Textbook Question
Two elimination products are obtained from the following E2 reaction:
a. What are the elimination products?
b. Which is formed in greater yield?
9. Alkenes and Alkynes
Zaitsev Rule
2:01 minutesProblem 83a
Textbook Question
Draw the major product obtained when each of the following alkyl halides undergoes an E2 reaction:
a. 
Verified step by step guidance1
Identify the alkyl halide structure and classify it as primary, secondary, or tertiary. This classification will help determine the reactivity and the likelihood of elimination (E2) over substitution (SN2).
Recall the E2 mechanism: it is a one-step, concerted reaction where a strong base abstracts a β-hydrogen (hydrogen on a carbon adjacent to the carbon bearing the leaving group), and the leaving group departs simultaneously, forming a double bond.
Locate all possible β-hydrogens in the alkyl halide. Identify the β-carbons (carbons adjacent to the carbon bonded to the leaving group) and determine which β-hydrogens can be abstracted by the base.
Apply Zaitsev's rule: the major product of an E2 reaction is typically the more substituted alkene (the alkene with the greater number of alkyl groups attached to the double-bonded carbons). However, if a bulky base is used, the less substituted alkene (Hofmann product) may dominate.
Draw the structure of the major alkene product by removing the β-hydrogen and the leaving group, forming a double bond between the α-carbon (carbon bonded to the leaving group) and the β-carbon. Ensure proper stereochemistry if applicable, as E2 reactions can lead to stereospecific products (e.g., anti-periplanar geometry).
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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 type of elimination reaction where a base removes a proton from a β-carbon while a leaving group departs from the α-carbon simultaneously. This concerted mechanism results in the formation of a double bond between the α and β carbons. Understanding the stereochemistry and the requirement for anti-periplanar geometry is crucial for predicting the major product.
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Alkyl Halides
Alkyl halides are organic compounds containing a carbon atom bonded to a halogen atom (F, Cl, Br, I). Their structure influences reactivity in elimination reactions, with primary, secondary, and tertiary classifications affecting the likelihood of E2 versus other mechanisms. The strength of the leaving group and the stability of the resulting alkene also play significant roles in determining the product.
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Base Strength and Sterics
The choice of base in an E2 reaction is critical, as strong bases facilitate the elimination process. Sterically hindered bases tend to favor elimination over substitution due to their inability to approach the substrate for nucleophilic attack. Additionally, the steric hindrance of the alkyl halide can influence the reaction pathway, with bulky substrates often leading to more substituted alkene products.
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