Textbook Question
Predict the products of the following reactions.
(c) benzyl bromide + sodium cyanide
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8. Elimination Reactions
SN1 SN2 E1 E2 Chart (Big Daddy Flowchart)
Problem 41c
Textbook Question
Given the reactants shown, what type of elimination would you expect to occur?
(c) 
Verified step by step guidance1
Analyze the reactants: The substrate is a tertiary alkyl bromide, and the base is lithium hexamethyldisilazide (LiN(SiMe3)2), which is a strong, bulky base.
Understand the elimination mechanism: Bulky bases typically favor the E2 elimination mechanism because they are sterically hindered and cannot easily approach the electrophilic carbon for substitution reactions.
Identify the β-hydrogens: In the substrate, locate the β-carbons (carbons adjacent to the carbon bonded to the bromine) and determine which β-hydrogens are available for elimination.
Consider regioselectivity: The bulky base will preferentially abstract the β-hydrogen from the less sterically hindered position, leading to the formation of the less substituted alkene (Hofmann product).
Predict the outcome: The reaction will proceed via an E2 elimination mechanism, resulting in the formation of an alkene. The major product will likely be the less substituted alkene due to the steric hindrance of the base.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Elimination Reactions
Elimination reactions involve the removal of a small molecule from a larger one, typically resulting in the formation of a double bond. In organic chemistry, these reactions can be classified as either E1 or E2 mechanisms, depending on the reaction conditions and the structure of the substrate. Understanding the type of elimination is crucial for predicting the products formed during the reaction.
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E2 Mechanism
The E2 mechanism is a concerted elimination process where the base abstracts a proton while the leaving group departs simultaneously, leading to the formation of a double bond. This mechanism requires a strong base and is favored by substrates that can stabilize the transition state, such as tertiary or secondary alkyl halides. The stereochemistry of the substrate also plays a significant role in determining the outcome of the reaction.
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Role of Bases in Elimination
In elimination reactions, the choice of base is critical as it influences the reaction pathway and the type of elimination that occurs. Strong bases, such as alkoxides or amides, are typically used to promote E2 eliminations, while weaker bases may lead to E1 reactions. The presence of bulky bases can also affect the regioselectivity and stereoselectivity of the elimination process.
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