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Questions and Answers
What is a characteristic feature of E2 reactions?
What is a characteristic feature of E2 reactions?
Which of the following factors favors elimination over substitution reactions in alkyl halides?
Which of the following factors favors elimination over substitution reactions in alkyl halides?
What is meant by regioselectivity in elimination reactions?
What is meant by regioselectivity in elimination reactions?
Which product is favored when using a strong bulky base in eliminations?
Which product is favored when using a strong bulky base in eliminations?
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In E2 reactions, what is the required stereochemistry involving the E-hydrogen and the leaving group?
In E2 reactions, what is the required stereochemistry involving the E-hydrogen and the leaving group?
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What does stereoselectivity indicate in an elimination reaction?
What does stereoselectivity indicate in an elimination reaction?
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How are alkene substitutions determined in terms of stability?
How are alkene substitutions determined in terms of stability?
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In the E and Z nomenclature system, how are priorities assigned?
In the E and Z nomenclature system, how are priorities assigned?
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Study Notes
Elimination Reactions of Alkyl Halides
- Elimination reactions involve the removal of a leaving group and a β-hydrogen to form a double bond.
- E2 reactions are concerted, meaning the base removes a β-hydrogen and the electrons form the C-C double bond simultaneously, causing the leaving group to depart.
- E2 reactions are favored by strong bases (pKa > 14) with a negative charge, such as hydroxide, and the presence of bulky substrates (2° or 3° alkyl halides). High temperatures (>40 °C) also favor E2 reactions.
- The major product in elimination reactions is typically the most substituted alkene due to its increased stability.
- The choice of base and substrate affects the reaction pathways.
Elimination vs. Substitution
- Elimination reactions are favored over substitution reactions by utilizing strong bases and bulky substrates as described above.
- In contrast, using a weak base such as water promotes a substitution reaction.
- Bulky bases favor elimination over substitution.
Regioselective and Stereoselective Reactions
- Regioselective reactions favor the formation of one constitutional isomer over another.
- In elimination reactions, the major product is usually the most substituted alkene due to its higher stability.
- E2 eliminations are regioselective.
- Stereoselective reactions favor the formation of one stereoisomer over another.
- E2 eliminations are stereoselective when the β-carbon has two or more labile hydrogens.
- E2 elimination reactions show stereoselectivity, often yielding the anti-coplanar product, where the β-hydrogen and the leaving group are opposing each other.
Determining Degree of Substitution in Alkenes
- The degree of substitution of an alkene refers to the number of alkyl groups attached to the carbons involved in the double bond.
- More substituted alkenes are generally more stable due to increased alkyl groups.
Hoffmann vs. Zaitsev Products
- The Zaitsev product is the most substituted alkene and is typically the major product, while the Hoffmann product is the least substituted alkene and is present in a lesser amount.
- The Hoffmann product is favored by using strong, bulky bases.
Stereochemistry of E2 Eliminations
- E2 eliminations proceed when the β-hydrogen and the leaving group are anti-coplanar.
- The relative orientation of the hydrogen and leaving group affects the stereochemistry of the product.
E and Z Nomenclature
- Z-isomers have the two highest priority groups on the same side of the double bond.
- E-isomers have the two highest priority groups on opposite sides of the double bond.
- Priorities are decided based on the atomic weight of the directly attached atoms.
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Description
This quiz covers the mechanisms of elimination reactions involving alkyl halides, focusing on E2 reactions and their preferences for strong bases and bulky substrates. It also contrasts elimination with substitution reactions to highlight different pathways. Test your understanding of these fundamental organic chemistry concepts.
