Textbook Question
Propose a mechanism for each of the following reactions:
a.
8. Elimination Reactions
Cumulative Substitution/Elimination
Problem 44d
Textbook Question
Paying close attention to the stereochemical outcome, predict the product of these elimination reactions.
(d) 
Verified step by step guidance1
Identify the reaction type: The presence of NaOH and H2O suggests that this is an elimination reaction, specifically an E2 mechanism due to the strong base (NaOH). In an E2 reaction, the base removes a proton from a β-carbon, and the leaving group (Br) departs simultaneously, forming a double bond.
Analyze the stereochemistry: In an E2 reaction, the anti-periplanar geometry is required. This means the β-hydrogen and the leaving group (Br) must be in opposite planes (one on a wedge and the other on a dash). Check the structure to identify the β-hydrogens that are anti-periplanar to the bromine.
Determine the major product: The elimination will form the most stable alkene. Stability is influenced by the degree of substitution of the double bond (more substituted alkenes are more stable due to hyperconjugation and inductive effects). Evaluate the possible double bonds that can form and select the most substituted one.
Consider regioselectivity: This reaction follows Zaitsev's rule, which states that the most substituted alkene is favored as the major product. Compare the possible alkenes and confirm which one is the most substituted.
Draw the product: Based on the stereochemical requirements and Zaitsev's rule, sketch the structure of the major product, ensuring the correct placement of the double bond and stereochemistry.
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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 often occur with alkyl halides, where a leaving group (like Br) is expelled, and a pi bond is formed between adjacent carbon atoms. Understanding the mechanism, such as E2 or E1 pathways, is crucial for predicting the products.
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Stereochemistry
Stereochemistry refers to the study of the spatial arrangement of atoms in molecules and how this affects their chemical behavior. In elimination reactions, the stereochemical outcome can lead to different products, such as cis or trans isomers, depending on the orientation of the leaving group and the hydrogen being removed. Recognizing stereochemical configurations is essential for predicting the correct product.
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Base Strength and Reaction Conditions
The strength of the base used in elimination reactions significantly influences the reaction pathway and product formation. Strong bases, like NaOH, favor E2 mechanisms, leading to the formation of alkenes through a concerted process. Additionally, the solvent (in this case, water) can affect the reaction's outcome, making it important to consider both base strength and conditions when predicting products.
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