involves a two-step process where the leaving group departs first, forming a carbocation intermediate, followed by nucleophilic attack. Sₙ2 (bimolecular nucleophilic substitution) is a one-step process where the nucleophile attacks the substrate simultaneously as the leaving group departs.
Step 2: Evaluate the nucleophile's strength. Strong nucleophiles (e.g., negatively charged species like OH⁻ or CN⁻) favor Sₙ2 reactions because they can directly attack the substrate. Weak nucleophiles (e.g., neutral molecules like H₂O or ROH) are more likely to participate in Sₙ1 reactions, as they rely on the formation of a carbocation intermediate.
Step 3: Consider the substrate structure. Sₙ1 reactions are favored by substrates that can stabilize a carbocation intermediate, such as tertiary carbons or allylic/benzylic carbons. Sₙ2 reactions are favored by substrates with less steric hindrance, such as primary or methyl carbons.
Step 4: Assess the solvent. Polar protic solvents (e.g., water, alcohols) stabilize carbocations and favor Sₙ1 reactions. Polar aprotic solvents (e.g., DMSO, acetone) enhance the nucleophilicity of strong nucleophiles and favor Sₙ2 reactions.
Step 5: Apply these principles to the given nucleophile. Determine whether the nucleophile is strong or weak, the substrate's structure, and the solvent conditions to predict whether the reaction will proceed via an Sₙ1 or Sₙ2 mechanism.
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3mKey Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Nucleophilicity
Nucleophilicity refers to the ability of a nucleophile to donate an electron pair to an electrophile during a chemical reaction. Stronger nucleophiles are more likely to participate in nucleophilic substitution reactions, influencing whether the reaction follows an Sₙ1 or Sₙ2 mechanism. Factors such as charge, electronegativity, and solvent can affect nucleophilicity.
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Sₙ1 Mechanism
The Sₙ1 mechanism is a two-step nucleophilic substitution process where the first step involves the formation of a carbocation intermediate after the leaving group departs. This mechanism is favored by tertiary substrates and polar protic solvents, as they stabilize the carbocation. The rate of the reaction depends only on the concentration of the substrate, making it unimolecular.
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Sₙ2 Mechanism
The Sₙ2 mechanism is a one-step nucleophilic substitution process where the nucleophile attacks the substrate simultaneously as the leaving group departs. This concerted mechanism requires strong nucleophiles and is favored by primary substrates and polar aprotic solvents. The reaction rate depends on both the substrate and the nucleophile, making it bimolecular.
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