Textbook Question
When 2-bromo-3-phenylbutane is treated with sodium methoxide, two alkenes result (by E2 elimination). The Zaitsev product predominates.
a. Draw the reaction, showing the major and minor products.
9. Alkenes and Alkynes
Zaitsev Rule
6:12 minutesProblem 96b
Textbook Question
For each of the following alkyl halides, indicate the stereoisomer that would be obtained in greatest yield in an E2 reaction.
b. 4-bromo-2,2,3,3-tetramethylpentane
Verified step by step guidance1
Analyze the structure of 4-bromo-2,2,3,3-tetramethylpentane. Identify the β-hydrogens (hydrogens on the carbons adjacent to the carbon bearing the bromine atom). Note that the E2 reaction requires a β-hydrogen to proceed.
Determine the anti-periplanar geometry required for the E2 elimination. In an E2 reaction, the leaving group (bromine) and the β-hydrogen must be in an anti-periplanar (coplanar but opposite) arrangement for the elimination to occur.
Examine the stereochemistry of the molecule. Identify which β-hydrogens can adopt the anti-periplanar geometry with the bromine atom. This will dictate which alkene product is formed in the greatest yield.
Apply Zaitsev's rule to predict the major product. Zaitsev's rule states that the more substituted alkene (the one with more alkyl groups attached to the double-bonded carbons) is generally favored in an E2 reaction.
Draw the structure of the most substituted alkene that can be formed based on the anti-periplanar geometry and Zaitsev's rule. This will be the stereoisomer obtained in the greatest yield.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
E2 Reaction Mechanism
The E2 (bimolecular elimination) reaction is a concerted process where a base abstracts a proton from a β-carbon while a leaving group departs from the α-carbon, resulting in the formation of a double bond. This mechanism typically requires a strong base and is stereospecific, favoring the formation of the more stable alkene product, often following the Zaitsev's rule.
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Stereochemistry of Alkenes
Stereochemistry refers to the spatial arrangement of atoms in molecules and is crucial in determining the properties of alkenes. In E2 reactions, the stereochemistry of the starting alkyl halide influences the stereoisomer of the alkene produced, with trans alkenes generally being more stable than cis due to reduced steric strain.
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Steric Hindrance
Steric hindrance occurs when bulky groups around a reactive center impede the approach of reactants or the transition state. In the case of 4-bromo-2,2,3,3-tetramethylpentane, the presence of multiple bulky methyl groups can influence the regioselectivity and stereoselectivity of the E2 reaction, favoring the formation of the less hindered alkene.
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