Questions and Answers
AH undergoes elimination reactions with Bronsted Lowry bases. The elements of HX are lost and an alkene is formed.
True
What is dehydrohalogenation?
A method to introduce a pi bond and prepare an alkene by removing elements from two adjacent atoms.
List some common bases used in dehydrohalogenation.
Na+ -OH, K+ -OH, Na+ -OCH3, Na+ -OCH2CH3, K+ -OC(CH3)3
What is the process to draw the product of dehydrohalogenation?
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Describe a carbon-carbon double bond.
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What defines a monosubstituted alkene?
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Cis isomers have two groups on opposite sides of the double bond.
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Trans alkenes are generally more stable than cis alkenes.
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Sp2 hybridized carbon atoms can donate electron density more easily than sp3 hybridized carbon atoms.
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What is the Zaitsev Rule?
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Alkynes are prepared by two successive dehydrogenation reactions.
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What does it mean for a reaction to be regioselective?
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What are vicinal and geminal dihalides?
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Strong bases favor E1 reactions over E2 reactions.
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Secondary substrates with strong bases and nucleophiles lead to a mixture of Sn2 and E2 products.
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Study Notes
Elimination Reactions Overview
- Alkyl halides (AH) undergo elimination with Brønsted-Lowry bases, losing HX and forming alkenes.
- Dehydrohalogenation: Common method for alkene synthesis involving the removal of HX from adjacent carbons.
Dehydrohalogenation Mechanism
- Involves a base removing a proton from the β carbon, generating a π bond between the α and β carbons.
- Common Bases: Include NaOH, KOH, sodium methoxide, sodium ethoxide, and potassium tert-butoxide.
- Configuration change results in double bond formation along with halogen as the leaving group.
Structural Features of Alkenes
- Carbon-Carbon Double Bond: Consists of a σ bond (from sp² hybrid orbitals) and a π bond (from 2p orbital overlap).
- Alkenes are classified based on substitution: monosubstituted, disubstituted, trisubstituted, tetradisubstituted.
Geometric Isomerism
- Cis Isomer: Groups on the same side of the double bond, restricting rotation.
- Trans Isomer: Groups on opposite sides of the double bond, enhancing stability.
- Stereoisomers can occur when groups at both ends of the double bond differ.
Stability of Alkenes
- Trans alkenes are generally more stable due to fewer steric interactions.
- Increased stability correlates with the number of alkyl substituents around the double bond, attributed to the electron-donating inductive effect.
Mechanism of Elimination
- E2 Mechanism: Bimolecular, involving simultaneous breaking of the C-X bond and formation of a π bond in one step; rate increases with a stronger base and better leaving groups.
- E1 Mechanism: Unimolecular, occurring in two steps; initial formation of a carbocation followed by elimination; favored by weak bases and polar protic solvents.
Regioselectivity and Stereoselectivity
- Reactions can be regioselective (favoring one constitutional isomer) and stereoselective (favoring one stereoisomer).
- E2 reactions favor formation of more substituted alkenes (Zaitsev Rule).
Considerations for Reactions
- Nucleophiles and bases affect whether the pathway is SN or E: strong bases favor E, weaker nucleophiles prefer SN.
- Tertiary substrates with strong bases lead to E2 reactions; with weak bases lead to a mixture of SN1 and E1 products.
- Primary substrates with strong nucleophiles typically result in SN2, while sterically hindered bases lead to E2.
Unique Cases
- Alkynes: Produced via two dehydrohalogenation reactions, requiring stronger bases than for alkenes.
- Vicinal and Geminal Dihalides: Refer to halogen atoms on adjacent or the same carbon atoms, respectively.
Special Arrangements
- E2 elimination often requires anti-periplanar conformation, especially in cyclohexanes, for optimal spatial arrangement.
- Syn-periplanar positioning achieves lower energy but is less common in practical eliminations.
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Test your knowledge of elimination reactions and key concepts in Organic Chemistry with these flashcards from Chapter 8. Dive into terms like dehydrohalogenation and explore how alkenes are formed through these reactions.
